Agronomy
D VA N C E S I N
VOLUME 92
Advisory Board John S. Boyer University of Delaware
Paul M. Bertsch University of Georgia
Ronald L. Phillips University of Minnesota
Kate M. Scow University of California, Davis
Larry P. Wilding Texas A&M University
Emeritus Advisory Board Members Kenneth J. Frey Iowa State University
Eugene J. Kamprath North Carolina State University
Martin Alexander Cornell University
Prepared in cooperation with the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America Book and Multimedia Publishing Committee David D. Baltensperger, Chair Lisa K. Al-Amoodi Kenneth A. Barbarick
Hari B. Krishnan Sally D. Logsdon Michel D. Ransom
Craig A. Roberts April L. Ulery
Agronomy D VA N C E S I N
VOLUME 92 Edited by
Donald L. Sparks Department of Plant and Soil Sciences University of Delaware Newark, Delaware
AMSTERDAM • BOSTON • HEIDELBERG • LONDON NEW YORK • OXFORD • PARIS • SAN DIEGO SAN FRANCISCO • SINGAPORE • SYDNEY • TOKYO Academic Press is an imprint of Elsevier
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Contents CONTRIBUTORS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PREFACE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ix xi
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS Robin D. Graham, Ross M. Welch, David A. Saunders, Ivan Ortiz-Monasterio, Howarth E. Bouis, Merideth Bonierbale, Stef de Haan, Gabriella Burgos, Graham Thiele, Reyna Liria, Craig A. Meisner, Steve E. Beebe, Michael J. Potts, Mohinder Kadian, Peter R. Hobbs, Raj K. Gupta and Steve Twomlow I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Food Systems, Diet, and Disease . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Farming for Health. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. The Bioavailability Imperative . . . . . . . . . . . . . . . . . . . . . . . . . . III. Agricultural Interventions to Deliver Individual Limiting Nutrients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. The Plant Breeding Strategy . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Fertilizer Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Zn Deficiency Is Important . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Diet Diversification Through Food Systems Approaches . . . . . . IV. Analysis of Subsistence Food Systems . . . . . . . . . . . . . . . . . . . . . . . A. The Rice–Wheat Food System . . . . . . . . . . . . . . . . . . . . . . . . . . B. Rice/Potato-Based Food Systems in the IGP . . . . . . . . . . . . . . . C. A Case Study: The Dysfunctional Rice–Pulse Food System of Southeast Bangladesh . . . . . . . . . . . . . . . . . . . . . . . . . D. Bean Food Systems in Central America . . . . . . . . . . . . . . . . . . . E. Potato-Based Food Systems, Huancavelica Department, Peru . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. Subsistence Food Systems of Eastern and Southern Africa . . . . G. The Rice–Fish System: The Value of Rice–Fish Farming Systems as a Nutrient Delivery System for Households and Communities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
v
2 6 7 7 8 9 10 11 13 14 17 23 27 31 36 44
54
vi
CONTENTS V. The Socioeconomic and Policy Environments . . . . . . . . . . . . . . . . . A. Household Incomes, Food Prices, and Agricultural Development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Additions of Essential Trace Elements to Soils. . . . . . . . . . . . . . C. Biofortification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . D. Introducing New Nutrient-Dense Crops into Cropping Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
60 60 63 65 66 66 67
POLYACRYLAMIDE IN AGRICULTURE AND ENVIRONMENTAL LAND MANAGEMENT R. E. Sojka, D. L. Bjorneberg, J. A. Entry, R. D. Lentz and W. J. Orts I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII. XIII. XIV. XV. XVI. XVII. XVIII.
Early Uses of Soil Conditioners . . . . . . . . . . . . . . . . . . . . . . . . . . . . Major Conditioner Types. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Synthetic Conditioner Uses and Application Strategies . . . . . . . . . . Overview of Current PAM Use . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAM Defined and Described . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAM Properties AVecting EYcacy. . . . . . . . . . . . . . . . . . . . . . . . . . Early Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Irrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sprinkler Irrigation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Infiltration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAM Safety, Field Retention, and Environmental Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAM EVect on Organisms in RunoV and Soil . . . . . . . . . . . . . . . . . PAM Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAM and Ca . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PAM for Construction Sites and Other Disturbed Lands . . . . . . . . Canal and Pond Sealing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biopolymers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
76 77 78 81 84 86 93 95 102 107 115 122 127 131 133 137 139 141 141
CONTENTS
vii
NUTRIENTS IN AGROECOSYSTEMS: RETHINKING THE MANAGEMENT PARADIGM L. E. Drinkwater and S. S. Snapp I. II. III. IV.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nutrient Management in Agriculture . . . . . . . . . . . . . . . . . . . . . . . . Internal Biogeochemical Processes in Agroecosystems . . . . . . . . . . . Toward an Ecosystem-Based Approach to Improving Nutrient Use EYciency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Using Plant Diversity to Restore Ecosystem Functions . . . . . . . B. Restoration of Ecosystem Function Through Plant–Microbial Interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Microbially Mediated Processes . . . . . . . . . . . . . . . . . . . . . . . . . V. Plant Adaptation to Ecosystem-Based Nutrient Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VI. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
164 165 167 168 172 174 175 177 178 179 180
RICE AND WATER B. A. M. Bouman, E. Humphreys, T. P. Tuong and R. Barker I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . II. Trends and Conditions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. The Rice Plant . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Rice Environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . C. Water Use and Water Productivity . . . . . . . . . . . . . . . . . . . . . . . D. Ecosystem Services . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . E. Environmental Impacts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . F. The Main Challenges Ahead . . . . . . . . . . . . . . . . . . . . . . . . . . . . III. Response Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . A. Varietal Improvement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . B. Improved Management Practices . . . . . . . . . . . . . . . . . . . . . . . . C. Options at the Landscape Level . . . . . . . . . . . . . . . . . . . . . . . . . IV. Summary and Recommendations . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
188 190 190 191 197 200 202 206 208 208 211 221 226 227 228
INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
239
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Contributors Numbers in parentheses indicate the pages on which the authors’ contributions begin.
R. Barker (187), International Water Management Institute, Colombo, Sri Lanka Steve E. Beebe (1), International Center for Tropical Agriculture, Cali, Colombia D. L. Bjorneberg (75), Northwest Irrigation and Soils Research Laboratory, USDA Agricultural Research Service, 3793N-3600E Kimberly, Idaho 83341 Merideth Bonierbale (1), International Potato Center, Lima, Peru Howarth E. Bouis (1), International Food Policy Research Institute, Washington, DC 20006 B. A. M. Bouman (187), International Rice Research Institute, Los Ban˜os, Philippines Gabriella Burgos (1), International Potato Center, Lima, Peru L. E. Drinkwater (163), Department of Horticulture, Cornell University, Ithaca, New York 14853 J. A. Entry (75), Northwest Irrigation and Soils Research Laboratory, USDA Agricultural Research Service, 3793N-3600E Kimberly, Idaho 83341 Robin D. Graham (1), School of Agriculture, Food and Wine, University of Adelaide, South Australia 5005 Raj K. Gupta (1), International Maize and Wheat Improvement Center, New Delhi, India Stef de Haan (1), International Potato Center, Lima, Peru Peter R. Hobbs (1), Cornell University, Ithaca, New York 14853 E. Humphreys (187), CSIRO Land and Water, PMB 3 GriYth, NSW 2680, Australia Mohinder Kadian (1), International Potato Center, Delhi, India R. D. Lentz (75), Northwest Irrigation and Soils Research Laboratory, USDA Agricultural Research Service, 3793N-3600E Kimberly, Idaho 83341 Reyna Liria (1), Instituto de Investigacio´n Nutricional, Lima, Peru Craig A. Meisner (1), International Fertilizer Development Center, Dhaka, Bangladesh Ivan Ortiz-Monasterio (1), International Maize and Wheat Improvement Center, Obregon, Mexico W. J. Orts (75), Byproducts Engineering and Utilization Research Unit, USDA Agricultural Research Service, Albany, California 94710 Michael J. Potts (1), International Potato Center, Kampala, Uganda David A. Saunders (1), Interag Pty. Ltd., Victor Harbor, South Australia 5211 ix
x
CONTRIBUTORS
S. S. Snapp (163), Department of Crop and Soil Science and W. K. Kellogg Biological Station, Michigan State University, East Lansing, Michigan 48824 R. E. Sojka (75), Northwest Irrigation and Soils Research Laboratory, USDA Agricultural Research Service, 3793N-3600E Kimberly, Idaho 83341 Graham Thiele (1), International Potato Center, Quito, Ecuador T. P. Tuong (187), International Rice Research Institute, Los Ban˜os, Philippines Steve Twomlow (1), International Crops Research Institute for the Semi-Arid Tropics, Bulawayo, Zimbabwe Ross M. Welch (1), Soil and Nutrition Laboratory, USDA-ARS Plant, Ithaca, New York 14853
Preface Volume 92 contains four outstanding and timely reviews focusing on aspects of food production and human health, agricultural sustainability, and preservation of the environment. Chapter 1 is a comprehensive review of nutritious food systems. The primary subsistence food systems globally are discussed and ways to improve cropping systems and enhance the nutritional value of the food crops are evaluated. Chapter 2 is a comprehensive review of polyacrylamide in agricultural and environmental land management. Chapter 3 deals with alternative methods for managing nutrients in the environment with emphasis on an ecosystem-based approach that would optimize nutrient reservoirs via microbially driven and plant-driven processes. Chapter 4 is a comprehensive review of rice and water. Rice environments, water use and productivity, environmental impacts, and varietal improvements are discussed. I am grateful to the authors for their excellent contributions. DONALD L. SPARKS University of Delaware Newark, Delaware
xi
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS Robin D. Graham,1 Ross M. Welch,2 David A. Saunders,3 Ivan Ortiz‐ Monasterio,4 Howarth E. Bouis,5 Merideth Bonierbale,6 Stef de Haan,6 Gabriella Burgos,6 Graham Thiele,7 Reyna Liria,8 Craig A. Meisner,9 Steve E. Beebe,10 Michael J. Potts,11 Mohinder Kadian,12 Peter R. Hobbs,13 Raj K. Gupta14 and Steve Twomlow15 1
School of Agriculture, Food and Wine, University of Adelaide, South Australia 5005 2 Soil and Nutrition Laboratory, USDA‐ARS Plant, Ithaca, New York 14853 3 Interag Pty. Ltd., Victor Harbor, South Australia 5211 4 International Maize and Wheat Improvement Center, Obregon, Mexico 5 International Food Policy Research Institute, Washington, DC 20006 6 International Potato Center, Lima, Peru 7 International Potato Center, Quito, Ecuador 8 Instituto de Investigacio´n Nutricional, Lima, Peru 9 International Fertilizer Development Center, Dhaka, Bangladesh 10 International Center for Tropical Agriculture, Cali, Colombia 11 International Potato Center, Kampala, Uganda 12 International Potato Center, Delhi, India 13 Cornell University, Ithaca, New York 14853 14 International Maize and Wheat Improvement Center, New Delhi, India 15 International Crops Research Institute for the Semi‐Arid Tropics, Bulawayo, Zimbabwe
I. Introduction II. Food Systems, Diet, and Disease A. Farming for Health B. The Bioavailability Imperative III. Agricultural Interventions to Deliver Individual Limiting Nutrients A. The Plant Breeding Strategy B. Fertilizer Strategies C. Zn Deficiency Is Important D. Diet Diversification Through Food Systems Approaches IV. Analysis of Subsistence Food Systems A. The Rice–Wheat Food System B. Rice/Potato‐Based Food Systems in the IGP C. A Case Study: The Dysfunctional Rice–Pulse Food System of Southeast Bangladesh D. Bean Food Systems in Central America E. Potato‐Based Food Systems, Huancavelica Department, Peru F. Subsistence Food Systems of Eastern and Southern Africa G. The Rice–Fish System: The Value of Rice–Fish Farming Systems as a Nutrient Delivery System for Households and Communities 1 Advances in Agronomy, Volume 92 Copyright 2007 Elsevier Inc. All rights reserved. 0065-2113/07 $35.00 DOI: 10.1016/S0065-2113(04)92001-9
2
R. D. GRAHAM ET AL. V. The Socioeconomic and Policy Environments A. Household Incomes, Food Prices, and Agricultural Development B. Additions of Essential Trace Elements to Soils C. Biofortification D. Introducing New Nutrient‐Dense Crops into Cropping Systems VI. Conclusions References
The major subsistence food systems of the world that feed resource‐poor populations are identified and their capacity to supply essential nutrients in reasonable balance to the people dependent on them has been considered for some of these with a view to overcoming their nutrient limitations in sound agronomic and sustainable ways. The approach discusses possible cropping system improvements and alternatives in terms of crop combinations, external mineral supply, additional crops, and the potential for breeding staples in order to enhance their nutritional balance while maintaining or improving the sustainability and dietary, agronomic, and societal acceptability of the system. The conceptual framework calls for attention first to balancing crop nutrition that in nearly every case will also increase crop productivity, allowing suYcient staple to be produced on less land so that the remaining land can be devoted to more nutrient‐dense and nutrient‐balancing crops. Once this is achieved, the additional requirements of humans and animals (vitamins, selenium, and iodine) can be addressed. Case studies illustrate principles and strategies. This chapter is a proposal to widen the range of tools and strategies that could be adopted in the HarvestPlus Challenge Program to achieve its goals of eliminating micronutrient deficiencies in the food systems of resource‐poor countries. # 2007, Elsevier Inc.
I. INTRODUCTION Life cannot exist without continuous supplies in adequate amounts of all essential nutrients. If even one nutrient is limiting or missing from the nutrient medium or diet of an organism, the organism will suVer and ultimately die. Thus, nutrient supplies form the basis of all life on earth. Humans are no exception. Indeed, massive numbers of people are dying each year because of a lack of suYcient nutrients to thrive. The magnitude of this crisis in human health is appalling with annually over 50% of all deaths on earth associated with malnutrition (World Health Organization, 2003). In 2003 this amounted to about 30 million deaths mostly among the resource‐poor people in developing countries (Muller and Krawinkel, 2005; World Health Organization, 2003). Incredibly, approximately one (i.e., 0.95) person dies of diet‐related diseases every second. No other causes
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
3
Occupational safety
Risk factor
Unsafe water, sanitation, hygiene
Alcohol
Unsafe sex
Tobacco
Diet-related diseases
0
5000
10,000
15,000
20,000
25,000
Number of Deaths (31000)
Figure 1 Global human deaths in 2002 related to various risk factors (data from the World Health Report, World Health Organization, 2002).
of death (Fig. 1) and related misery even come close. Importantly, these deaths are preventable if sustainable solutions to malnutrition are implemented. Micronutrient malnutrition alone [e.g., iron (Fe), iodine (I), selenium (Se), zinc (Zn), and various vitamin deficiencies] aZicts over 3 billion people worldwide (Mason and Garcia, 1993). The consequences to human health, felicity, livelihoods, and national development are staggering, resulting in increased mortality and morbidity rates, decreased worker productivity and poverty. These circumstances are associated with and indeed begin with diminished cognitive ability and lower educational potential in children born to deficient mothers (Bhaskaram, 2002; World Health Organization, 2002). Dr. Bro Harlem Brundtland (Director General, World Health Organization, United Nations) declared at the World Economic Forum in 2000 that: Nutrition is a key element to any strategy to reduce the global burden of disease. Hunger, malnutrition, obesity and unsafe food all cause disease, and better nutrition will translate into large improvements in health among all of us, irrespective of our wealth and home country. (World Health Organization, 2002) Further, the World Health Organization’s 2002 World Health Report states that inadequate food and its poor nutritive value lead to a downward spiral of increased susceptibility to illness and loss of livelihood, ending in
4
R. D. GRAHAM ET AL.
premature death. Micronutrient deficiencies continue to increase in many nations. For example, the global burden of Fe deficiency has risen from about 35% of the world’s population in 1960 to over 50% in 2000 (World Health Organization, 2002) and the population during this time had almost doubled. Fe deficiency among poor women is increasing at an alarming rate in many developing countries. Current intervention programs (i.e., food fortification and supplementation programs) to alleviate the problem have not proven to be eVective or sustainable in many countries (Darnton‐Hill, 1999). Furthermore, globally these programs have been limited to addressing only Fe, I, and vitamin A deficiencies, often singly, with no global programs currently planned for addressing other limiting essential nutrients in the diets of the poor, not to mention the necessity of addressing all limiting nutrients together. The US National Academy of Sciences held a workshop in 2003 titled ‘‘Exploring a vision: Integrating knowledge for food and health’’ where attendees recommended a new paradigm for agriculture, one that closely links agriculture to human health. At that meeting, Dr. Charles Muscoplat, Dean, College of Agriculture, Food, and Environmental Sciences, University of Minnesota stated: It is time for the United States to shift to a new agricultural paradigm—one based on both what is good for the consumer and profitable for farmers. The workshop recommended that new research was needed to address the gaps at the intersection of agriculture and food, food and people, and people and health (Fig. 2). At the 57th World Health Assembly in 2004, malnutrition (both undernutrition and nutritional deficiencies) was acknowledged as a major cause of death and disease globally (World Health Organization, 2004). Noncommunicable diseases were recognized as being in crisis proportions in developed countries and rapidly increasing in developing nations. In 2001, chronic diseases (many diet related) accounted for almost 60% of the 56 million deaths annually and 47% of the global burden of disease. It was recommended that national food and agricultural policies be consistent with the protection and promotion of public health. Member States were asked to take healthy nutrition into account in their agricultural policies. In 2004, leading economists from around the world met in Copenhagen, Denmark to rank the 10 most important global challenges that nations should invest in. From that meeting the Copenhagen Consensus was born which included the top 10 challenges facing the world today. Two of the top 10 challenges in the Copenhagen Consensus (numbered 2 and 5) included: malnutrition—providing micronutrients to meet human needs, and development of new agricultural technologies to address malnutrition (Copenhagen
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
What influences consumer choice?
Food
What makes food healthful?
Agriculture
5
People
How does what people eat impact health?
Health
Making a closer connection between agriculture and health will require that research address gaps at the intersection of agriculture and food, food and people, and people and health. Figure 2 Table to farm: A new agriculture paradigm, linking agriculture to human health, modified from Rouse and Davis (2004).
Consensus, 2004). Thus, the world’s most respected economists recognized the importance of linking agriculture to human health. This global crisis in malnutrition is the result of dysfunctional food systems that cannot deliver enough essential nutrients to meet the requirements of all. Because agriculture is the primary source of all nutrients (excluding water and oxygen) for humans, agricultural systems must be contributing to this failure to meet nutritional needs (Welch et al., 1997). How can agricultural systems be changed in ways that will result in enough nutrient output of farming systems to assure adequate nutrition for all? Importantly, if agricultural technologies are directed at improving the nutritional quality of food crops, they must encompass a holistic food system perspective to assure that the intervention will be sustainable, and adopted by farmers and consumers. Further, the agriculture sector must adopt a specific goal of improving human nutrition and health, and the nutrition and health sectors must adopt agricultural interventions as a primary tool to fight malnutrition (Welch and Graham, 1999).
R. D. GRAHAM ET AL.
6
II. FOOD SYSTEMS, DIET, AND DISEASE Humans require at least 51 known nutrients (Fig. 3), in adequate amounts, consistently, to live healthy and productive lives. Unfortunately, global food systems are failing to provide adequate quantities of all of these essential nutrients to vast numbers of people. Advances in crop production, incurred during the Green Revolution, were dependent mostly on improvements in cereal cropping systems (rice, wheat, and maize) and resulted in greatly increased food supplies for the world, preventing mass starvation in many nations. However, cereals as normally eaten only supply needed carbohydrates for energy, a modest amount of protein but few other nutrients in required amounts. This change in agricultural production toward systems of cereal monoculture and away from more varied cropping systems appears to be contributing to micronutrient deficiencies by limiting food crop diversity (Welch, 2001a). This has had the unforeseen consequences of reducing available micronutrient supplies to resource‐poor populations
The known 51 essential nutrients for sustaining human life* Air, Water and Energy (3)
Protein (amino acids) (9)
Lipids-Fat (fatty acids) (2)
Macrominerals (7)
Trace elements (17)
Vitamins (13)
Oxygen
Histidine
Linoleic acid
Na
Fe
A
Water
Isoleucine
Linolenic acid
K
Zn
D
Carbohydrates
Leucine
Ca
Cu
E
Lysine
Mg
Mn
K
Methionine
S
I
C (Ascorbic acid)
Phenylalanine
P
Threonine
Cl
Tryptophan
F Se Si Mo
Valine
Co (in B12) B** Ni**
B1 (Thiamine) B2 (Riboflavin) B3 (Niacin) B5 (Pantothenic acid) B6 (Pyroxidine) B7/H (Biotin)
Cr**
B9 (Folic acid, folacin)
V**
B12 (Cobalamin)
As** Li** Sn** *Numerous other beneficial substances in food are also known to contribute to good health. ** Not generally recognized as essential but some supporting evidence published.
Figure 3 The known essential nutrients for sustaining human life, modified from Welch and Graham (2004).
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
7
formerly dependent on more diverse cropping systems which provided more traditional micronutrient‐rich food crops such as pulses, fruits, and certain vegetables that are now in low supply and no longer aVordable to this sector of society (Graham et al., 2001; Tontisirin et al., 2002). Nutrition transitions in rapidly developing nations are also causing increased rates of chronic diseases (e.g., cancer, heart disease, diabetes, obesity, osteoporosis, and so on) where people are shifting from traditional diets to more calorie‐rich diets derived from adopting developed nations’ food systems (Clugston and Smith, 2002). There is an urgent need to tightly link the agricultural and food processing sectors to human health to find ways to reduce the burden of diet‐related diseases in the world.
A. FARMING
FOR
HEALTH
There are numerous ways in which agriculture can contribute to improving human nutrition and health. Of high priority is increasing the output of micronutrients in staple food crops from farming systems to meet human needs, utilizing genetic variation within crop germplasm banks (biofortification), applying fertilizers, diversifying food systems, and increasing income (Graham et al., 2001). Agriculture must be closely linked to human health if we are to find sustainable solutions to nutrient deficiencies aZicting the lives and health of massive numbers of people globally.
B. THE BIOAVAILABILITY IMPERATIVE ‘‘You are not what you eat. You are what you eat and do not excrete.’’ This is because not all of the nutrients consumed in meals can be absorbed and utilized in the body (i.e., not all nutrients in diet matrices are ‘‘bioavailable’’) because of various interactions that operate in the digestive system. Various antinutrients (substances, especially in staple plant foods, which inhibit the bioavailability of nutrients) can dramatically reduce the amount of a nutrient that is absorbable from a meal. However, other substances (promoters) can counteract the negative eVects of antinutrients on nutrient bioavailability. Thus, it is imperative that agriculture not only increases the levels of nutrients in staple plant foods but also attend to the eVects of antinutrients by enhancing content of promoters. In this chapter, there is not enough room to discuss the implications of bioavailability to improving the nutritional quality of plant foods. Various review articles are available for those interested in pursuing this aspect of the nutritional quality of staple plant foods (refer to Lopez and Martos, 2004; van het Hof et al., 2000; Welch, 2002a,b).
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III. AGRICULTURAL INTERVENTIONS TO DELIVER INDIVIDUAL LIMITING NUTRIENTS There are several strategies that can be deployed in agriculture to increase the delivery of nutrients to people dependent on its production. Plant breeding is a highly favored strategy because appropriate new varieties are readily accepted by farmers. This has been well demonstrated by the large numbers of small producers who accepted and benefited from the new, high yielding varieties of the Green Revolution. On the whole, this approach requires the least change in behavior on the part of the subsistence farmer, so impact can be high. Of course, to be acceptable, these new, more nutritious varieties must satisfy the profit criteria that any other new variety must in order to be widely grown: high yield and acceptable cooking/eating quality. Nutrient content also can be increased by use of fertilizers, both mineral and organic. Fertilizer use is common in Asia but much less so in Africa in spite of the fact that Africa’s soils are often highly infertile. Fertilizers are relatively expensive and all other determinants of production must be reasonably well optimized for fertilizer use to be economic, and so resistance on the part of farmers is more common than with the use of new varieties. Nevertheless, for some nutrients commonly deficient in agricultural soils [nitrogen (N), phosphorus (P), potassium (K), sulfur (S), Zn], it is possible to boost nutrient content more by fertilizer use than through plant breeding. If this also increases yield, it is generally profitable for farmers to use fertilizer for the yield and profit advantage, and the superior nutrient content is a bonus without additional cost to the farmer. Other agronomic practices can also aVect nutrient concentration in foodstuVs: gypsum, lime, green manures, minimum tillage, and intercropping. The remaining intervention of concern in this concept paper is to alter the cropping mix to create or recreate diet diversity in order to balance the nutrient supply to the diet by exploiting the diVerences in nutrient content of various crops. Diet diversity was lost during the Green Revolution with its emphasis on high yielding cereals. As the population continues to grow, although fortunately not at such high rates (Lutz et al., 2001), it is quite diYcult to turn the clock back to a time when diet diversity was much richer than now. However, we argue here that it is possible with the use of best‐practice agronomy on the main staples to maximize yields, allowing the subsistence farmer the luxury of devoting less land to cereal production and more to secondary crops that diversify the diet and help to balance the nutrients delivered by the food system. Combining all three tools, breeding, fertilizers, and diversifying diets, is obviously complicated to implement with sustainability, economic viability, and societal acceptability, but their complementarities oVer the best
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prospects of eliminating micronutrient deficiencies and reducing diet‐related chronic diseases that threaten more than half of the world’s people.
A. THE PLANT BREEDING STRATEGY A plant breeding strategy is currently employed by the HarvestPlus Challenge Program to improve diets in resource‐poor populations by enhancing micronutrient density (coined ‘‘biofortification’’) in major staple crops (Graham et al., 2001). The HarvestPlus strategy focuses on enriching 6 major staples for Fe, Zn, and b‐carotene density, and collecting preliminary genetic data on these traits for 10 other important staples. Enriched new varieties must have high yield and cooking quality to ensure they are widely grown. Genetic variation for each of these traits has been found in all crops investigated (Graham et al., 1999) but with Fe and Zn that are nutrients not only for humans but for the crop too, a high genotype environment interaction has been found that makes the breeding eVort more complicated and breeding progress slower. For b‐carotene that is synthesized by the plant and is not a nutrient taken from the soil (that varies from place to place), the breeding is much simpler; moreover, the yellow pigment in the edible parts is easy to see so that once the pigmentation has been confirmed to be due to b‐carotene, selection in segregating populations can in large part be done rapidly by eye or simple color meter. Across a number of crops the inheritance of b‐carotene or other carotenoids appears to be simple, frequently due to one or two genes in any given cross. On the other hand, for Fe or Zn in seeds or grains, several uptake, translocation, and grain‐loading genes may be involved in each of these components of the pathway from soil to seed. Moreover, as mentioned already, the content and availability of these nutrients in each soil and the eVects of climate and season all impinge on the final concentration measured in the seed. We argue that a better strategy is to breed for more of certain plant‐ synthesized substances that promote the absorption by the gut of the Fe and Zn present in the grain, or to breed to decrease the inhibitors of absorption. Among the promoters are vitamins A and C, sulfur amino acids, and prebiotic nondigestible polysaccharides such as inulin and resistant starch. These prebiotics pass the small intestine undigested but get metabolized in the colon by ubiquitous strains of beneficial bacteria (e.g., bifidobacteria and lactobacilli) that are able to degrade these indigestible carbohydrates and through release of short chain fatty acids, and induced transporters in the mucosal cell membrane can enhance absorption of calcium (Ca), magnesium (Mg), Fe and Zn (Van Loo, 2004; Yeung et al., 2005). Vitamin C, usually high in fresh root and tuber staples, is a strong promoter of absorption of Fe from seeds and grains. Vitamin A and b‐carotene also enhance
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the utilization of Fe and Zn from bound forms in grains, as do sulfur amino acids under some conditions. On the other hand, certain tannins and other polyphenols inhibit the absorption of Fe from staples. Low‐polyphenol varieties have shown better bioavailability of Fe in tests (Glahn et al., 2005), although health benefits in relation to chronic diseases may accrue from many of these so‐called antinutrient polyphenolics (Hasler, 2002), so to breed for low content is debatable.
B.
FERTILIZER STRATEGIES
The nutrient requirements of higher plants diVer materially from those of humans shown in Fig. 3 because most higher plants require only certain minerals [N, P, K, Ca, Mg, S, Fe, boron (B), Zn, copper (Cu), manganese (Mn), molybdenum (Mo), nickel (Ni), chlorine (Cl), possibly cobalt (Co) and Se], water, oxygen, carbon dioxide, and solar radiation from which they can synthesize all organic compounds they need. On the other hand, animals and humans require even more minerals than are known to be needed by plants, and in addition, they require some 25 or more preformed organic compounds in amounts varying from large to very small (Fig. 3). Plants are able to supply all the known essential minerals for human diets even though they may not necessarily require all of them for their own growth. In particular, plants contain Se, I, and Co (in column 5 of Fig. 3) and their concentrations may be enough to satisfy human requirements fully if the soils on which they grow are not too poor. However, probably half of all soils are deficient in one of these three ultra‐micronutrients (daily requirements about 100 times less than those of Fe and Zn) and although plant production is not restricted, humans dependent on crops for most of their diet can be deficient. Well over 1 billion people live in areas of Se‐deficient soils, and a similar number live in areas of I‐deficient soils. These soils can be fertilized with Se or I as the case may be in order to prevent these deficiencies in humans, but crop production is unlikely to be increased because of the smaller requirements for these elements by plants, if they are required at all. In these cases, there is no incentive for the farmer to use Se and/or I fertilizers unless the consumers are willing to pay more for the enriched product. The alternative is for government to legislate for I and/or Se to be added to fertilizers, such as urea, phosphate, or compound fertilizers, which may be used in these deficient areas. The fertilizer strategy has a particular advantage in that the nutrient has to pass through the crop and/or grazing animal to enter the human diet and this will ensure that overdose through misuse or misunderstanding is highly unlikely. This approach has been used
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with spectacular success using I in far western China (Cao et al., 1994) and Se in Finland (Makela et al., 1993). Cobalt fertilizer may need to be used in some subsistence food systems where soil‐available Co is low because Co is a constituent of cobalamin, vitamin B12. The diets of many resource‐poor people in developing countries is so low in animal products that vitamin B12 deficiency is becoming of increasing concern to nutritionists (Stabler and Allen, 2004). Vitamin B12 for humans comes mostly from animal tissues. It is synthesized by rumen bacteria in ruminants and by other bacteria in nature, but not by plant or animal cells. All other organic requirements of humans can be sourced from plant products, and if vegetarians eat some animal products such as milk‐based foods or honey, they can obtain enough vitamin B12 for health, provided soils have suYcient Co in the first place. In all probability, essential micronutrients remain to be discovered, but it is already clear that these would be needed in only minute amounts, probably in the order of the 2 mg day1 requirement of vitamin B12, a requirement that may be satisfied by eating small amounts of diverse and exotic fresh vegetables, herbs, and spices. No strategy discussed in this chapter deals directly with these unknown requirements, but increasing diet diversity per se will be dealt with after the discussion of Zn.
C.
Zn DEFICIENCY IS IMPORTANT
The most widespread known nutritional deficiency in humans worldwide is that of Fe, but it is particularly ineVective as a fertilizer because it is quickly oxidized and made insoluble in soil. Significantly, Fe deficiency in humans can be due to causes other than Fe‐deficient soils and low‐Fe food crops. It can also be caused by Zn, vitamin A, b‐carotene, I, Se, folate, or vitamin B12 deficiencies, as well as by certain gut bacteria, intestinal worms, and other human parasites and pathogens. But of all of these, Zn deficiency is the most widespread problem (Hotz and Brown, 2004; Wuehler et al., 2005). Half of the world’s soils are deficient in Zn (Sillanpaa, 1982, 1990) but Zn fertilizers are remarkably eVective; a single application may last for several years. Zn deficiency occurs in most environments and soil types, inorganic reactions with S and P controlling its solubility, whereas in biological systems, its solubility is controlled by complexation with amino, sulfhydryl, phosphate esters and carboxylic compounds and related polymers. In turn, Zn determines the functionality of many macromolecules by conformational changes that are extremely important in biology. Zn‐containing enzymes are involved with DNA synthesis and repair and RNA synthesis and editing,
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and transcription, translation, and feedback control of these systems. It further binds to over 900 human proteins (11 times more than does Fe) and more than 500 proteins in plants (Gladyshev et al., 2004), largely determinant of function. It is not surprising therefore the symptoms of Zn deficiency are many and varied, depending on the genotype. The concentration of Zn in average rock and soil is perhaps 100 times less than that of Fe, yet Zn acquired from soil participates in almost all processes and pathways in living organisms. It can be deemed the most important metabolic promoter among the 51 essential nutrients. Because Zn interacts with such a vast number of proteins, symptoms of Zn deficiency in humans are many and overlapping, and consequently many diVerent disease states are associated with its deficiency. Under Zn deficiency, humans lose muscle mass to release Zn for maintenance of the Zn concentration in blood and vital organs. So unlike Fe, Zn deficiency is not easily diagnosed by blood analysis. In these respects, it is not surprising that Zn deficiency has been exceedingly diYcult to diagnose in humans and animals. Deficiency of Zn is the ultimate ‘‘hidden hunger.’’ However, a survey by Hotz and Brown (2004) of Zn in global diets resulted in a map of putative Zn deficiency (dietary Zn deficiency), and these authors estimated 2.6–3 billion people at risk. A map from this seminal work (Fig. 4) shows a distribution of low‐Zn diets that is heavily concentrated on South Asia, Southeast Asia, and Africa, and reasonably mimics, for data so inherently diVerent in nature, the map of Zn‐deficient soils (Alloway, 2004).
Risk of zinc deficiency based on the prevalence of childhood growthsturing and absorbable zinc content of food supply
Low Intermediate High Insufficient data for risk category
Figure 4 Global distribution of diets low in Zn (Hotz and Brown, 2004).
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Among the changes in food/cropping systems brought about by the Green Revolution are the following:
Loss of diet diversity toward more cereal‐based diets lower in Zn Shift to higher pH soils and lower rainfall characteristic of cereal production Use of P fertilizers that tend to decrease Zn uptake Use of N fertilizers that tend to reduce Zn retranslocation from leaves to seeds
Each of these changes can decrease Zn in the diet, which may be viewed as an unexpected consequence of the Green Revolution. The rise of micronutrient deficiencies generally appears to be among covarying events of the Green Revolution and include, as well as Zn deficiency, Fe deficiency anemia and vitamin A deficiency in humans. We propose that the rise in Zn deficiency is fundamental and so a possible underlying causal factor in the other changes. For all these reasons, addressing Zn deficiency is of the highest importance and excellent fertilizer strategies are available for almost any agroecosystem. Moreover, the widespread availability of the inductively coupled plasma (ICP) optical emission spectrometer makes certain the diagnosis of Zn‐deficient crops and the safe use of Zn fertilizers (Graham, 2006).
D. DIET DIVERSIFICATION THROUGH FOOD SYSTEMS APPROACHES The incidence of some micronutrient deficiencies can be dealt with by breeding more nutrient‐dense crops (especially for Fe, Zn, and vitamin A deficiencies), whereas deficiencies of Zn, Se, and I can be best addressed by fertilizer strategies. Both these strategies are available to address Zn deficiency. However, for the two most important cereals, wheat and rice, and for beans, no genetic variation has been found in nature for b‐carotene so for these crops currently conventional breeding programs are not an option, but food systems can be changed to introduce pro‐vitamin A carotenoids into the diet from complementary foods. This is probably the single most important change that can be made to food systems by way of diet diversity. The study that follows in Section IV.C is a special case of a dysfunctional food system in a poor area of Bangladesh where Ca is deficient in the diet especially for children. However, the general strategy can be used to address any dysfunctional food system, one that fails to deliver all essential nutrients in reasonable balance. The approach is first to deal with the agronomy of the staple crop production. This position paper takes the view that low‐input agriculture is unsustainable in the modern world because the population has increased beyond what can be sustained without modern inorganic fertilizers. Global ecologists have estimated that the sustainable human population of the
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planet is 2 billion (Evans, 1993, 1998), meaning in eVect that more than 4 billion people could not survive without artificial fertilizers. If that is accepted, then even subsistence farmers need some fertilizer to compete eVectively—to match the overall lower costs of production from the use of fertilizers, allowing farm families to pay for access to better education and health opportunities, as well as for diet diversification and better nutrition. Of course, this may require certain interventions by governments in facilitating access to credit and the manufacture/distribution of appropriate fertilizers, and the availability of experienced agronomists, although in many instances, all these services may be supplied by the private sector if the needs are properly identified. Therefore, we advocate first optimizing the nutrition of the existing staple crops. When the crop is well nourished, the yield advantage should make production more profitable for the farmer. Moreover, dealing with micronutrient deficiencies as well as the basic NPK, lime, and S requirements means that many of the real or potential deficiencies in humans should be dealt with, but there are exceptions already mentioned: vitamins A and B12. Vitamin A can be dealt with by introducing a b‐carotene‐rich crop into the cropping system, which should be readily achievable because the increased productivity of the fertilized primary staple will allow some land to be freed for crop diversification. Field crops rich in b‐carotene include selected varieties of pulses like cowpea, chickpea, the cereals maize and sorghum, root crops like cassava and orange‐fleshed sweetpotato, and a variety of yellow, orange, and red vegetables and fruits. The other vitamin that is not addressed in the cropping system is vitamin B12, mentioned earlier, that needs to come from very small amounts of dairy, egg, or meat sources that ultimately depend on soil Co taken up by plants. It is necessary of course that the food system is developed with the community to ensure its sustainability, economic viability, and its acceptability in terms of foods provided and labor requirements.
IV. ANALYSIS OF SUBSISTENCE FOOD SYSTEMS Food systems encompass activities related to production, acquisition, and utilization of foods that aVect human nutrition and health (Bernstein, 2002). They comprise subsystems such as cropping systems (Christiansen, 1967; Pumisacho and Sherwood, 2002) and form part of wider livelihood systems (Ellis, 2000). Food systems strategies seek to address problems of food insecurity and malnutrition by understanding requirements for the production of and access to diverse foods to increase their supply, aVordability, and their consumption.
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Subsistence food systems evolve to optimize food production and quality in the face of limitations and variability in the environment, seed material, availability and aVordability of inputs, and numbers of people dependent on them. In the last 40 years, there has been an acceleration of change in food systems following the population explosion of the human race that began after World War II. Under the threat of potential mass starvation, an international eVort later dubbed the Green Revolution greatly increased production of cereals, especially rice, wheat, and maize, and in the two decades 1960–1980 restored the world to overall food suYciency. Cereals now dominate food systems more than ever before. They have partly replaced legumes in food systems because of their greater yield, tolerance to biotic stresses, and wide adaptability: Fig. 8 shows that cereal production has increased at a faster rate and pulse production at a much slower rate than has population growth. In this chapter, the major food systems are identified by the dominant cereal component(s) and further by secondary staples. Rice is the most important cereal; rice–based food systems feed over half of the human population, most notably in South, East, and Southeast Asia, but also provide a complementary staple throughout the world. Rice‐based food systems include rice, rice–wheat, rice–pulse, and rice–fish systems that are discussed here. These days, rice is most commonly eaten after polishing, a process of abrading away the outer layers of the grain (after removing the hull or husk to convert ‘‘paddy’’ into ‘‘brown rice’’). These outer layers, called the ‘‘bran,’’ include the pericarp, seed coat, testa, and the nutrient‐rich aleurone layer. In polishing, the aleurone and the germ, also rich in nutrients, are lost to the bran. The bran thus contains much of the Fe, Zn, Ca, vitamins, phytate, and some of the protein (Lauren et al., 2001). Before the advent of electric milling machines in the villages, rice was processed for cooking by pounding or parboiling. Pounding is a milder process than modern milling and basically removes the pericarp and seed coat to allow faster water penetration during cooking, an important step as the cost of energy for cooking rice is significant for many resource‐poor rice farmers. Pounding, by removing the tough seed coat and pericarp, also imparts a more refined taste. With the germ and aleurone largely intact, some farmers today still claim that pounded or brown rice is more sustaining and nutritious, essential for the hard work of growing the crop itself (Drs. Apichart Vanavichit and Girish Chandel, personal communication). In much of India and Bangladesh, brown rice is parboiled by steaming and redrying before milling. This brief hydration and redrying allows a milder milling so that more nutrients are retained in the rice while still achieving a product acceptable in appearance and eating quality. In India, 61% of rice is parboiled (Department of Agriculture and Cooperation, 2002), undoubtedly a positive factor in the nutrition of a largely vegetarian population.
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Milling is a food processing issue of importance at least as great in terms of nutritional impact as other interventions discussed in this chapter. The Philippines has created a Foundation for Promotion of Eating Brown Rice (Cuyno, 2003) that has increased consumption of brown rice to about 10% of the total in a short time. Similar advocacy and education are needed for parboiled rice, wheat, yellow maize, and cassava and other staples processed into flour. Wheat is mostly milled before use. In subsistence food systems, this is done by stone grinding in which all the components of the grain, including the aleurone and germ, are retained in the cooked (wholemeal) products. Modern milling to white flour may be even more drastic than rice milling as pure endosperm (break flour) can be separated. On the other hand, sophisticated mills can now separate by various means all of the diVerent layers of the wheat grain making possible flour of chosen nutritional composition, potentially a significant advance for nutrition. Major wheat food systems include rice–wheat and wheat–pulse. The rice–wheat food system of South Asia and adjacent China covers 17 million ha and feeds in whole or in part about 1 billion people. Although wheat is necessarily grown on drained land (upland), most rice is wet (paddy) rice in which the land is flooded. This system allows rice to be grown in the wet season when the risk of flooding is high and wheat would fail, whereas wheat can be grown in the ‘‘dry,’’ cooler season less conducive to high rice yields. Maize is the most widely grown and productive cereal but more than half is used to feed animals, and so feeds humans indirectly. There are, however, important subsistence farming systems based on maize, notably in its origins of Central America and South America and in Africa. Maize is also an important crop in Asia where soils are too sandy and/or infertile for rice. Important maize‐based subsistence food systems are: maize, maize–pulse (Africa), maize–bean, and maize–cassava (Mesoamerica). Other important subsistence food systems are cassava, cassava–bean (Africa), potato, sweetpotato (Melanesia), and Andean potato‐based mixed staple. A selection of these major subsistence food systems has been described in this section, with the recognized agronomic and nutritional problems of the communities dependent on them. We describe examples of how the cropping system could be changed in potentially sustainable ways to deliver a more nutritionally balanced food system for improved health of the people dependent on the system. We discuss several major food systems important in their own right because they feed billions of people, and others that are smaller in size but illustrate our concept of increasing nutrient output of food systems to improve human health. The principal concept held here is that a food system is only sustainable if it adequately supplies, year round, from internal or
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external sources, all the nutrients required for good health by the people who are dependent on the food system, including those who in subsistence systems are the drivers of the food system. An in‐depth analysis of a system includes productivity, agronomic management practices, sustainability issues, and food composition values for the major crop components, as well as genetic and environmental variability and reliability of food composition data. Our aim is to identify strengths, weaknesses, and opportunities within a particular food system for enhancing its value to human nutrition and health of the dependent communities where problems are known to exist. The first system discussed is the large rice–wheat food system of South Asia and China.
A. THE RICE–WHEAT FOOD SYSTEM 1.
Introduction
Rice and wheat are the world’s two most important food crops, contributing 45% of the digestible energy and 30% of total protein in the human diet (Evans, 1993). The rice–wheat cropping system is one of the most important in the developing world with approximately 17 Mha in South and East Asia. The distribution of this cropping system includes India, 10.5 Mha; Pakistan, 2.2 Mha; Nepal, 0.6 Mha; Bangladesh, 0.5 Mha; and China, 3.2 Mha (Dawe et al., 2004; Timsina and Connor, 2001). This section will concentrate on the analysis of the rice–wheat cropping system in the Indo‐Gangetic Plains (IGP). Before the Green Revolution many of the wheat and particularly the rice varieties were photosensitive and/or of long duration resulting in diYculties in matching the planting dates of rice and wheat in a double cropping system. There was more crop diversity in the common mixed cropping systems where a number of crops were planted together like wheat with chickpea, mustard, lentil, flax, and others all in the same field. The intensification of the ricewheat cropping system became feasible with the adoption of the shorter season, nonphotosensitive, higher yielding rice and wheat varieties, which, together with the development of new irrigation systems, increased fertilizer and pesticide use, resulted in the Green Revolution. Rice production in South Asia grew from 67 Mt in the early 1960s to 144 Mt in the early 1990s (Hobbs and Morris, 1996). During the same period, wheat production increased from 15 to 73 Mt. As productivity increased, farmers’ incomes grew, staple food costs declined for low‐income consumers, employment of landless laborers grew, and—indirectly—development of small rural industries was stimulated (Hobbs et al., 1997). Although the benefits of the
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Green Revolution are clear, India is now a country that has high levels of malnutrition despite large stocks of food grains.
2.
Breeding Potential
Wheat screening has shown that the germplasm with the highest levels of Fe and Zn in the grain are the wild relatives, primitive wheats and landraces. There are reports of some entries with levels around 90–100 mg kg1 of Zn (Cakmak, 2002; Monasterio and Graham, 2000). The HarvestPlus Challenge Program has adopted the target of increasing the concentrations of Zn in wheat grains by at least 10 mg kg1, and approximately 25 mg kg1 for Fe in order to have measurable impact, assuming the percentage of bioavailable nutrient is similar (HarvestPlus website: www.harvestplus. org). Under the conditions of northwestern Mexico, Pakistan, and northwestern India, this means that the levels of Zn will have to be increased from the current 35 to a target 45 mg kg1, while for Fe it will have to be increased from 35 to 60 mg kg1. That the donor parents for high levels of Fe and Zn in the grain have levels above those required by human nutritionists suggests that breeding the required levels of Fe and Zn in the grain in modern varieties is an achievable goal even though there is significant environmental eVect on Fe and Zn concentrations in wheat and numerous genes involved. The challenge is to be able to maintain the high levels of Fe and Zn present in the primitive and wild wheat, while keeping the high yields of the improved modern varieties. The adoption of biofortified varieties will not take place on the basis of micronutrient concentration in the grain, but rather in terms of their yield potential, disease resistance, and/or consumer acceptability. There are reports of genetic variability for micronutrient concentration in brown rice with a range of 7.8–24.4 mg kg1 in Fe and 15.9–58.5 mg kg1 in Zn (Gregorio et al., 2000). However, the overall mean values and variances found in polished rice seem to be much smaller, which suggests that using conventional breeding to increase levels of Fe and Zn in polished rice will be a bigger challenge than in wheat and may require a biotechnological approach. While most urban people in the IGP prefer to eat polished rice, there appears to be a trend developing for rural people to recognize and value the superior nutritional qualities of brown rice, pounded rice, or parboiled rice (less polished rice) that they consumed in earlier times Department of Agriculture and Cooperation (2003). Screening for b‐carotene in wheat and rice has not revealed any promising natural variability for this trait. All the yellow wheat samples analyzed so far present higher levels of lutein but not of b‐carotene. The lack of natural variation for b‐carotene in rice led to the development of genetically
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modified organism (GMO) rice varieties with high levels of b‐carotene, which came to be known as ‘‘golden rice.’’ The lack of variability observed for b‐carotene in wheat so far suggests that a biotechnological approach similar to that used for rice may also be required in wheat. A preliminary study found limited variability in Se and I concentrations in the grain of wheat and white rice (Lyons et al., 2005a,b). This suggests that improvement for these nutrients through plant breeding would be slow, and probably impracticable where soils are low in these elements. On the other hand there are successful fortification initiatives to make I more available to the population using salt as a vehicle, and as these two elements are quite eVective as fertilizers in increasing concentrations in grain, Se‐ and I‐fortified fertilizers have the potential to be an even more sustainable strategy.
3.
Agronomic Practices
Long term experiments on the rice–wheat rotation in South Asia and China have shown that there has been a significant decline in input productivity (Hobbs and Morris, 1996; Pingali et al., 1997), resulting in farmers having to apply more inputs to obtain the same yields as previous harvests. This suggests that the sustainability of the rice–wheat cropping system should be reevaluated not only in terms of agronomic performance but also in terms of a food system that can provide the necessary nutrients to the target populations. In India, it is estimated that 47% of the soils are Zn deficient (Katyal and Vlek, 1985). For example, a soil survey in India reported that out of 90,218 soil samples collected in the states of Bihar, Haryana, Punjab, Uttar Pradesh, and West Bengal, 51% of the soils were considered Zn deficient, 10% Fe deficient, 3% Mn deficient, and 2% Cu deficient. The state with the highest incidence of Zn and Fe deficiency was Haryana, while the lowest was West Bengal (Singh, 1999). Zn deficiency is the main micronutrient problem reported in the soils of the IGP. This problem is more prevalent in rice than in wheat and this seems to be associated with two factors: (1) flooding of rice fields reduces Zn availability and (2) rice is more sensitive than wheat to Zn deficiency. However, problems of Zn deficiency have also been reported in wheat that can be corrected by soil applications of zink sulfate. Experiments in wheat have shown that soil, seed, and/or foliar application of Zn can be eVective in increasing levels of Zn concentration in the grain (Cakmak, 2002). Moreover, high Zn seeds produce more vigorous seedlings in the next crop. Similarly, fertilizer studies have shown good responses in grain concentrations of
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Se and I, indicating that crop fertilization is a viable strategy for Zn, Se, and I biofortification (Lyons et al., 2005b). Heavy rates of P fertilizer application to soils low in available Zn can induce Zn deficiency (Robson and Pitman, 1983). There are some indications of overfertilization with P (diammonium phosphate) in some areas of the IGP. In a survey of 105 soil samples, 60% contained levels of available P ranging from 35–120 kg ha1. This excess P application is likely to result in reduced accumulation of Zn in the grain of wheat and rice and a less favorable phytate to Zn ratio in the grain (Cakmak, 2002). Reports of Fe deficiency in the IGP rice–wheat system are restricted to rice in light‐textured, high pH, and calcareous soils.
4.
Nutritional Analysis of the Food System
The segment of the population with the most problems of micronutrient malnutrition is the poor, and within this group, especially children and women. According to Dixon et al. (2001) the typical poor family in the rice– wheat cropping areas has access, in addition to rice and wheat, to some vegetables and milk. While it is estimated that up to 50% of the milk production is consumed by the family (Hemme et al., 2003), still, the production of vegetables and milk remain in short supply and not suYcient to cover all the nutritional needs of the family. There are also the landless poor in the rural areas and the urban poor that cannot grow their own crops and mostly buy them. The urban poor could also potentially benefit from biofortification of food staples. Vitamin A deficiency is a serious problem in the rice–wheat area (Table I). This could be addressed by bringing into the food system a b‐carotene‐rich pulse crop, such as cowpea (summer) and chickpea (winter), or a cereal crop like high b‐carotene yellow maize, or root crops like yellow/orange cassava and sweetpotato, and a variety of yellow, orange, green, and red vegetables and fruits. Cassava or sweetpotato in the IGP is found usually on the upper parts of the topography where rice is not grown and drainage is better. Many farmers in Bihar grow maize as a winter staple. In other areas maize is mainly grown as a fodder crop or for roasted sweet corn. All in all, there is suYcient potential to increase the b‐carotene supply in this food system: promoting suitable yellow varieties and educating mothers and their families of its critical importance may be the needed drivers for change in this direction. Another solution could be the adoption of golden rice, which is currently being tested in field trials. Golden rice is genetically engineered to contain b‐carotene in the rice endosperm, which is later converted by the body into vitamin A; 70 g day1 would provide the recommended daily allowance (RDA)
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Table I Fe, Zn, and Vitamin A Deficiency Estimates in India and Pakistan (Hotz and Brown, 2004; UNICEF and the Micronutrient Initiative, 2004) Nutrient deficiency country
Number per year of child deaths
Fe deficiency anemia India 22,000 Pakistan – Vitamin A and Znb deficiencies India 330,000 Pakistan 56,000
Children under 6 years (%)
Population at risk (%)
75 56
51a 59a
57 35
26b 11b
a b
Risk in women 15–49 years. Risk of inadequate Zn intake, entire population.
of vitamin A (Paine et al., 2005). However, golden rice is yellow‐orange in color and it is a GMO, so problems with adoption and acceptance must be overcome. The prevalence of Fe deficiency anemia mainly in children and women is another important micronutrient malnutrition problem for the populations that live in the rice–wheat cropping system (Table I). The increase in anemia in the IGP during the Green Revolution has been attributed to the reduction in the consumption of legumes (Fig. 8). Although there is some consumption of legumes and vegetables, these are not suYcient. Legumes and green leafy vegetables need to play a more important role in the food system due to their content of Fe bioavailability promoters (Sections II and III). A large proportion of the population in the rice–wheat area does not eat meat. This makes it more vulnerable to vitamin B12 deficiency, which needs to come from dairy, egg, or meat sources. However, recent increases in milk production in parts of India where the rice–wheat rotation is important promise to help reduce the problem with this vitamin, and also with I deficiency (Hemme et al., 2003). Protein content in legumes is significantly higher than in rice and wheat. In addition, there is an important diVerence in the amino acid composition of the protein. Wheat is deficient in the amino acid lysine, while rice has inadequate levels of lysine and threonine. In contrast, grain legumes are deficient in the S‐containing amino acids, methionine, and cysteine. Combined consumption of cereals and grain legumes is common in South Asia, which results in almost complete essential amino acid balance and a nutritional improvement over cereal‐based diets. Maximum protein nutrition
R. D. GRAHAM ET AL.
22
is obtained when the grain legume content is about 10% in a wheat‐legume diet, and about 20% in a diet with rice, maize, or barley (Lauren et al., 2001) but these are levels that have not been adequately met since the Green Revolution (Fig. 8).
5.
Diversification of the Rice–Wheat Rotation
In the analysis of the food system, the importance of diversifying the rice– wheat system to improve human nutrition was addressed. In this section, the agronomic feasibility of diversifying the system is discussed. In the Trans‐Gangetic Plain and in the western part of the Upper‐Gangetic Plains, rice–wheat systems mostly include an indica‐type monsoon rice and a spring wheat because there is generally insuYcient time for a third crop. In the Punjab, covering parts of northern India and Pakistan, in addition to Haryana and parts of western Uttar Pradesh, Basmati rice is a popular cash crop. There, the rice crop is generally transplanted from May to July and harvested from late October to late November. Wheat is then grown from November/December and even into January to March in warmer areas and to May in cooler parts of Pakistan (Timsina and Connor, 2001). In this area, the best chance for diversification would be to replace part of either rice or wheat with a diVerent crop. In the eastern part of Upper‐Gangetic Plains, and in the Middle‐ and Lower‐Gangetic Plains where temperatures are generally higher, monsoon rice is grown from June/July to October/November, and wheat from November/December to March/April. There, rice–wheat systems often include an additional crop [e.g., mungbean (Vigna radiate), cowpea (V. unguiculata), dhaincha (Sesbania spp.)], after wheat or before rice and less frequently, cowpea, mustard (Brassica juncea), and potato (Solanum tuberosum) after rice or before wheat (Timsina and Connor, 2001). Rice–potato is now one of the most common rotations in the IGP (Section IV.B). In the northeast and eastern IGP, unpredictable heavy rains during sowing and emergence (ca. 20% of years) reduce establishment of legumes and cause ineVective root nodulation, while more frequent rains (ca. 80% of years) during reproductive growth cause abortion of flower buds, and pods, and reduce grain yield and quality. Thus, high rainfall and waterlogging appear to be a major constraint to the successful inclusion of legumes as premonsoonal crops in rice–wheat systems of the Middle‐ and Lower‐Gangetic Plains. Therefore, new cultivars are required of legumes and other crops that can germinate, emerge, and establish under transient waterlogging, and are also able to complete grain filling and maintain grain quality under heavy rain. Planting legumes on raised beds should alleviate waterlogging problems (Timsina and Connor, 2001). There are reports of 15–47% increase
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
23
in the yield of legumes when planted on raised beds compared to planting on the flat in the IGP (Connor et al., 2003). Inclusion of legume crops in the rice–wheat cropping system could potentially have a number of agronomic advantages, for example, biological N fixation if there is good nodulation, nutrient cycling from deeper soil layers, and breaking of weed and pest cycles. In the mid and lower reaches of the IGP, there is usually a 60–70 day period between wheat harvesting and rice transplanting that could potentially be used for the inclusion of a third crop in the rice–wheat cropping system. The third crop could be an edible legume, (Ahlawat et al., 1998; Dwivedi et al., 2003), and development of short‐duration summer varieties of crops like black gram, green gram, mungbean, and pigeon pea, with adequate insect and disease resistance, could potentially be used to diversify the rice–wheat cropping system, particularly when grown on a raised bed system. There is little information about the potential of fruits and vegetables for diversification in the rice–wheat system, but wider adoption of them should also be beneficial. However the rice–wheat system is diversified, if it is to have a nutritional benefit to the rural communities, the additional food crop must be consumed and become a part of the normal diet, not merely marketed. On occasions, this may require some inputs into education of the health benefits and aspects of consumption.
B. RICE/POTATO‐BASED FOOD SYSTEMS 1.
IN THE
IGP
Introduction
Although rice–wheat is the dominant system in the IGP, there are many other important cropping systems practiced by farmers for sustainable livelihoods. The kharif (wet season) rice–potato–boro (summer season) rice is the emerging cropping system in Eastern IGP (Bangladesh and West Bengal, India). Since the early 1970s, potato production has increased more rapidly in the irrigated subtropical low lands of Asia than in any other part of the world. The most impressive example of the area expansion occurred in West Bengal, where land cultivated to potato expanded fivefold over the past 25 years (Bardhan Roy et al., 1999).
2. Cropping Systems The role of potato relative to wheat in the cropping systems is influenced by temperature that in turn depends on latitude and altitude. Irrigation within
R. D. GRAHAM ET AL.
24
Month Latitude
May June July Aug
Sept Oct
Nov Dec
Jan Feb
Mar
Apr May June
Wheat
Temperate
Potato Rice Wheat Rice
Wheat
Subtropical Rice
Potato
Wheat
Rice
Potato
Potato
Rice Tropical
Potato
Rice
Rice Rice
Figure 5 The roles of potato and wheat in rice‐based cropping systems by latitude.
the temperate to subtropical zones in Asia can support all the cropping patterns of rice, wheat, and potato described in Fig. 5. More than 80% of potato is cultivated after kharif (wet season) rice in the IGP. In selected agroecologies of North and Northwest India (Punjab, Haryana, and West Uttar Pradesh) and Punjab of Pakistan adjoining India, where the winter is prolonged, some farmers practice potato–wheat–rice. The early bulking potato varieties are planted in September/October and harvested in November/December. After potato harvest, the farmers sow wheat. The costs for land preparation and fertilizer requirements for wheat are reduced following potato. Where there is a longer winter, farmers can grow two crops of potato due to low aphid populations and economic bulking rate in both crops under the cooler temperatures. However, this pattern cannot be followed in the short, mild winters of Eastern and Central IGP. The cultivation of 80‐ to 90‐day potato varieties does not allow suYcient time for wheat. Consequently, farmers plant either wheat or potato.
3.
Crop Diversification
Fertile land and enhanced irrigation facilities provide farmers with opportunities for crop diversification and intensification. With the introduction of new, high‐yielding cultivars and improved technologies, the cropping systems are changing rapidly. For example in the IGP of West Bengal,
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
25
rice–potato–sesame, rice–potato–mustard, and rice–potato–jute cropping systems are being replaced by rice–potato–boro rice, rice–potato–groundnut, and rice–potato–vegetables (Bardhan Roy et al., 1999). Diversification currently aims to increase productivity by the introduction into the rotations of alternate vegetables, field crops, fruit crops, flowers, and other options (Modgal, 1998), but ideally, diversification should also aim at improving diet diversity and nutritional balance for the farm families and the local population dependent on them. This will require further technology development and extension to accompany the new options at the farm level.
4.
Limitations to Crop Diversification
Crop diversification depends on many factors such as physical, economic, marketing facilities, consumer demand, location specific technology, and suitable crop varieties. The IGP largely consist of hot subhumid and hot humid eco‐regions with alluvium‐derived soils (ICAR, 1990). The abiotic factors (rainfall, temperature, photoperiod, nutrients), availability of location‐ specific appropriate technologies, socioeconomic conditions of farmers, and marketing opportunities are factors that define the cropping pattern in a region. For example, the wide adoption of boro rice (summer rice) and groundnut due to increased productivity and enhanced income of farmers in some parts of West Bengal have changed the cropping pattern from rice–jute and rice–mustard to kharif rice (wet season)–potato–boro rice and kharif rice–potato–groundnut. The expansion of potato production has been impressive in the irrigated lowlands of South Asia. This growth in potato production can be attributed to the crop’s ability to produce large amounts of food in short periods of time under conditions of land scarcity fuelled by population pressure. Illustrating the potato’s contribution to satisfying food demand from population pressure, production in Bangladesh has tripled since the mid‐1960s and about 40% of potatoes are now produced in Dhaka (Munshigoni), the most populous district of the country and an area where potato production share was slightly more than 10% prior to the Green Revolution.
5.
Profitability and Sustainability of Potato–Rice System in the IGP
With the help of staV of the International Potato Center, activities in the IGP now take a commodity perspective and address the following issues: the potential for intensifying potato production, suitable varieties, displacement eVects caused by introducing potato into various cropping systems, interactions between potatoes and other crops in the system, and the implications
R. D. GRAHAM ET AL.
26
for natural resource management of intensifying potato production. For example, potato was introduced with maize as an intercrop with rice in North Bihar and between rice–wheat and rice–onion in South Bihar, based on the results of participatory trials that optimized varieties and management practices for specific locations, leading to greater productivity and enhanced profitability. In Bangladesh, potato was introduced into the wheat–rice cropping system in Dinajpur district, northern Bangladesh where the enhanced income from the rice–potato cropping pattern soon became apparent.
6. Intensification of Potato and Rice by Double Transplanting Technology Rice is a staple food and potato an important cash crop for farmers in the subtropical Eastern IGP. The areas under rice and potato are 15 and 0.8 million ha, respectively. In the last 5 years, potato area has increased by about 10%, although the farmers face overproduction about every 3–4 years, and also potato is more vulnerable to abiotic and biotic stresses. The farmers continue with potato cultivation due to the net return from potato being, on average, significantly higher than for other crops. Kharif rice–potato–boro rice and kharif rice–boro rice are the cropping systems that dominate this region. Cultivation of potato and boro rice in sequence aVects productivity of both crops due to an early harvest of potato or a delayed planting of boro rice. Researchers introduced double transplanting of boro rice in West Bengal with an objective to enhance the productivity by more eYcient management of natural resources. Briefly the double transplanting system is as follows: rice nursery seedbeds are sown 8 days after potato planting, 40–45 days later the seedlings are sown into a larger plot, 10 seedlings per hill at 1515 cm spacing, 40–45 days later the rice is transplanted for the second time, 5 tillers per hill at 2015 cm spacing immediately after potato harvest at about 90 days after planting. All this allows summer rice to be established in main field after potato by the end of February to obtain maximum yields. The interpolation of user‐friendly double transplanting technology in the kharif rice–potato–boro rice has provided better use of natural resources, leading to a potential per hectare benefit of US $143 compared to rice–potato–rice using traditional planting.
7.
Nutritional Benefits of Potato
Potato provides carbohydrates, proteins, minerals, vitamin C, B group vitamins, carotenoids (in yellow types), and high‐quality dietary fiber.
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
27
The net protein utilization or biological value of potato protein (about 71% that of whole egg) is higher than the other components of this food system, wheat (53%), maize (54%), peas (48%), and beans (46%), and is comparable to cows’ milk (75%) (Gopalan et al., 1972). The vitamin C of potato can enhance the bioavailability of nonheme dietary Fe such as phytate‐bound Fe in co‐ingested cereal or legume seeds, an important benefit for farmers growing potato who would otherwise use the potato as a cash crop rather than a beneficial component of the family diet. A detailed analysis of potato is presented in Section IV.E.
C. A CASE STUDY: THE DYSFUNCTIONAL RICE–PULSE FOOD SYSTEM OF SOUTHEAST BANGLADESH The rice–pulse food system is a major subsistence farming system in eastern South Asia, including Bangladesh, Bihar, West Bengal, and Assam. While flooded rice dominates the fields of these areas, the secondary staple comes from one or more leguminous grain crops grown in the dry season, while in the same region an alternative secondary ‘‘staple’’ to rice comes from small dried fish caught in the rice paddies. In the southeast panhandle of Bangladesh that has double monsoonal rainfall peaks each year and population density is high, this food system has changed in the last two or three decades. Under the pressure of high population, subsistence farmers have been forced to grow another rice crop in lieu of the pulse (in this case, cowpea) in order to produce enough calories for the growing numbers of mouths to feed (FAO, 1999). Whereas in their culture, these people describe themselves as ‘‘rice and pulse eaters,’’ pulses have not kept up with the increasing population pressures, retaining the low productivity of decades ago (Fig. 8). While the near year‐round rainfall can support up to three crops of rice per year; yields are low, yet the yield from the rice crop is generally higher and more environmentally stable than that from pulses. Modern rice generally has higher potential yield and less susceptibility to diseases. Under these circumstances, the change to less pulses and more rice is understandable but population growth and consequent changes in the food system have wrought a heavy cost in health and well‐being, as the outside world discovered about 12 years ago (Cimma et al., 1997). About 10% of the children in the Chakaria district of the Bangladeshi Panhandle have rickets, weak and distorted bones, with potentially up to 50% of children with lesser symptoms. Rickets is a disease well known in northern European countries a century ago where it was shown to be due to deficiency of vitamin D, a cofactor in deposition of Ca in bones, the disease resulting from too little sunlight for suYcient biosynthesis of vitamin D in
28
R. D. GRAHAM ET AL. Table II Ca in Crops of Chakaria Village (Welsh, 2001b)
Crop Rice Rice Chick pea Lentil Black gram
Ca concentration (mg kg1)
References
89 107 458 330 1808
Cimma et al. (1997) Welch (2001b) Welch (2001b) Welch (2001b) Welch (2001b)
Cowpea also appears to be up to 15 times more concentrated in Ca than polished rice after cooking—from food composition tables in the United States (USDA‐ARS, 2001). The pulse samples were from the Chakaria market, imported from India.
exposed skin. However, the Bangladeshi children had high levels of vitamin D and it was eventually established that they and their diet were deficient in Ca itself (Cimma et al., 1997; Fischer et al., 1999). It is significant that no one over about 25 years of age in that area had symptoms of rickets, and as the abnormalities last for life except through orthopedic surgery, it appears that the diet began to change around 25 years ago, which was also when the cropping system began to change toward continuous rice culture. That this association in time is cause‐and‐eVect is underpinned by the diVerences in Ca added to the diet by the pulses, vis‐a`‐vis rice (Table II), and in a later supplementation trial in these children, their Ca status could be brought up to within the normal range by supplementation with as little as 50 mg Ca day1 (Abed and Combs, 2001). As little as 25 g of cowpea could supply this daily amount of Ca, provided the forms in the seeds were bioavailable. More extensive surveys in Bangladesh now suggest that Ca deficiency rickets in the children of Chakaria is but the tip of the Ca‐deficiency iceberg of eastern South Asia and also of parts of Africa (Meisner et al., 2005; Thatcher et al., 1999).
1. Analysis of the Food System The soils of these high‐rainfall tropical areas are acid‐sulfate soils and generally low in nutrients owing to leaching by rain passing vertically through the soil profile into groundwater and eventually into the ocean. Deficiencies of N, P, and K are widespread (Table III), regardless of soil type, and soils are mostly very acidic. In addition, deficiencies of micronutrients are often limiting to crop production but may not be identified: deficient micronutrients include Zn (overall, 49% of all soils globally), B (31%), Cu (14%), and molybdenum (Mo) (15%). Therefore, for good productivity and nutrient
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
29
Table III Percentage of Nutrient‐Deficient Soils in Parts of Bangladesh (A) (Morris et al., 1997) in Comparison with 190 Soils Worldwide (B) (Sillanpaa, 1990) Nutrient A. Bangladesh Extent of deficient soils (%) B. The world Extent of deficient soils (%)
N
P
K
B
Cu
100
22
85
73
Fe
Mn
Mo
Zn
2
69
3
1
24
15
85
55
31
14
3
10
15
49
composition, crop production is likely to require N, P, K, lime or dolomite (to counter soil acidity and supply Ca and Mg), Zn, B, Mo, and, on highly organic soils, Cu. Productivity in the Chakaria area is low and rice, itself poor in nutrients especially after milling, dominates the food system. 2.
Remedial Food System Strategies
Possible strategies to eliminate rickets in the Chakaria area are several. The first is the traditional nutritional intervention of supplying Ca pills; as demonstrated in the Ca supplementation trial (Abed and Combs, 2001), as little as 50 mg Ca day1 may be suYcient, but while supplements are ideal for acute cases, they are generally not sustainable. Another approach (Meisner et al., 2005) is the addition of a teaspoon of ground limestone to the rice during cooking, a strategy similar to that in Mexico in the making of tortillo. Lime or dolomite could be added to the soil both to decrease soil acidity, a benefit to crop productivity in general, and especially to pulse production, and to increase the Ca and Mg levels in the crops. Even with a rice‐ dominated culture, this can be expected to increase the Ca concentration of rice itself, although the increase is insuYcient of itself to supply the required additional 50 mg day1 of Ca (Meisner et al., 2005). However, if this is coupled with new, high‐Ca rice varieties, it could well solve the Ca deficiency problem. In the International Rice Research Institute, rice varieties have been discovered with at least twice the Ca concentration as that of the widely distributed Green Revolution varieties. The rice analyzed from the Chakaria market averaged 89 mg kg1. A rice with 214 mg kg1 Ca would supply an extra 30 mg Ca day1 to children eating 150 g dry rice daily. This would seem to be a viable option, and would likely be sustainable if a Ca‐enriched rice variety were introduced to the area with agronomic and organoleptic characteristics similar in most respects to the existing varieties. No change in behavior would be required at all, and so this strategy has the distinct advantage of sustainability. From our survey of Ca concentrations
R. D. GRAHAM ET AL.
30
in the rice germplasm, this breeding program seems entirely feasible (Gregorio and Graham, unpublished). The bioavailability of Ca in the diet could be further increased by deploying rice varieties high in inulin (Welch, 2001b), inulin can promote absorption via the colon of Ca, Fe, Zn, and Mg (Van Loo, 2004; Yeung et al., 2005). From the little evidence so far available, breeding a high‐inulin rice variety would be feasible. It is noted here that resistant starch, which does not get digested in the small intestine is also a prebiotic like inulin and rice varieties with resistant starch are already known. A study of Ca bioavailability from such rice in the Chakaria context is warranted. Human nutrition literature suggests that increasing Ca intake alone may not resolve the problem. Other nutrients if deficient may interact to aVect the eYcacy of supplied Ca: not only are Cu, Zn, and Mn required for deposition of Ca into bone (Saltman, 1996), but B may also be required (Nielsen, 1996). This is in addition to vitamin D that in this case study was not a limiting factor as it has been in cases of rickets elsewhere. Interestingly, B and Zn are highly likely to be deficient in this population that depends largely on the low‐nutrient rice staple grown on nutrient‐poor acidic soils. Notably, in this context, pulses are generally higher in B, Zn, Cu, and Mn than rice, as well as being much higher in Ca (Table II). Reintroducing pulses to the food system would seem to be an ideal solution as it is obviously part of the culture of the people, but how could this be achieved in the face of the opposite trend during the last 20 years or so?
3.
Preferred Nutrient‐Balanced Food System
A food systems strategy to change the diet in sustainable and acceptable ways could be developed along the following lines: We must first target rice production to make it more eYcient and productive so that the needed rice could be produced on less area, leaving land available to the pulse crop, especially in the boro (dry) season. What is most needed are fertilizers to increase rice yield on these soils of poor fertility, and varieties adapted to the environment, and ideally higher in Ca and inulin. The fertilizers adopted must address all the deficiencies in the soil that limit crop productivity materially. Only with balanced plant nutrition will the economics become so favorable that the returns in yield are more profitable for the farmer than using no fertilizer. At the same time, balanced fertilizers are important for better nutrition of the subsistence farmers and their families. However, increased use of fertilizers will require credit facilities to allow the leap from a low‐cost, low‐yield system to a moderate‐input, high‐yield one. Increasing the existing pulse crop on the land no longer needed for rice will materially balance the diet, and balancing the nutritional requirements of the pulse crop as well will decrease the additional risk inherent in growing
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
31
pulses as they will be better able to resist disease and abiotic stresses. Then, the supply of vitamins A and C must be addressed. It is likely that soup greens obtained locally from home or village gardens may be suYcient, but if not, yellow forms of cassava, sweetpotato, guc, or fruits may be needed. Some varieties of cowpea supply pro‐vitamin A (B. Reddy, personal communication) so it is important that pro‐vitamin A‐containing varieties are available and mothers are made aware of their importance. Finally, the adequacy of I, Se, and Co (B12) in the diet or in blood/urine samples from the population should be checked. If any is inadequate, these three elements can be included in the pulse fertilizer mix to provide enough in the staples (Section III). While global evidence suggests that we have considered those nutrients most commonly deficient, others may be inadequate in a given food system, and the adequacy of other essential minerals and vitamins, particularly vitamins B1, B12, and E should be considered. Vitamins B1 and E are found already in this food system in the rice bran (aleurone layer and germ) and ways of adding a little bran to the diet in acceptable ways needs to be explored because the extent of beriberi (deficiency of B1) appears to be increasing (P. Newton, Laos, personal communication). Vitamin B12 as already discussed requires small amounts of (small) animal products. Taking care of this last group of vitamins is largely a matter of education of the value of small amounts of bran and animal products, rather than further changes to the underlying food system.
D.
BEAN FOOD SYSTEMS
IN
CENTRAL AMERICA
Traditionally in Central America beans have formed part of a food system with maize as the principal cereal, or in dryer regions, with sorghum. Maize and beans have been traditional staples in Central America since precolonial times. In recent years and with increasing urbanization, the food system has become more diversified, although in rural areas and among urban poor, beans and maize remain the basis of the diet. 1.
Nutritional Status Across Countries
Table IV presents data on several common nutritional and health parameters for all five Central American countries. Among these, Costa Rica is exceptional for its superior level of economic development and its public services, including its health system. At a glance the food system(s) in Central America would appear to be functioning reasonably well in terms of protein and caloric intakes. All countries report average intakes at acceptable levels (at or above the recommended minima of 55 g day1 protein and 2200 kcal day1 energy). However, in spite of an apparently positive overall
R. D. GRAHAM ET AL.
32
Table IV Nutritional and Health Parameters in Central America
Country Guatemala El Salvador Honduras Nicaragua Costa Rica
Caloric Anemia in Infant intake Stunting in Urban Protein children mortality Rural (kcal childrena under 5 years (per 1000 povertya povertya intake (g day1)a day1)a (%) (%) live births)a (%) (%) 55 65 58 60 72
2190 2550 2350 2280 2860
49 19 29 20 6
26b 19b 34c 28b 26d
35 32 32 30 8
27 43 56 30 19
74 55 46 68 25
a
http://www.fao.org/es/ess/faostat/foodsecurity/Countries http://www.fao.org/es/ESN/nutrition/profiles_by_country_en.stm c http://www.jsi.com/intl/omni/up_3_98.htm d Cunningham et al. (2001). b
nutritional scenario for the whole region, Costa Rica reports levels of stunting and infant mortality far below the other four countries, suggesting serious problems still to be addressed by the other nations. National data conceal internal variability among regions within countries that can present parameters of health diVering widely from national averages. Indeed, Latin America is the region with the greatest inequities in income distribution in the developing world (London˜o and Sze´kely, 1997), so national averages are bound to hide significant disparities. For example, in Nicaragua, which is one of the poorest countries in the hemisphere, the departments of Jinotega and Madriz present 39% and 49% stunting of children, respectively, compared to a national average of 28% (FAO, 2001), while in the northeast of Guatemala stunting can reach 70%, versus a national average of 49% (FAO, 2003). Almost all countries report higher levels of poverty in rural areas (Table IV), and this is often accompanied by more negative health parameters, as well as far poorer intakes of protein and calories. For example, in Nicaragua caloric intake among the poorest of the rural poor can fall below 1000 kcal day1, and protein intake can also be inadequate (FAO, 2001). Prevalence of stunting among children in Honduras increased in the decade of the 1990s when the economic state of the country was especially depressed (ACC/SCN, 2000). Poverty is an important determinant of nutritional status and of food consumption. Again in Nicaragua, bean consumption among rural dwellers varies from 17 to 36 kg per person per year, increasing with higher income level (FAO, 2001). Furthermore, other social conditions have influenced nutritional parameters temporarily. During the civil war in El Salvador, food availability suVered and the nutritional state of many worsened (FAO, 2002).
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
33
However, it is noteworthy that Fe status, estimated by rates of anemia among preschool children, does not vary so widely across countries as other parameters (Table IV). While data from Honduras suggest the most critical situation, even in Costa Rica as many as 26% of preschoolers present anemia. Among public health problems for which data are readily available, it seems that anemia, and probably Fe‐deficiency anemia, are among the most intractable, and a failing of the food system per se. Vitamin A and I deficiencies have been addressed by food fortification but still occur at moderate levels, given imperfections in the fortification and delivery systems (see Sections D.3, D.4). Information on other deficiencies is sketchy but Zn deficiency often accompanies Fe deficiency. A recent report of the International Zinc Nutrition Consultative Group highlights Guatemala, El Salvador, Honduras, and Nicaragua as having high risk of Zn deficiency (Hotz and Brown, 2004). These countries present an estimated 41–49% of the population at risk of inadequate intake of Zn. As in the case of Fe, this is associated with the low consumption of animal products. Costa Rica, on the other hand, holds a medium level of risk with an estimated 29% of the population at risk. As more attention is directed to Zn and its levels of deficiency, it may well emerge as still another weak link in the present food system.
2. Bean Production Beans are both a crop for home consumption and for market, and among traditional crops, are the most important income generator in Central America. Thus, market demands and commercial criteria for grain type and quality play an important part in farmer varietal preference. Although a wide array of colors is found in landraces, the small, light‐red, shiny‐seeded types are now the dominant class, followed by small opaque‐seeded blacks. Two rainfed planting seasons, in May and September, account for more than 80% of production, while a third crop planted in December or January develops under residual moisture or requires irrigation. Drought, poor soil fertility (P and N) and diseases (fungal, viral, and bacterial) are important limitations. Although the yield potential of the crop is above 3000 kg ha1, regional yields average around 750 kg ha1, with El Salvador presenting the highest yields of about 950 kg ha1. Depending on the region and season, beans may be interplanted with maize in May (one row of maize for every 3–5 rows of beans), or may be planted in relay as the maize crop is maturing in September. Fertilizer is widely available but its use depends largely on capital resources of the producers, and beans often receive no added fertility. In the case of relay planting with maize, farmers will often apply fertilizer to maize and the beans will be produced on residual fertility. Where beans are fertilized directly,
R. D. GRAHAM ET AL.
34
rates of 65–130 kg ha1 are typically used, most commonly with 18,200 (NP) although 121,012 (NPK) is also available in some areas. There is evidence that micronutrients (especially Zn) can be limiting in soils of Central America (ArrozGua, 2004; Bornemisza and Peralta, 1981). However, micronutrients are not applied to beans, and there is little awareness among producers of potential micronutrient deficiencies. It would probably be necessary to demonstrate a positive response to Zn fertilization across several crops to induce fertilizer producers to incorporate Zn into formula fertilizers, unless government policy demanded it.
3.
The Role of Beans in the Diet
Work at the Institute of Nutrition of Central America and Panama (INCAP) in Guatemala established proportions of about 2.5:1 maize–bean for optimal amino acid balance (Navarrete and Bressani, 1981), although in fact beans are consumed in a lower proportion than this. Bean consumption levels vary widely between rural and urban areas. National statistics typically suggest per capita consumption of 15–20 kg year1, but both anecdotal accounts and reports of surveys testify to levels that can be twice as high in rural areas (FAO, 2001). It is not clear that all home consumption is calculated in production figures, and this is consumption that would escape oYcial statistics. Most protein in the Central American diet is of plant origin (FAO, 2001, 2002, 2003). This is particularly the case among the poor for whom animal protein represents less than 20% of protein intake, and as little as 10% among the poorest. Although maize is by far the most important source of energy (38–57% among countries), beans supply as much as 14% of calories, and are the third or fourth most important source. Beans are also a source of dietary Fe, and are particularly important when consumption of animal products is low. If beans are consumed at a rate of 20 kg per person per year, this is the equivalent of 55 g day1 of beans. At a concentration of 50 mg kg1 Fe, this consumption level would provide 2.75 mg Fe day1. It has been observed that rural populations willingly consume double this quantity of beans if price and availability permit. Furthermore, the HarvestPlus Challenge Program has set itself the goal of doubling the concentration of Fe in bean cultivars through breeding. If both consumption and Fe concentration were doubled, this would result in 11 mg day1 of Fe from beans, or an additional 8.25 mg day1 of Fe in the diet. The nutritional benefits from this achievement will depend on bioavailability of the Fe, which is assumed to be low, but it is probable that bioavailability of the additional Fe will be equivalent to that in beans consumed at present, and that the total bioavailable Fe will, in any case, be proportional to the total Fe consumed.
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
35
It must be noted that to fulfill the HarvestPlus strategy and to maximize the benefits of biofortified beans, both Fe concentration and production must increase significantly. In the case of the poorest of the poor, whose yields are limited by a range of biotic and abiotic stresses, this implies a significant eVort in crop improvement. In particular, abiotic stress (drought and poor soil fertility) must be overcome. Poor farmers often occupy the most diYcult environments, and for them improving yield is more urgent than improving nutritional quality. Very promising results have been obtained at the International Center for Tropical Agriculture (CIAT) in Colombia in improving drought tolerance and combining this trait with disease resistance and commercial grain color. These traits must now be combined with improved Fe and Zn concentrations. Addressing low yields due to poor soil fertility requires a coordinated eVort of agronomic management and improved genetic adaptation. The option of fortifying NP fertilizers with Zn to redress overt Zn deficiency, increase tolerance to drought and disease, increase the eYciency and adoption of carrier NP fertilizers, and increase Zn density may be widely successful in this food system where Zn deficiency in diets hovers around 50%, because it is likely that yield benefits will accrue that will cover the extra cost of Zn, and because both maize and bean are generally considered sensitive and responsive to fertilizer Zn. In this maize‐based food system, the greatest contributions from a HarvestPlus Challenge Program perspective must come from increasing Fe and Zn in beans and increasing pro‐vitamin A in the maize crop. Beans should also be explored as a source of Se. Deficiency of I may be addressed by traditional fortification of salt but the agricultural strategy of deploying I‐enriched NP fertilizers remains an option that may be more sustainable in the long run.
4.
Other Components of the Food System
As is common, fruit and vegetable consumption is highly dependent on income, with very low levels among the poor in Nicaragua (FAO, 2001), but with much higher intakes across income levels in Guatemala (FAO, 2003). Horticultural crops have great potential to contribute vitamins to the diet. For example, chili peppers are an excellent source of vitamin C and are consumed in parts of Central America. Vitamin C can improve Fe absorption significantly and can play an important role in Fe nutrition. There is some evidence that pro‐vitamin A (carotenoids) can have a similar positive eVect on Fe absorption, especially when subjects are vitamin A deficient (Garcia‐Casal et al., 1998). If this is confirmed, the role of carotene‐rich foods such as orange‐fleshed sweetpotato, local varieties of squash, or fortified sugar could be important. However, at present consumption data
36
R. D. GRAHAM ET AL.
usually are not discriminated by specific fruits and vegetables to estimate potential contributions. Furthermore, food preparation methods will greatly aVect vitamin content and nutritional contribution. The role of complementary foods is a significant gap in our knowledge to analyze the food system realistically, and to plan eVective interventions. Food fortification has been employed for many years in Central America for vitamin A, using sugar as the vehicle. This has been successful, for example, in reducing vitamin A deficiency in Honduras from 40% to 14% since 1965 (OMNI, 1998). Sugar has the advantage of having few or no substitutes in the mass market (assuming that ‘‘light’’ products have little attraction among the malnourished), and therefore enjoys a ‘‘captive audience.’’ On the other hand, Guatemala was the first country to implement sugar fortification in the region, and although 85% of sugar is fortified, fortification is not uniform (FAO, 2003). Rural populations benefit relatively less, and indigenous populations of the Guatemalan plateau continue to have serious problems. El Salvador reports 36% of the population as deficient in vitamin A across age groups, based on serum retinol (FAO, 2002). Iodized salt is widely used in the region, although in Guatemala quality of fortification is not optimal and intakes are moderately insuYcient (FAO, 2003). Fortification of wheat flour with Fe has been contemplated, but rural populations that consume mostly maize tortillas would appear to benefit only marginally from this strategy. 5.
Information Gaps
A more complete analysis of a bean–maize based food system in Central America requires specific data, including: better discrimination of consumption levels across rural versus urban areas, and across economic levels; more detailed data on consumption, preparation methods, and micronutrient bioavailability of specific complementary foods, especially chilies, vegetables, fruits, and animal products across age and gender groups; more localized data on nutritional status to target interventions; and broader data on micronutrient deficiencies in soil and potential response of crops to fertilization. This information and potential agricultural interventions in the food system must also be viewed in light of the potential of standard fortification strategies and their scope of adoption and impact.
E. POTATO‐BASED FOOD SYSTEMS, HUANCAVELICA DEPARTMENT, PERU A multidisciplinary study at the International Potato Center (CIP) of high‐altitude cropping and food systems in the Andes included base line studies, cropping systems analysis, poverty research, participatory GIS
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
37
surveys, and nutrition studies in eight farmer communities of the Department of Huancavelica, Peru. Diversity across the department and the communities in which research was conducted makes this a relevant setting for exploring potato‐based food systems more generally. The department of Huancavelica covers over 2 million ha (6.1%) of the Peruvian Andes mostly at elevations between 3500 and 4500 m. The total population is just under 500,000 of whom three‐quarters are rural and 97% are economically dependent on agriculture and livestock (INEI, 1994). Huancavelica is often considered the poorest of Peru’s 24 departments; chronic malnutrition aVects 52% of the children while average infant mortality reaches 112 per thousand in rural areas (MEF, 2001; Rubina and Barreda, 2000). Potato is the most important crop, covering more than a quarter of the total annual cropping area, followed by maize, barley, wheat, faba beans, and peas, which together account for another 27% (Rubina and Barreda, 2000). Agroecological classification of the study area was based on Pulgar Vidal’s framework (1996) of natural zones. Although Huancavelica has five agroecological zones, Puna, Suni, Quechua, Yunga, and Selva Alta, only three of these contain potato production systems (Table V). About 87% of the department’s population lives in the Quechua and Suni zones where potato agriculture is concentrated (Rubina and Barreda, 2000). Farmers manage several production zones simultaneously, disbursing species, varieties, labor, and risk across environments. Sectoral fallowing systems, called laymes, are maintained in most high‐altitude communities: all farmers plant the same crop in the same communally defined geographical sector. Rotation cycles begin with potatoes followed by barley and/or fallow (natural pasture). Fields at lower altitudes are managed by individual households. Each production zone has a distinctive potato species and cultivar portfolio: bitter species and landraces at higher extremes (P2, P3, P4), diverse nonbitter landraces (four subspecies) at intermediate altitudes (P3, P4, S1, S2), improved varieties at lower levels (S1, S2, Q1), and commercial landraces in fields dedicated to a single variety (S1, S2). Farmers in all communities also maintain chaqro fields (production zones P3, P4, S1) in which mixtures of nonbitter/noncommercial landraces are managed together. These receive only locally available organic inputs as do bitter landraces, while commercial plantings (landrace and improved varieties) receive chemical fertilizers. Farmers generally use their own seed, but occasionally purchase additional seed after poor harvests. The informal potato seed systems cover most communities, maintain reasonable quality, are highly flexible, and can distribute seed widely; therefore, it is an important seed resource for farmers (Thiele, 1998). Crop damage caused by abiotic factors such as frost, hail, and drought is more important than insect pest and disease damage at the upper altitudinal
38
Table V Agroecological Zones, Potato Crop Production Systems, Biophysical Conditions, and Component Crops Within the Department of Huancavelica Area (%)
Altitude range (m asl)
Puna
15
4200–4350
P2: Sectoral fallowing (communal land)
4000–4200
P3: Sectoral fallowing (communal land)
Rain dependent Very high incidence of frost/hail Rain dependent High incidence of frost/hail
4000–4200
P4: Mixed cropping dominated by potato (private use)
Rain dependent High incidence of frost/hail
3750–4000
S1: Mixed cropping dominated by potato (private use)
Rain dependent Moderate incidence of frost, hail, and late blight
3500–3750
S2: Mixed cropping dominated by potato (private use)
Mostly rain dependent Intermediate incidence of late blight
2300–3500
Q1: Mixed cropping dominated by potato (private use)
Rain dependent or irrigated High incidence of late blight
Suni
Quechua
35
32
Production system
Biophysical conditions
Component crops Bitter potatoes followed by natural pastures Mixed potato landraces, either bitter or nonbitter, followed by barley and natural pasture Mixed potato landraces, either bitter or nonbitter, commonly rotated with barley, wheat, oats, and/or minor tubers Mixed or single nonbitter potato landraces and improved varieties commonly rotated with barley, wheat, oats, quinoa, tarwi, and/or minor tubers Improved potato varieties and/or commercial nonbitter landraces, commonly rotated with barley, wheat, quinoa, faba beans, tarwi, and cultivated pastures Improved potato varieties rotated with diverse crops: wheat, faba beans, tarwi, maize, and cultivated pastures
R. D. GRAHAM ET AL.
Zone
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
39
limits of agriculture as practiced in the Huancavelica department. Farmers manage these factors through field‐scattering practices (Goland, 1993), and consider that mixed landrace stands or chaqro fields reduce damage and loss; semi‐traditional practices such as burning cow dung near fields to prevent frost damage and the use of fireworks for hail prevention are common. Farmers harvest by hand; men usually dig up tubers and women start selection, separating out damaged and small tubers for processing into chun˜o by freeze‐drying; large tubers for sale, and medium size tubers for seed. Seed and tubers for fresh consumption are taken to homesteads for storage while bitter potatoes are often left in the fields so that chun˜o processing can begin once nightly frosts become frequent. Cultural dimensions and local knowledge systems of potatoes far exceed a simple catalogue of varieties and include terminology for the crop’s agricultural and social ecology (Brush, 2004). Potato‐related practices, knowledge, and culture including a characteristic Andean ‘‘cosmovision’’ are highly dynamic, continuously reinvented and frequently molded into new expressions at harmony with modernity. So it is not uncommon to find an improved variety with a local name being used for an indigenous process such as freeze‐drying. This is typical of Andean agriculture, which purposefully absorbs new technologies and knowledge to reshape these for local utility.
1.
Malnutrition in the Huancavelica Department
Data collected in six of the research communities showed that one in four children presented global malnutrition (weight for age). Only 7% of the children showed normal height for age ratios, while 20% were found to be severely malnourished, 43% moderately and 30% slightly so (chronic). The percentage of acute malnutrition (weight for height) was found to be minimal (INEI, 2000). Farmers in all communities produce crops and animal products for home consumption. Potato in particular is often produced, yet rarely purchased. Strictly, subsistence farmers are virtually nonexistent in Huancavelica and although household consumption is largely determined by on‐farm production, food systems are still dependent on purchase of some additional inputs. Andean peasants are highly articulated with the ‘‘outside world’’ (Mayer, 2002; Mayer et al., 1992), and markets have played an important role in household economies for many centuries (Contreras and Glave, 2002). Farmers from all the study communities sell part of their produce, such as potatoes, tarwi (Lupinus mutabilis), maca (Lepidium meyenii), wool, and meat at regional markets. With the income, they buy certain foods, ingredients, and
R. D. GRAHAM ET AL.
40
other necessities. Food expenditure varies within and between communities depending on resources. Nonmonetary exchange through reciprocal labor relations or barter of foods is still common. OV‐farm employment has become much more common in recent decades for all of Huancavelica’s rural communities and provides many households with complementary resources to enrich the food system. The dynamics of migration and oV‐ farm employment have changed rural food systems, increasing possibilities to purchase new foods such as rice and pastas through monetary exchange. Families with access to oV‐farm employment were rarely ranked as being poor by local standards.
2. Access to and Availability of Food There are three main pathways for rural households to obtain food: on‐ farm production, exchange or purchase, and food donation programs from the government or regional NGOs. Table VI provides an overview of foods commonly obtained through each pathway. Access and availability of food is related to a household’s relative wealth. Nonpoor households have more livestock while poor households are more crop dependent. The consumption of barley gruel, barley soup, and freeze‐ dried potatoes were mostly associated with being poor while consumption of
Table VI Common Foods Obtained Through Three Basic Pathways by Families Surveyed in Huancavelica On‐farm produced foods Potato and chun˜o Barley Wheat Oats Oca Mashua Olluco Faba beans Maize Peas Sheep meat Alpaca or Llama meat Guinea pig meat Cow meat Milk Cheese
Purchased foods
Donated foods
Bread Rice Pastas Salt Sugar Baking oil Maize Faba beans Peas Fruits Vegetables Tuna fish Cheese
Milk (powder/tins) Oatmeal (cereal) Fortified cookies Rice Pastas
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
41
Table VII Foods Associated with Being Poor Versus not Poor Among Families Surveyed in Huancavelica Community Allato Pongos Grande Villa Hermosa Pucara Dos de Mayo Libertadores Huayta Corral Tupac Amaru
Foods of those who are poor
Food of those who are not poor
Barley gruel, barley soup Barley soup, potato soup, chun˜o Barley soup, zanco (flour with pig fat) Barley gruel, water from well Barley gruel, barley soup Barley soup, mashua, weedy vegetables (yuyos, berros) Barley gruel, chun˜o Barley gruel, chun˜o
Cheese, eggs, milk, meat Eggs, cheese, meat, legumes Meat, eggs, vegetables Rice, milk, meat, tap water Faba beans, meat Vegetables, zanco (flour with pig fat), potato, meat Meat, eggs, cheese, rice, pastas Rice, pastas
Source: Participatory poverty analysis workshops (2005).
rice, pastas, eggs, meat, and vegetables were commonly associated with not being poor (Table VII). Poor households on average spend S./157 (US$46) per month (52%) on food. Nonpoor household’s total monthly spending is higher with S./262 (US$79) per month (44%) on food. Of the poor households, 68% consume only two meals a day, and consume more barley and less rice, pasta, milk, and fruit.
3.
Diet and Nutrition
The nutritional contribution of potato in the diet of women with children from 6‐ to 36‐month old was studied in six communities during the periods of abundance and scarcity (Burgos, 2006). Potato was dominant in the diets of both adult women and children, represented by a mean daily consumption of 839 and 645 g (women) and 202 and 165 g (children) during periods of abundance and relative scarcity, respectively. During the period of abundance the total diversity of potato cultivars consumed was higher than during the period of relative scarcity, both for women and children: 90 versus 61 cultivars for women and 81 versus 41 cultivars for children. Within the group of ‘‘other roots and tubers,’’ carrots and olluco (Ullucus tuberosus) were most frequently consumed during the period of abundance, while carrot intake alone was more frequent during the period of relative scarcity. Barley, rice, oats, and pastas were the most frequently consumed cereals by both women and children during both periods. For legumes, both women and children most frequently consumed faba beans and peas in both periods. Broadly, vegetable consumption was
R. D. GRAHAM ET AL.
42
Table VIII Dietary Coverage from Potato and Total Diet in Huancavelica Compared to Recommended Intakes (FAO/WHO, 2001) Period of abundance Coverage by total diet (%)
Period of relative scarcity
Coverage by potato (%)
Coverage by total diet (%)
Coverage by potato (%)
Women Children Women Children Women Children Women Children (n ¼ 76) (n ¼ 75) (n ¼ 76) (n ¼ 75) (n ¼ 77) (n ¼ 78) (n ¼ 77) (n ¼ 78) Energy Protein Fe (mb) Fe (lb) Zn (mb) Zn (lb) Ca
88.7 96.4 59.6 29.5 152.0 76.0 38.2
84.0 183.9 88.2 40.4 62.4 29.6 36.6
38.6 38.2 13.1 6.5 45.2 22.6 6.2
29.2 57.8 16.8 7.7 15.9 7.5 3.2
87.3 104.5 71.8 35.5 170.4 85.2 42.3
85.6 193.0 118.7 54.4 87.3 41.6 46.0
28.7 28.0 9.9 4.9 39.2 19.6 5.5
23.0 43.7 13.6 6.2 14.8 7.0 3.3
Source: Nutrition surveys 2004/2005; mb ¼ medium bioavailability; lb ¼ low bioavailability.
infrequent, mainly onions and garlic during both periods of inquiry, and sacha col or yuyo (Brassica rapa), a weedy vegetable, during the period of relative scarcity. Overall frequencies of fruit, meat, milk, and egg consumption were found to be extremely low for both groups and periods of inquiry. The overall diet was found to be generally deficient in energy, Fe, Zn, and Ca. Potato provided over a quarter of the recommended total energy requirements for adult women and children, and contributed significant amounts of protein: 38% and 28% for women and 58% and 44% for children for the period of abundance and relative scarcity, respectively. The contribution of potato to Fe intake was calculated at both low and medium bioavailability. At low Fe bioavailability, total mean potato Fe intake by women and children contributed 7% and 8%, respectively, of required intake during the period of abundance and 5% and 6% during the period of scarcity, whereas for the medium Fe bioavailability scenario, the corresponding figures are 12% and 15% (women, Table VIII) and 17% and 14% (children). The contribution of potato to Fe status may be more than these figures indicate if in fact potato is consumed together in the same meal with Fe from cereals, owing to potato’s relatively high vitamin C content. The corresponding figures for low bioavailable Zn are higher (than for Fe) in women, about 23% and 20% but only 8% and 7% for children, for abundant and scarce conditions, respectively (Table VIII); for moderately bioavailable Zn, the corresponding figures are 45% and 34% of the required intakes (women) and 16% and 15% for children under abundant and scarce conditions,
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
43
respectively. For both Fe and Zn, co‐consumption with carrots that are high in b‐carotene may enhance eYciency of absorption and utilization. The data confirm that potato is a main staple that sustains food security for rural households and contributes significantly and positively to nutritional balance during periods of abundance and relative scarcity. Potato contributes more than 95% of the vitamin C requirements for both groups in the two periods, and contributes significantly to the recommended requirements for energy, protein, Ca, Fe, Zn, and vitamin C (Table VIII). Other food categories, especially cereals, also provide substantial contributions to the recommended requirements.
4.
Potentially Beneficial Interventions
Malnutrition is a serious problem in Huancavelica. Food system interventions are needed to increase availability of energy, Fe, Zn, Ca, and pro‐ vitamin A to vulnerable groups including mothers and children. While overall protein intake is adequate, there is a need to enhance dietary diversity to satisfy the essential amino acids required. Average potato yields of less than 8.6 tons a hectare are low. Increasing crop yields should increase food availability (potato, barley, and others cereals). This can be achieved through technological innovations such as fertilizer, soil management, integrated pest management, improved varieties, and water management. The challenge is to obtain significant yield increases on a sustainable basis and build on the rich ecological and agrobiodiversity that makes Andean cropping systems resilient. A wide diversity of native Andean crops rich in protein, essential amino acids, and minerals are available to be incorporated into the cropping systems, including maca, quinoa (Chenopodium quinoa), and tarwi. Maca has a particularly high content of Ca (258 mg per 100 g) and Fe (15 mg per 100 g). It is also a good source of energy and protein (Herna´ndo Bermejo and Leo´n, 1994). Quinoa has high quality protein; it contains relatively more of the essential amino acids—lysine, arginine, histidine, and methionine. Tarwi is an exceptional source of protein (42% in dry grain, 20% in cooked grain, and 45% in flour) and fat (16% of dry grain). Selection from existing varieties, or breeding, for nutritionally superior potato or barley varieties rich in Ca, Fe, Zn, and pro‐vitamin A may have a positive impact if these varieties comply with other major preferences farmers look for, for example, taste, productivity, storability, and resistance to important biotic and abiotic stresses. Enhanced processing of potatoes may potentially increase food security, especially for those months of the year that are critical in terms of availability of staples. This may be achieved by improving the sensitive steps and
44
R. D. GRAHAM ET AL.
conditions in the process chain, for example, the kind of surface on which chun˜o is prepared. The introduction of greenhouses and vegetable cropping has been promoted by several NGOs as a strategy to diversify diets and improve nutrition. Potential exists to expand and promote small livestock such as guinea pigs, rabbits, and chickens. Food assistance programs can also make a positive contribution to improving the nutritional status of the population in Huancavelica if carefully managed, for there is a risk of a shift to foods that cannot be produced locally such as rice, promoting dependence and potentially undermining local food systems. The farmers of Huancavelica are not passive recipients of knowledge and technology from outside, as has been described. ‘‘Interventions’’ by outsiders should build on this potential and involve farmers in reinventing their local food systems to make them more nutritious. Education about nutritional needs is one important component that can help farmers modify cropping systems and diets to include more nutritious foods, and could be included with farmer field schools and other participatory extension and training approaches that have typically focused on cropping systems rather than the food system as a whole. The challenge is to work with farmers on a diverse range of options and develop robust local food systems that not only provide more nutritious food but do so in a culturally appropriate way that strengthens the ecological and genetic diversity that characterizes the Andes.
F.
SUBSISTENCE FOOD SYSTEMS
OF
EASTERN
AND
SOUTHERN AFRICA
1. Potato and Sweetpotato/Bean Food Systems in the Great Lakes Region The Great Lakes region of Central Africa, comprising Burundi, Rwanda, eastern parts of the Democratic Republic of the Congo (DRC), and southwest Uganda, covers approximately 65,000 km2 lying between latitudes 2 N and 5 S. Altitudes range from 800 m on the shores of Lake Tanganyika in the south to over 3000 m along the high points of the mountainous Nile/Congo Divide. The predominantly rich volcanic soils of the Divide give way to acid lateritic soils to the east. The region has a bimodal rainfall pattern with the ‘‘short rains’’ occurring between mid‐October and December and the ‘‘long rains’’ between March and June. Rainfall is concentrated along the Divide, with less and a marked June to September dry season on the eastern edge.
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
45
Month Cropping season Short rains
Long rains Dry season
Oct
Nov
Dec
Jan
Feb
Mar Apr
May
June July Aug
Sept
maize/sorghum, potato, coco yam beans, peas, wheat Rice, vegetables, coco yam
Bananas and cassava are primarily planted at the beginning of the short rains, but grown and harvested throughout the year.
Figure 6 Principal cropping seasons in the Great Lakes region of East Africa.
a. Cropping System. The region is densely populated with most of the approximately 20 million people living in the mountainous areas. Agriculture accounts for the livelihood of over 90% of the population. Farms are generally small, averaging 0.5 ha. Few animals are kept, except on the lower eastern plains. There are two main cropping seasons corresponding to the short and long rains with a third during the June to September dry season in the limited wet valley bottoms. Bananas (Musa spp.) are grown by over 70% of households and sweetpotato (Ipomea batatas L.) and cassava (Manihot esculenta L.) each grown by over 60%. Potato (Solanum tuberosum L.) is grown at elevations above 1600 m asl by approximately 40% of such households. Phaseolus beans are the main protein source, and also an important energy source, grown by over 80% of households. The cereals, sorghum (Sorghum bicolor L.) and maize (Zea mays L.), are grown on about 50% of farms. Coco yams (Colocasia esculenta L.), wheat (Triticum aestivum L.), and peas (Pisum sativum L.) are also grown seasonally. The principal growing seasons are shown in Fig. 6. The three crops a year allow some continuity of food supply, but shortfalls occur at the end of the dry seasons, particularly where farmers have no access to the valley bottoms. The situation is aggravated by the perishable nature and poor storability of the root and tuber crops. Crops are rarely grown as a sole crop, except for wheat, peas, recently introduced climbing beans, and recently some cash crops such as potato. More commonly, they are grown as complex poly‐crops designed to reduce risk. Six or more crops are frequently grown on any small parcel of land and planted in a predetermined spatial and temporal arrangement to maximize the use of land, water, and light and ensure an extended harvest. Exact areas
R. D. GRAHAM ET AL.
46
Table IX Area and Production of Selected Crops in Great Lakes Region, Central Africa Potato
Sweetpotato
Beans
Country
Area planted (ha)
Production (t)
Area planted (ha)
Production (t)
Area planted (ha)
Production (t)
Burundia D. R. Congob Rwandac Uganda (southwest)d
10,000 20,000 133,418 32,034
26,091 92,300 1,072,772 315,084
125,000 44,000 163,070 64,167
834,394 224,450 908,306 276,265
244,000 200,000 319,349 108,767
220,218 109,340 198,220 76,088
a
FAOSTAT Data (2004). Bouwe, personal communication. c MINAGRI (2002). d UBS (2003). b
devoted to each crop are diYcult to assess, but recent estimates are shown in Table IX. The major constraints to production are the limited land holdings, leading to intensive cropping and land and soil degradation; low soil fertility away from the volcanic ridge and the almost complete lack of fertilizer use; lack of aVordable inputs, particularly suYcient quantities of quality planting material; drought and unpredictable rains resulting in increasing cereal‐crop failure; and disease, particularly the recent pandemics of cassava mosaic virus and banana bacterial wilt, which have reduced crop yields by as much as 100% over large areas, thus not only decreasing production but severely curtailing cropping options. Exacerbating these constraints has been the recent civil unrest that has not only reduced the labor force but resulted in a reduction in the animal population by over 60% with the concomitant loss of manure, the main source of soil fertility. b. Cash Crops. CoVee, tea, and cotton are the major cash crops of the region. However, low prices and the domination of these crops by men means that little of the income derived from them enters the household budget, a possible exception being to pay school fees. General household income is derived from sales of excess bananas (usually for beer), potatoes, and beans. Such income is generally minimal from the small land‐holdings per household. c. Human Nutrition. Most areas in the Great Lakes region are considered capable of producing enough energy and protein for the population under peaceful conditions (United Nations World Food Program, WFP),
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
47
Table X Range of Seasonal Energy and Protein Intakes Among Women Aged 21–48 Years in Six Provinces of Burundi (Nkunzimana et al., 1995b)
Cropping season Short rains Long rains Dry season
Energy intake (kcal per person day1)
Protein intake (g per person day1)
1287–1661 1469–1833 1688–1987
33–49 50–67 56–69
but FAO estimates that 50% of the population of Central Africa is undernourished (FAO, 2003). Approximately 3% of the region’s food requirements are imported, mainly in the form of wheat for bread making. However, severe imbalances and deficiencies, often seasonal, in intake of both the major and minor nutrients among some sections of the population, critically women and young children, are common. A survey by Nkunzimana et al. (1995b) in six provinces of Burundi indicated that roots and tubers (55%) and legumes (35%) provided approximately 90% of the daily energy intake: cereals (6%) and fish and meat, vegetables and fats (approximately 1% each) provided the balance. Also highlighted in this survey was the seasonal variation in dietary energy and protein intake (Table X). The low intake levels, 51–78% and 49–101%, of the energy and protein RDA during the short rains coincides with a period of food shortage after reserves have been depleted during the dry season. In addition to the dietary intake of the major nutrients, numerous studies have shown that Fe and vitamin A intake in the most vulnerable groups (pregnant and lactating women and children from 6 to 59 months of age) are also suboptimal. While data on actual intake are very unreliable, Table XI shows recent estimates. In one recent study, Donnen et al. (1996) showed that vitamin A deficiency coexists in East Kivu with protein‐energy malnutrition (PEM) and is characterized by a high prevalence of severe biochemical depletion and a low prevalence of clinical signs. It is interesting to note that despite the high proportion of children covered by vitamin A supplementation programs, over one‐third of children still have a subclinical deficiency, even where post‐conflict activities have been intense. A survey in Burundi (Nkunzimana et al., 1995a,b) found that 74% of nonpregnant women, 95% of pregnant women, and 51% of lactating women received less than the RDA of Fe suggesting that the figures in Table XI may be underestimates. Nkunzimana et al. (1995a,b, 1996) propose that Fe deficiency in these regions may be partly explained by a typical diet poor in highly bioavailable heme Fe and in promoters of nonheme Fe absorption
R. D. GRAHAM ET AL.
48
Table XI Estimates of Vitamin A Deficiency and Fe Deficiency Anemia in the Great Lakes Regiona (Anon, 2003, 2004) Vitamin A deficiency
Country Burundi D. R. Congo Rwanda Uganda
Fe deficiency anemia
Children receiving Women 15–49 Children <6 years at least one dose Children <5 years years with with VAD (%) vitamin A per year (%) with IDA (%) IDA (%) 44 58 39 66
95 80 94 37
82 58 69 64
60 54 43 30
a Due to diYculty in collecting data, all figures are estimates. VAD, vitamin A deficiency; IDA, Fe deficiency anemia.
and concluded that the lack of heme Fe and ascorbic acid in diets should be regarded as the main determinant of the low‐potential Fe bioavailability in the Imbo region of Burundi. A major cause is the almost total lack of animal and fish products (approximately 4.5 kg per person year1 or 3.5 g per person day1) in the diet, except for a narrow strip bordering Lake Tanganyika, and a low consumption of all groups of vegetables that provide minerals, ascorbic acid, and other vitamins. Such conditions apply to the whole region, with the exception of some parts of East Kivu, where leafy vegetables, including cassava and bean leaves, are eaten almost daily (Yazawa and Hirose, 1989). At higher elevations, Fe deficiency is often less and may be a result of the increased consumption of potato. The low levels of meat intake also aVect adversely the ability of the body to absorb the fat‐ soluble b‐carotene from the intestine since oils are rarely used for cooking. While Fe and vitamin A are the most widely reported deficiencies in the region, there are often also deficiencies of I, Se, and Zn, commonly due to their low levels in soil. Systematic studies of the micronutrient status of Central African soils have not been completed; but there is evidence that all these elements are deficient to some degree and would be reflected in the human nutrition of these highly subsistence communities. All of these micronutrients interact in human metabolic processes and have important benefits to human vigor and immune competence. Thus ensuring adequate levels of any one may partially oVset deficiencies in others. Deficiency of I, endemic throughout the region, is now largely controlled by the use of iodized salt. However, the use of iodized salt does nothing for potential deficiencies of I for livestock production, the products of which would greatly improve the nutritional status of the population and increase
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
49
farmers’ income allowing for more diet diversification from purchased foods. About 42% of the population of Burundi continues to be aVected by I deficiency but less than 15% in Rwanda and Uganda (Anonymous, 2003). Deficiencies of Se have been recorded in East Kivu (Fondu et al., 1978) and are considered to play an important role in anemia. Zn, found in red meat, legumes, and whole grain cereals, is not recognized as a major health problem, but adequate Zn is known to promote Fe and vitamin A utilization. Furthermore, soils in the region are deficient in Zn (Hotz and Brown, 2004), a situation exacerbated by the root crop‐based diet low in Zn. d. Nutrition and Disease. The population in the Great Lakes region has a high incidence of HIV/AIDS, malaria, acute respiratory infections, tuberculosis, and diarrhea resulting in very high levels of morbidity and mortality in children and women of childbearing age. Deficiencies in macro‐ and micronutrients aVect the body’s ability to resist disease. Conversely, malaria, parasites and infectious diseases aVect the absorption of micronutrients. The increased incidence of these ailments severely impinges on the individual’s and household well‐being, reducing the ability to work the land, impairing child development, and cognitive ability with an increase in the number of days lost at school, thus reducing the individual and country’s potential. e. Implications of the Cropping/Nutrition Balance. War, civil unrest, and HIV/AIDS have resulted in many woman‐headed households throughout the region. In Burundi, for example, it is estimated that households headed by women are now 18%. Field labor for food crop production is supplied mostly by women. Much of the land preparation is carried out at the end of the dry season, when many marginal families are restricted to only one meal a day. Thus women, at a time when maximum energy reserves are required, are weakened and liable to illness. This situation is severely aggravated, when the rains begin and the mosquito population increases, by a seasonal outbreak of malaria. Scarce financial resources are often used to employ outside labor. Micronutrient deficiencies, particularly vitamin A deficiency, which lowers immunity to disease, exacerbate the situation. Timely planting and weeding are often missed and yields reduced. It is estimated that the economic impact of vitamin and mineral deficiencies alone amounts to 2.5% and 1.1% of GDP in Burundi and Rwanda, respectively (Anonymous, 2004). Children, especially young girls, are pressured to fulfill their mother’s role resulting in lost educational opportunities. A high rate of population increase, currently over 2.5% throughout the region, returning refugees, pressure for land and movement to marginal
50
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areas, failure of traditional staple crops due to disease, drought, and unpredictable rains are all likely to place pressure on traditional short‐duration, energy and protein crops, potato, sweetpotato, and bean. The question arises as to how these cropping systems can supply a suYcient and balanced diet. f. Breeding for Nutrition. Sweetpotato is potentially a major source of b‐carotene. Cultivars of sweetpotato have been identified that are capable of providing the RDA of b‐carotene from only 100 g day1 of fresh root and breeding is under way to incorporate this character into locally adapted cultivars. Already, locally selected cultivars with increased b‐carotene contents are being grown in neighboring areas of Kenya, Uganda, and Tanzania. Bean cultivars with high Fe and Zn contents have also been identified (Welch et al., 2000) and similar intensive breeding programs should make available locally adapted, high Fe and/or Zn cultivars. However, the proportion of Fe and Zn that can be absorbed from beans is low due to their antinutrient content, especially certain polyphenols. Bollini et al. (1999) have shown the possibility of breeding for a reduction in antinutrients. Wild potato species with high carotenoids, vitamin C, or Fe contents have also been identified and are currently being assessed for the possibility of incorporating these traits into improved genetic material. If consumed with beans, high levels of these constituents may enhance the absorption of bean Fe. g. Future Scenarios. Immediate improvement in the PEM and the incorporation of a better nutrient balance is most likely to arise from intensification of current cropping systems and will rely heavily on the currently grown crops, particularly potato, sweetpotato, and beans. Traditional polyculture techniques are likely to be preferred as they reduce risk and the time taken for cultural practices such as weeding and pest control. New cultivars combined with optimal use of animal manures and compost will form the basis of intensification. Dietary mineral and vitamin deficiencies could be addressed through the incorporation of vegetables, including leaves of sweetpotato and bean, fruits rich in minerals, vitamins, and essential oils, and small livestock or fish that promote the bioavailability of plant sources of Fe, Zn, and pro‐vitamin A. The longer term trend within the region is market orientated and the need for a consistent, quality product. Monocropping and the intensive use of inputs, such as new cultivars, improved seed, pesticides, poles for climbing beans, is becoming more common; as is the use of low levels of fertilizer through which soil micronutrient deficiencies may eventually be corrected. However, many inputs remain beyond the financial resources of farmers, making them liable to failure. The changing practices have significant environmental and human health implications. The success of a future market‐ and nutrient‐orientated farming system will depend heavily on the development
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
51
of integrated methods for the control of the major pests and diseases, while at the same time safeguarding the environment and developing sustainable market linkages. Ultimately, however, the marketing of crops with better nutritional qualities will benefit the region at large as the fast‐increasing urban population, particularly the poor, has access to them. The change to a market orientation may have serious social consequences if the income is confined to the male head of household and the women have even fewer resources with which to maintain the household, a situation exacerbated by ever smaller land holdings. Fortunately, in Rwanda at least, where due to the civil conflict, women outnumber men by 60% to 40% in many areas, recovery has resulted in a more equitable sharing of both workload and income. h. Conclusions. A review of 30 agricultural projects by Berti et al. (2004) showed that most agricultural interventions increased production, but not necessarily nutrition or health within the participating households. Of the interventions that did have some positive eVect, most invested in four or five types of ‘‘capital’’: physical, natural, financial, human, and social. Those interventions that invested in nutrition education and considered gender issues were particularly successful. Also, interventions involving ‘‘home gardens’’ and vegetable production were more likely to be successful. Traditional agricultural and health projects have not fully benefited the peoples of the Great Lakes region. The traditional root and legume crops are the most suited to the diYcult environmental conditions and have the potential to address both PEM and micronutrient deficiencies. To maximize the value of these crops a holistic intervention is needed that not only introduces nutrient‐rich cultivars, but improves understanding of the interactions of micronutrients on nutrition and health, increases total production, and addresses the important social issues of gender, nutrition education, including food preparation and the use of green leafy vegetables, and the partitioning of household income.
2.
A Case Study: Maize–Cowpea Intercrop System, Zimbabwe
During the period 1996–2002, a development project was implemented in communal areas in low rainfall (300–500 mm) parts of southern Zimbabwe. In the communal areas, farmers are allocated small parcels of land, 1–3 ha, by the traditional chiefs, but have no title deed to that land and consequently have no collateral through which loans for production development might be accessed. Grazing areas are communally held. Only 15% of farmers are food self‐suYcient in most years, 25% in some years, while 60% are never self‐suYcient.
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During an extensive series of participatory rural appraisals focused on community resource management, agricultural development problems and possible solutions, low or declining productivity of the maize crop was signaled as the most universal cropping problem. Mostly the soils are coarse granitic sands having poor moisture‐holding capacities, with a hard pan formed at about 12‐cm depth due to many years of superficial moldboard tillage with cattle. Deep tillage to allow roots to explore more soil volume for moisture and nutrients was not a practical option. Low soil fertility was implicated—the few available soil analyses indicated most elements were low, but fertilizer application studies utilizing compound fertilizers had rarely resulted in economically viable responses. Further, due to their financial circumstances and a poorly developed distribution system, very few farmers (7–12% depending on district/rainfall) either used inorganic fertilizers or had tried and rejected them. Although some farmers (who had cattle) had attempted fertilizing with farmyard manure, the majority had rejected its use due to crop burning, particularly under the erratic rainfall conditions; it was found that they were applying manure at rates recommended for higher rainfall conditions. Studies of N use, either mineral or organic (farmyard manure) at low rates, were shown to give yield increases of 30–100% but cost and availability meant that these were options for only the best resourced farmers (10–20%). Other options were needed for the poorer farmers. In the past, cowpeas were a regular component of the cropping system in the low rainfall, communal areas of Zimbabwe. Severe droughts in the 1980s and early 1990s caused the loss of cowpea seed either directly from crop failure or consumption of seed cowpeas. Lack of a seed distribution network in the dry areas resulted in the almost complete disappearance of cowpeas. Cowpea was reintroduced to the cropping system in an experimental way with farmers each conducting an experiment on their own plot to compare cowpea intercropped with maize to maize monoculture. Nearly 700 farmers participated in this study. Cowpeas were available from a seed company in Harare and the study team provided an initial distribution network and later established local seed multiplication initiatives utilizing innovative farmers. The question became, how to plant the cowpeas—as a sole crop or as an intercrop in maize? Older farmers spoke of maize–cowpea intercropping in earlier years but more recently extension messages had discouraged the practice. Further, the few experiments which had been carried out on a dryland research station near the study areas suggested very large maize yield losses due to intercropping, depending on seasonal conditions (Shumba et al., 1990), although the sole maize yields were very much greater than expected by farmers. On the other hand, most farmers were constrained by the lack of draft animal power such that no more land could be cultivated for cropping. This latter factor and
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53
Table XII Yield Results (t/ha) from 685 Farmer‐Managed Maize–Cowpea Intercropping Trials in Three Districts, Zimbabwe, 1998–2001 (Lough et al., 2002)
Maize without cowpea Maize with cowpea Increase with cowpea Cowpea yield
Shurugwi district
Zvishavane district
Mberengwa district
1.10 1.75 0.65 0.14
0.75 1.24 0.49 0.22
0.50 0.80 0.30 0.08
shortages of labor for weeding an additional crop convinced the team to cautiously opt for intercropping. Collaborating farmers were given small packs of various varieties, both long and short season and determinate and indeterminate types. These they planted in participatory adaptive trials—completely farmer managed, the innovation (cowpea) management dictated by the farmer’s management techniques. This allowed a range of outcomes to be observed and analyzed. Generally, maize was planted in 90‐ to 100‐cm‐wide rows with 30 cm between plants. Depending on the type, cowpeas were planted in one (indeterminate) or two (determinate) rows between the maize rows, normally at first weeding (about 4–5 weeks after maize planting). No fertilizer application to either crop was the norm. Surprisingly, there were substantial yield increases obtained by the farmers (Table XII), admittedly from a low base. Furthermore, they indicated almost no runoV after rainfall, with accompanying elimination of erosive action, and a reduction in weeding required, particularly with indeterminate types. In one administrative area (Ward 5) in Shurugwi district, there was nearly 100% adoption of this technology by the year 2000, the highest of all wards in the district. After harvest in that year, the headmaster of the Ward 5 Primary School remarked that since the children had been bringing lunches comprising cowpeas and maize rather than maize alone, the children had developed noticeably longer attention spans that he attributed to the cowpeas. The following year, this previously unexceptional school was the highest achiever in terms of academic results in the whole district! The local extension worker, Mrs. Dominica Shumba, working with more than 1000 farmers in 350‐ to 500‐mm rainfall country won the National Extension Worker of the Year Award. It is worth considering what improvements to the food system could possibly underpin the putative causal link between cowpea in the diet and improved scholastic performance identified by the headmaster. For a start, it is clear that the system is deficient in N that the cowpea can contribute through N fixing nodules on its roots and its consequently higher protein level in the
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grain. While legume protein is commonly deficient in sulfur amino acids, it can complement maize protein with more lysine. More importantly for scholastic performance, cowpeas are not only higher in Fe and Zn than maize, but they contribute promoters of absorption and utilization of both these two elements that have been widely linked to cognitive ability in children but which are widely deficient in resource‐poor populations. The leaves of cowpea if consumed fresh in soup with Fe from either maize or cowpea itself, as is common practice, contain vitamin C that strongly promotes Fe absorption; moreover, selected varieties contain significant pro‐vitamin A (also in yellow, orange, and red varieties of maize but not in white types common in Zimbabwe) that promotes absorption and/or utilization of Zn as well as Fe. In addition, addressing likely deficiencies of vitamin A, Fe, and Zn will greatly improve immune competence that should result in less absenteeism through childhood diseases like diarrhea and colds and flu. Cowpea thus makes a major contribution to the quality of the food system as well as to the agronomy of the cropping system in terms of food productivity. It is important to consider how this food system could be further improved with respect to the health of the population, especially children. On a global scale, after Fe, Zn, and vitamin A, the most common deficiencies in subsistence diets are of I and Se, both deficiencies are widespread in East Africa generally. Deficiency of I is endemic in Zimbabwe, being most severe in the north and northeast of the country. These two deficiencies can be corrected through the food system by use of I‐ and Se‐fortified fertilizers that have been successful in other countries, but this strategy depends on widespread and extensive use of base fertilizers that is not the case in this target area. As only minute amounts of these two nutrients are required, it may be practical to spread them by air over large areas in a matter of hours, an initiative that would need to be undertaken by the government after consultation with FAO, WHO, and the communities. The advantage of this method is that no change in behavior would be needed from the farming community. The benefits in vigor, immune competence, and resistance to the spread of HIV are discussed in earlier sections. Finally, encouraging the use of small amounts of fresh, homegrown vegetables, herbs, and spices and small livestock can contribute functional components to the diet both known (especially vitamin B12) and not yet known to science.
G. THE RICE–FISH SYSTEM: THE VALUE OF RICE–FISH FARMING SYSTEMS AS A NUTRIENT DELIVERY SYSTEM FOR HOUSEHOLDS AND COMMUNITIES Rice–fish farming systems have been in practice for over two centuries by Asian communities where monsoon rains ensure the reliability of rain‐fed rice–fish systems. However, with the expansion of aquaculture and further
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
55
rice–fish system research, these systems have expanded into other areas throughout the world, particularly those areas having high human population densities or where native fish naturally found in ponds and rivers have been heavily fished. This section will summarize some of the potential productivity levels that have been reached successfully, the agronomic/aquaculture practices necessary for sustainable productivity while emphasizing the potential for these rice–fish systems to deliver important nutrients and diet diversity to communities where malnutrition still exists. Constraints to the systems’ expansion will also be discussed and possible improvements suggested as to its sustainability and dietary, agronomic, and societal acceptability.
1.
Productivity Levels
Rice–fish systems can be either ‘‘capture’’ or ‘‘cultivated’’ depending on whether fish are allowed to enter from a surrounding pond or fields to reproduce within the rice field or are stocked from fingerlings released to the rice field. Mixing fish culture with rice cultivation often provides an increase in rice productivity and profits due to fewer pesticides required, and the contribution of fish to rice soil fertility. Some have reported fewer weeds in shallow rice–fish cultivation, adding to productivity and profit of the system. China, where rice–fish systems developed centuries ago, had over 138,000 ha area in 1986. This represented only 1.5% of all rice lands of which 50% are claimed to be suitable for rice–fish. The productivity of the fish then was 183 kg ha1 (Defu and Maoxing, 1995; Renkui et al., 1995). This increased dramatically during the next decade to 1.2 million ha with an average productivity of over 310 kg ha1 fish and generally 10–20% increases in rice yields in most locations (Table XIII). There are many countries that record rice–fish production, some having expansion rates up to 34% , for example in Egypt (Fig. 7). Productivity levels vary widely among the countries, depending on the intensity of cultivation and length of maturity of the rice varieties. In China, yields of fish above 3 t ha1 have been recorded for a range of conditions (MacKay, 1995). However, even with the averages recorded in Table XIII, there is substantial benefit from fish cultivation concurrent with rice. One study recorded fish yields ranging between 62 and 81 kg ha1 were suYcient to ‘‘break‐even’’ with the additional resource investment (Gupta et al., 1998).
2.
Agronomic Management Practices
There have been many papers describing the various technical details of the physical modifications required to create an environment suitable for rice–fish cultivation [Gregory, 1997; Halwart, 1998 (lists many of those
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Table XIII Countries Having Rice–Fish Systems Listed, Although not All Countries Record Data
Country
Area (ha) in rice–fish cultivation
China
1.2 million
Egypt Indonesia Thailand India
173,000 138,000 25,500 Not determined but practiced Not determined but practiced Not determined but practiced 13,400
Sri Lanka Philippines Madagascar
Malawi Vietnam Bangladesh
Not determined but practiced 40,000 Not determined but practiced
Rice–fish productivity (kg ha1) 310 (average) 150–450
25
References FAO, 1997 Renkui et al., 1995 Halwart, 1998 Siregar et al., 1998 FAO, 1997 Halwart, 1998 Halwart, 1998 Halwart, 1998
100
90–155 212–233
Randrianiarana et al., 1995; WARDA, 2003 WARDA, 2003 Lazard and Cacot, 1997 Gupta et al., 1998
review papers); Halwart and Gupta, 2004]. However, the practices common to most systems are listed here. For rice varieties, the choice of modern varieties or landraces does not oVer any constraints to the system—both work equally well as long as they are locally adapted and suitable (Halwart, 1998). Many species of fish can be used with varying commercial and nutritional values. Obviously the grass carp and other fish that consume plants are not encouraged. Also, carnivorous fish need to be cultivated singly or not at all for obvious reasons. Integrated pest management (IPM) is practiced by most rice–fish cultivators owing to concern about the use of pesticides on fish cultivation. Fish themselves can further reduce pest populations by their pest consumption, oVering further savings of pesticides. Fish feeding activities has been shown to reduce weed growth in shallow rice–fish systems (Piepho, 1993). Increased supplemental fertilization in this system over rice monoculture is usually practiced. Fish have been shown to increase the soil fertility, thus increasing rice yields in many locations (Gupta et al., 1998). Additional irrigation is required for areas where rainfall is unreliable. In Bangladesh, the average additional water requirement for fish was
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57
Figure 7 Map of the world showing areas where rice–fish and/or rice–crustacean farming is practiced (Halwart and Gupta, 2004).
estimated at 26% more than for rice alone (Gupta et al., 1998). Aquaculture alone now accounts for 31% of fish consumed but will expand further as captured fish from ponds and rivers is becoming depleted.
3.
Sustainability Issues
Reliable rainfall is necessary for cultured fish in the rice fields. Sometimes fields can have depressions where fish can ‘‘escape’’ when rainfall level drops or supplemental irrigation is unavailable. Growers with larger land properties can have the rice fields open up into a pond to avoid any temporary drought so the fish can escape to the pond and be self‐released when the rice paddy is again flooded. Thus, marginal farmers with less land become less involved in rice–fish due to the risks, labor time, inability to obtain credit, and less land available for rice–fish to allow for ‘‘escapes’’ in times of drought (Gupta et al., 1999). Theft of fish in the rice fields is a concern where there are too few growers practicing aquaculture or the rice–fish system. Table XIII indicates those countries having or trying rice–fish cultivation, but whose area is so low that it is not oYcially recorded. Literature indicates potential for expansion in all these countries in the future. The case study of rice–fish expansion in Madagascar may be studied as ‘‘lessons learned’’ since external assistance had ended (Halwart, 1998).
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58
4.
Food Composition Values Emphasizing Genetic Variability in and Reliability of Food Composition Data
Much of the data concerning food composition will concentrate on Bangladesh, where much nutritional literature and valid data can be found (Table XIV). In Bangladesh, only 6% of the daily protein intake of 48 g per capita came from fish in 1983 (Ahmad and Hassan, 1983). However, of the protein derived from animals, 59% is from fish. Thus, the potential remains for rice–fish to address malnutrition. The addition of fish to a rice‐based diet has potentially huge benefits nutritionally. Rice, especially milled, white rice, is relatively low in protein, very low in Fe, low in Zn, low in Ca, supplies no vitamins A and C, and is further deficient in fat, B vitamins, including B12, folate, I, and Se. Fish, especially small fish such as mola that are eaten whole, can complement rice in covering practically all of its nutritional limitations with the possible exceptions of the last two in the case of freshwater fish (sea fish are valuable sources of I and Se). It is important that such subsistence farmers realize the great nutritional advantage in consuming these small fish rather than using them only as a cash crop. In an I‐deficient area, freshwater fish are likely to be as deficient as the people so other sources of I must be
Table XIV Nutrient Intake (per capita day1) in Bangladesh During 1962–2001 Nutrient Energy (kcal) Protein (g) Fat (g) Carbohydrate (g) Ca (mg) Fe (mg) Vitamin A (I. U.) Thiamin (mg) Riboflavin (mg) Niacin (mg) Vitamin C (mg)
1962–1964a
1975–1976b
1981–1982c
1995–1996d
2000–2001e
2118.0 55.3 20.1 Not reported 286.7 9.4 1670 1.35 0.54 20.5 39.9
2094.0 58.5 12.2 439 305.0 22.2 730 1.65 0.87 22.21 9.51
1943.0 48.4 9.8 412.0 260.0 23.4 763.0 1.38 0.68 13.15 13.00
1868.0 46.9 15.9 384.0 335.3 11.4 1668.0 1.17 0.48 18.34 32.8
2080.0 64.5 19.6 421.2 379.6 17.6 1892.0 2.68 0.87 – 71.1
a Nutrition Survey of East Pakistan (1962–1964), Ministry of Health, Government of Pakistan in collaboration with the University of Dhaka, Dhaka, 1966. b Nutrition Survey of Rural Bangladesh (1975–1976), Institute of Nutrition and Food Science, Dhaka University, 1977. c Nutrition Survey of Rural Bangladesh (1981–1982), Institute of Nutrition and Food Science, Dhaka University, 1983. d Jahan and Hossain (1998). e Computed from the reported food intake in ‘‘Household Income and Expenditure Survey (2000), Bangladesh Bureau of Statistics, Dhaka (2003).’’
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59
found. Iodized salt is the most likely but seaweed from the ocean can suYce if available and part of the culture. If the area is Se deficient, a solution from outside is necessary although garlic has the capacity to concentrate Se and much of its odor can be due to Se compounds. External sources are obviously foods from Se‐adequate or Se‐rich areas, such as wheat from North Dakota where soils are unusually high in Se and wheat Se is an unusually bioavailable source of this essential mineral. Fertilization with Se as sodium selenate is very eVective but it is not widely used as no yield advantage can be expected. Among fish, small indigenous fish are usually consumed whole and thereby provide a substantial source of Ca from their bones. Vitamin A content varies widely among fish species: from more than 15,000 RE kg1 raw edible part for mola, a small indigenous species Amblypharyngodon mola, to less than 1000 RE kg1 for most of the cultivated carp fish species (Roos et al., 2002, 2003). Most of the vitamin A comes from the retinol in the fish eyes and viscera. In their study, fish consumption was unaVected by the domestic aquaculture production indicating that fish cultivation did not change household fish consumption. However, 84% of the total fish consumed was from the small indigenous species, contributing 40% of the recommended vitamin A intake at the household level. Thus, while fish consumption may be unaVected by rice–fish or aquaculture, the species of fish such as mola can be integrated in existing carp culture without negative eVects and can contribute to increased household vitamin A intake. It is noteworthy here that in high arsenic (As) areas, mola also contain high concentrations of As but being entirely in the organic form, it is nontoxic to humans. Halwart and Gupta (2004) reviewed papers relating to nutritional advantages to rice–fish cultivation and concluded that improvements of a farming household’s nutrition as a result of culturing fish in the rice fields may just be an incidental and perhaps even indirect eVect such as being able to buy meat or chicken as a result of the extra cash earned from fish. The main benefit of rice–fish farming is often seen as providing an opportunity to earn cash, so an education program of the nutritional advantages of household consumption of some of the fish harvested is required.
5.
Socioeconomic and Policy Environments and Constraints
Because of the increased system benefits of rice–fish to both enterprises, most studies reviewed have shown very favorable increases to profitability with rice–fish cultivation with few of the net benefits from rice–fish culture being lower than that of rice monoculture alone (Gupta, 1998; Halwart and Gupta, 2004; MacKay, 1995). Reliable rainfall or external water sources must be maintained for rice–fish culture to continue to expand. When water becomes limiting, many studies
R. D. GRAHAM ET AL.
60
show that rice–fish cultivation loses its appeal due to the risks. Additionally, most literature reviewed indicates the major constraint to the systems’ expansion is the lack of fingerlings at the time of rice–fish cultivation (Halwart, 1988; Little et al., 1996). Where this system is well established, fingerlings are available and rice–fish cultivation is more widespread, such as in China.
6.
Conclusions (Improvements to the System)
The main beneficial eVects of rice–fish systems are related to environmental sustainability, system biodiversity, farm diversification, and household nutrition (Rothuis et al., 1998). Research results on rice–fish systems indicate that with proper classification of rice‐producing areas for their suitability for rice–fish farming, and consideration of the capacity of the irrigation infrastructure, general soil characteristics, physical requirements as well as the socioeconomic situation, its area can expand. This expansion can make impacts in maintaining marketable fish within the communities but when these same communities are empowered with knowledge of nutrients found in the varying fish species, their choice of species can aVect their household and community nutritional status. Hidden benefits of rice–fish farming such as risk reduction through diversification of the farming system may have a strong attraction to many farmers and their families (Halwart and Gupta, 2004).
V. THE SOCIOECONOMIC AND POLICY ENVIRONMENTS A. HOUSEHOLD INCOMES, FOOD PRICES, AGRICULTURAL DEVELOPMENT
AND
Interventions to improve the minerals and vitamins supplied by the cropping system at any given time should be understood in the context of agricultural and economic development over time. In this context, per capita intakes at the household level are generally a function of that household’s income and food prices. We first examine income. Table XV shows per capita energy intake and share of food expenditures by broad food groups by income group for three countries. At low incomes the poor give priority to purchasing food staples, the most inexpensive source of energy, to keep from going hungry. Then at the margin as income increases, they buy non‐staple plant foods (e.g., lentils, fruits, vegetables) and animal products (including fish) because of a strong underlying preference for the tastes of these foods.
Bangladesh
Kenya
Income tercile Per capita energy intake Staples Non‐staple plant All animal Total Food budget share (%) Staples Non‐staple plant All animal Total
1
2
3
1805 1903 1924 281 347 394 44 61 89 2130 2311 2407 46 32 22 100
41 35 24 100
36 36 28 100
Philippines
Income quartile All households 1879 340 64 2283 40 34 26 100
Derived from data of Bouis (1996) and Bouis et al. (1992, 1998).
1
2
3
Income quartile 4
All households
1283 1371 1388 1394 256 348 363 464 112 120 161 187 1651 1839 1912 2045 Data not available
1360 357 145 1862
1
2
3
4
1361 1431 1454 1381 197 229 304 395 67 102 118 207 1625 1762 1876 1983 43 30 27 100
36 36 28 100
28 39 33 100
24 37 39 100
All households 1406 281 124 1811 33 35 32 100
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS
Table XV Per Capita Energy Intakes (kcal day1) and Food Budget Shares by Broad Food Group by Income Group for Three Countries
61
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Although diets are expressed in Table XV only in terms of energy (and not minerals and vitamins), because non‐staple plant foods and animal products are denser than food staples in bioavailable minerals and vitamins, percentage increases in mineral and vitamin intakes rise much more sharply with income than do energy intakes. Animal products are the most expensive source of energy, but the richest sources of bioavailable minerals and vitamins. There is a natural underlying tendency, then, for dietary quality to improve as economic development proceeds. As household income rises and demand for non‐staple plant foods and animal products rises, prices for these better quality foods will tend to rise, all things being equal. These price signals, in turn, will give rise to supply responses from agricultural producers. The essence of economic (in this case agricultural) development is that technological improvements will be stimulated (e.g., development of higher yielding varieties either through public or private investments in agricultural research) which in turn will lead to more eYcient production, faster supply growth rates, and eventually lower non‐staple food prices. It is the role of public food policies to influence this long‐run process so that aggregate growth is rapid and so that all socioeconomic groups (importantly the malnourished poor) share in the benefits of this growth. With this as background, we now briefly examine food prices and the role of the Green Revolution in influencing food prices. Figure 8 shows the percentage increases in developing country population, in cereal production, and in pulse production between 1965 and 1999. Developing country population doubled during this period. It is the great achievement of the Green Revolution that cereal production more than doubled due to rapid technological change. After adjusting for inflation, real cereal prices have fallen over time despite the doubling of developing country population. As suggested in Table XV, the poor spend a high percentage of their income on food staples, and lower cereal prices frees up income that eases their burden and can be spent on a range of necessities, including better quality food. Pulse production in Fig. 8 is representative of increases in production for any number of non‐staple plant foods. Production increased significantly, but did not keep pace with growth in demand—due both to population growth and income increases as developing country economies have grown. There was no commensurate technological change in the non‐staple food sector. Consequently, inflation‐adjusted prices of many non‐staple foods have increased over time. This change in relative prices—lower food staple prices and higher non‐ staple food prices—has made it even more diYcult for the poor to achieve mineral and vitamin adequacy in their diets. Certainly, in the absence of knowledge among the poor about the importance for health of a nutritious diet and what relatively inexpensive non‐staple foods can provide in terms of
NUTRITIOUS SUBSISTENCE FOOD SYSTEMS Cereals
Change in production and population (%)
250
Pulses
63 Population
200
150
100
50
Developing
World
Developing
Bangladesh
Pakistan
India
Developing
Bangladesh
Pakistan
India
0
Figure 8 Percentage changes in cereal and pulse production and in population, 1965–1999.
minerals and vitamins, for those poor whose incomes have remained constant, price incentives have shifted the diet more and more toward reliance on food staples. As described in previous sections, this has led to a worsening of mineral and vitamin intakes for many segments of developing country populations, micronutrient malnutrition, poor health, and much misery. To reiterate, the long‐run task of public food policy is to stimulate growth in the non‐staple food sector (sometimes referred to as ‘‘high‐value’’ agriculture) through any number of instruments‐agricultural research, education, building infrastructure, improving markets for agricultural inputs and outputs to name a few. However, this is a several‐decades‐long process. In the meantime, again as described in previous sections, there are specific, cost‐ eVective steps that can be taken to utilize agriculture to improve mineral and vitamin intakes. Policy aspects are discussed below for three interventions already discussed in agronomic terms in earlier sections: (1) additions of essential trace elements to soils, (2) biofortification, and (3) introduction of novel, nutrient‐dense crops into the food system.
B. ADDITIONS
OF
ESSENTIAL TRACE ELEMENTS
TO
SOILS
As documented in previous sections, minerals severely lacking in human diets can be added to food systems by additions to the soil—using commercially produced fertilizers applied on the ground, by air and through irrigation
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systems. To justify implementation of any variants of this strategy, the basic economic principles are (1) that the benefits to human health and greater agricultural production are greater than costs of implementation and (2) that where the private sector is involved there are proper incentives for participation and equal treatment of companies.
1.
Direct Additions Through Public Action
The DeLong study in western China (Cao et al., 1994) already discussed provides evidence that a strategy of adding I to an irrigation system provides a simple, low‐cost method for getting I into the food system and into the diets of people who are I deficient. In this well‐documented case, the benefits to public health are substantially higher than costs. Moreover, there are benefits to livestock production as well. While this one case demonstrates a principle, much of agriculture, however, is rainfed and often it will not be possible to use an irrigation system as a delivery mechanism. Other means of delivery such as through spraying by air and adding the nutrient to other fertilizers already in use are practicable and may be cost‐eVective in given situations. To the extent that both human and plant health are limited by the availability of the trace mineral being applied, Zn for example, benefits will be high. To the extent that a single application may be suYcient to generate benefits over several years, costs will be low (unlike N that needs repeated applications). Economies of scale in delivery and lack of alternative means of delivery (e.g., in areas of Africa where fertilizer markets are poorly developed or nonexistent) may provide the rationale for public intervention as compared with relying on private incentives to improve soil fertility. Carefully researched pilot activities which successfully document high benefits and low costs will be required to convince policymakers that innovative solutions to supplying trace minerals to soils should be implemented.
2.
Fortification of Fertilizers
In areas where fertilizer use by farmers is already widespread, it is technically feasible to ‘‘fortify’’ fertilizers with specific trace minerals—as a vehicle for getting limiting trace minerals into soils, then into plants, and the overall food system. In the case of Zn where the profits from increased yields can far exceed the extra cost of Zn‐enriched fertilizers, it may be enough through public research simply to document the benefits of Zn for yields to jump‐ start private sector eVorts to develop supplies and a market for fertilizers with Zn—as happened in Turkey (Cakmak, 2002).
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Where an increased supply of trace minerals could contribute to public health, but not to agricultural productivity, two related strategies are possible. The government could require (pass laws) that all fertilizers be fortified with particular trace minerals to certain minimum standards. This will increase fertilizer costs to farmers (fertilizer producers will pass the increased production costs on to their customers) and necessitate establishing institutions so that the law will be enforced. If fertilizer prices were significantly aVected, the government could choose alternatively to subsidize these additional production costs so that fertilizer prices would remain unchanged; laws mandating fertilizer fortification would still require enforcement and subsidy payments would require oversight. Again, carefully researched pilot activities which successfully documented high benefits and low costs would be required to convince policymakers to implement such laws and to incur the cost of such subsidies.
C. BIOFORTIFICATION Technical issues associated with biofortification and the potential economic benefits of biofortification have been discussed in previous sections and elsewhere (Graham et al., 2001). With this as background, there are two primary policy issues: (1) should the government require that all new releases (public and private) for given crops contain minimum levels of specific minerals and vitamins and, related to this, (2) what level of public resources should be invested in biofortification as part of its investment in agricultural research? As more scientific knowledge and experience is gained with biofortification, as more eYcient methodologies are developed, the costs of including mineral and vitamin density as part of ‘‘standard practice’’ in breeding may become quite manageable. If so, then there will be strong incentives in terms of improved public nutrition and health to require minimum levels of minerals and vitamins in the edible portions of new releases. What levels to investments in biofortification are appropriate will depend in part on the costs of alternative instruments which are available for providing nutrients in the food system—for example, supplementation, fortification, fertilizer strategies discussed above, and introduction of new crops in cropping systems which is discussed below. Biofortification may be conceptualized as a subset of this last activity—introducing a new crop line into the food system if not a completely novel crop. It has the advantage over the supplementation, fortification, and fertilizer strategies that most costs are incurred up front. That is, they do not involve the same recurrent costs year after year.
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D. INTRODUCING NEW NUTRIENT‐DENSE CROPS CROPPING SYSTEMS
INTO
To illustrate this strategy, consider the case of the introduction of orange‐ flesh sweetpotato (for which there are specific lines very dense in b‐carotene) into a food system that is severely deficient in vitamin A. In some cases, it may be that white sweetpotato varieties are already being consumed. In other areas, sweetpotato may be a completely novel crop. In either case, a communication strategy would need to be developed, directed not only at users but at policymakers and diVusers of this technology (diVusers ultimately report to policymakers who provide, or do not provide, an enabling environment to implement the dissemination strategy). EVective communication creates demand for vines, ensures suppliers, and markets to link supply. Demand would need to be motivated by a message of improved nutrition. Finally, after the initial public investment introducing the new crop into the food system, at some point public activities would need to be withdrawn, leaving in place a supply–demand marketing chain operating within the market economy.
VI.
CONCLUSIONS
Many national food systems have become dysfunctional, failing to supply all the nutrients required for healthy crops and for healthy people dependent on those crops for nutriture. This is a global problem aVecting most people, rich and poor. However, agricultural strategies and tools are available to redress these problems that appear to date back to the loss of diet diversity induced by changes in agricultural systems during the Green Revolution. Agronomists, and indeed the whole agricultural sector, need to understand the implications their activities have on the nutrient delivery of the food systems they work with, and to consider their role as developing, in partnership with nutritionists and primary healthcare oYcials, sustainable cropping systems that can deliver nutrients in balance to whole populations. Much of this new agenda will need to focus on micronutrient inputs and outputs of farming systems, and will require access to analytical laboratories and careful attention to nutrient interactions in soils, crops, animals, and humans. The complexities of food systems, of micronutrient chemistry and biology, as well as the opportunities available in molecular and digital technologies oVer exciting challenges to agronomists willing to tackle, collaboratively, one of the world’s most pressing issues—malnutrition and the devastating eVects it has on human health and well‐being and on societies as a whole.
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POLYACRYLAMIDE IN AGRICULTURE AND ENVIRONMENTAL LAND MANAGEMENT R. E. Sojka,1 D. L. Bjorneberg,1 J. A. Entry,1 R. D. Lentz1 and W. J. Orts2 1 Northwest Irrigation and Soils Research Laboratory, USDA Agricultural Research Service, 3793N‐3600E Kimberly, Idaho 83341 2 Byproducts Engineering and Utilization Research Unit, USDA Agricultural Research Service, Albany, California 94710
I. II. III. IV. V. VI. VII. VIII. IX. X. XI. XII. XIII. XIV. XV. XVI. XVII. XVIII.
Early Uses of Soil Conditioners Major Conditioner Types Synthetic Conditioner Uses and Application Strategies Overview of Current PAM Use PAM Defined and Described PAM Properties AVecting EYcacy Early Contributions Surface Irrigation Sprinkler Irrigation Infiltration PAM Safety, Field Retention, and Environmental Impacts PAM EVect on Organisms in RunoV and Soil PAM Degradation PAM and Ca PAM for Construction Sites and Other Disturbed Lands Canal and Pond Sealing Biopolymers Conclusions References
Anionic polyacrylamide (PAM) has been sold since 1995 to reduce irrigation‐induced erosion and enhance infiltration. Its soil stabilizing and flocculating properties improve runoV water quality by reducing sediments, N, dissolved reactive phosphorus (DRP) and total P, chemical oxygen demand (COD), pesticides, weed seeds, and microorganisms in runoV. PAM used for erosion control is a large (12–15 Mg mol1) water‐soluble (non‐ cross‐linked) anionic molecule, containing <0.05% acrylamide monomer. In a series of field studies, PAM eliminated 80–99% (94% avg.) of sediment in runoV from furrow irrigation, with a 15–50% infiltration increase compared to controls on medium to fine‐textured soils. Similar but less dramatic results occur with sprinkler irrigation. In sandy soils infiltration is often unchanged 75 Advances in Agronomy, Volume 92 Copyright 2007, Elsevier Inc. All rights reserved. 0065-2113/07 $35.00 DOI: 10.1016/S0065-2113(04)92002-0
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R. E. SOJKA ET AL. by PAM or can be slightly reduced. Typical seasonal application totals in furrow irrigation vary from 3 to 7 kg ha1. Research has shown little or no consistent adverse eVect on soil microbial populations. Some evidence exists for PAM‐related yield increases where infiltration was crop‐limiting, especially in field portions having irregular slopes, where erosion prevention eliminated deep furrow cutting that deprives shallow roots of adequate water delivery. Modified water management with PAM shows great promise for water conservation. High eVectiveness and low cost of PAM for erosion control and infiltration management, coupled with easier implementation than traditional conservation measures, has resulted in rapid adoption. About 800,000 ha of US irrigated land use PAM for erosion and/or infiltration management. In recent years, PAM has been deployed for uses beyond agricultural erosion control, including construction site erosion control, use in storm water runoV ponds to accelerate water clarification, soil stabilization and dust prevention in helicopter‐landing zones, and various other high‐ traYc military situations. Among the newest topics being researched is the use of PAM to reduce ditch, canal, and pond seepage, using specific application protocols that take advantage of its increase of water viscosity at higher # 2007, Elsevier Inc. concentrations.
I. EARLY USES OF SOIL CONDITIONERS Understanding the current success of and growing attention to polyacrylamide (PAM) and related synthetic and biopolymers for land care uses and environmental protection is easier if seen in the context of soil conditioner technology development. Animal and green manures, peat, crop residues, organic composts, lime, and various other materials have been used as soil conditioners for thousands of years. Conditioner identification, technology, and use have been largely a marriage of convenience between agriculture’s need for chemical and physical maintenance or improvement of the soil, and for disposal or management of waste materials from the full range of human activities. Over time, the spectrum of materials used as soil conditioners has expanded to include composted manures, sawdust, or other milling residues as well as other organic industrial wastes such as food, textile, and paper processing wastes. Mineral materials such as rock phosphates, gypsum, coal dust, rock flour, and sand have also been used. The terminology and concept of soil amendments and conditioners have become associated primarily with physical conditioning. Chemical conditioning for provision of plant nutrients to soil is largely ascribed to materials termed fertilizers. Clearly, however, there is overlap. Lime, for example, neutralizes acid, aVecting pH, some structural phenomena such as aggregation, and fertility by aVecting ion balances and reducing rooting impairment
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from acid soil conditions. Coal can provide humic acids which stimulate stabilization of soil organic fractions (Dzhanpelsov et al., 1984). Many fertilizers directly and indirectly aVect soil physical properties and many conditioners directly and indirectly aVect soil fertility. The overlap occurs because of the intimate association of soil physicochemical processes and their linkage to soil‐supported biotic processes, cycles, and functions. The designation of fertilizer versus conditioner tends to be based on the dominant eVect. Categories are often assigned by law, based on the chemical analysis and/or the proof of claims for the materials. The development of soil conditioner technology has been comprehensively reviewed (De Boodt, 1975, 1990, 1992; Gardner, 1972; Stewart, 1975; Wallace, 1995, 1997, 1998a,b; Wallace and Terry, 1998; Wallace and Wallace, 1995).
II. MAJOR CONDITIONER TYPES There are three major classes of soil conditioners: natural organic materials, inorganic or mineral materials, and synthetic materials consisting primarily of chemical polymers and surfactants. Organic conditioners have typically been used to increase infiltration and retention, promote aggregation, provide substrate for micro‐ and mesobiological activity, improve aeration, reduce soil strength, and resist compaction, crusting, and surface sealing. EVects, such as increased infiltration and water retention, are often evident immediately on soil incorporation, whereas other eVects, such as aggregation, depend on chemical and biological processes occurring over weeks or months. Mineral conditioners can modify chemical or physical processes. Lime, for example, raises soil pH. Gypsum or lime is often used to increase soil base saturation or reduce soil exchangeable sodium percentage (ESP) of retained cations. Both are good Ca sources that increase flocculation of primary particles and stabilize aggregates and other structural features and reduce dispersion and seal formation. In saline soils, especially if irrigation water contains significant amounts of Na, Ca sources are used to reduce water sodium adsorption ratio (SAR) and soil ESP. Mineral conditioners are especially important for management of arid or tropical soils where high temperatures promote rapid biooxidation of incorporated organic material. Oxides of iron have been used to promote aggregation in low organic matter soils (Rhoton et al., 2002; Schahabi and Schwertmann, 1970; Vampati and Loeppert, 1986). Ferric hydrides are common water‐treatment and industrial process waste products. The third category of conditioner includes synthetic materials designed to produce specific physical and chemical interactions in soils. These are
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usually highly eVective materials that produce physical eVects with very small amounts of material added. The mode of action of these materials can be targeted to a particular physical process or property of soil. The two most common classes of synthetic amendments are surfactants and flocculants. Surfactants aVect the surface tension of water and are most commonly used to enhance the wetting and infiltration of treated soils. Flocculants are materials that enhance the cohesive attraction among dispersed fine particulates. In aqueous media this leads to formation of loose aggregates known as flocs that achieve suYcient size and weight to settle out of suspension, leading to clarification of the suspension. When applied to consolidated soil these materials tend to enhance existing structural stability and, in the presence of flowing fluids, increase shear strength and reduce detachment. Most synthetic conditioners achieve their desired eVects at applications of 100 kg ha1 or less, compared to tonnes per hectare, as is the case for most organic or mineral conditioners. Some synthetic conditioners can have substantial eVects on soil processes at kilogram per hectare rates or less, depending on application protocols.
III.
SYNTHETIC CONDITIONER USES AND APPLICATION STRATEGIES
Soil conditioner use is limited by economics, often related more to transportation and application costs of bulky materials than to material price. Thus, organic and mineral soil conditioner use in production agriculture has been largely limited to a few highly eYcacious materials such as lime, gypsum, and manure. High‐value nursery operations, cash crops, turf, and landscape applications are less constrained by costs. Organic and mineral conditioners are used more often if available gratis as a means of by‐product disposal from industrial processes, or if farms are located near sources, lowering transport cost. Synthetic conditioners, despite higher eYcacy per unit of material, have seen limited use because of higher material cost, oVsetting savings from lower application rates and transportation costs. Cost is also related to the strategies of use and application. Until the 1990s the strategy for conditioner use was to change the overall physical and/ or chemical makeup of a significant portion of the soil profile. This might mean attempting to condition a tillage slice, typically to a depth of 10–15 cm or even to a typical rooting depth of 30–45 cm. To modify such a depth of soil usually demands application of large amounts of conditioner. This strategy is inexorably linked to the cost considerations of material amount, bulk, and transportation, as well as application equipment, convenience, time, and labor. The advent of chemical polymer conditioners changed the
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logistics of this strategy because of the high eYcacy of these conditioners. However, the strategy remained expensive because of the high cost of polymer conditioners, especially in the early decades of polymer conditioner development. During World War II water‐soluble polymers were used to stabilize soils for road and runway construction (Wilson and Crisp, 1975). Experimentation with these conditioners for agricultural applications began following World War II. Minsk et al. (1949) patented an industrial process for polymerizing acrylamide (AMD) molecules which made synthesis of a wide variety of water‐soluble polymer compounds economical for industrial and environmental uses. Since the 1950s soil scientists have explored using synthetic polymeric conditioners to alter physical and, in some cases, chemical and biological soil properties for improved agricultural performance (Allison, 1952; Bear, 1952; Chemical and Engineering News, 1951; Fuller and Gairaud, 1954; Hedrick and Mowry, 1952; Martin, 1953; Martin et al., 1952; Quastel, 1953, 1954; Ruehrwein and Ward, 1952; Sherwood and Engibous, 1953; Weeks and Colter, 1952). Wallace (1995) cited 16 reports of water‐soluble polymer soil conditioners by 1952, and 99 reports by 1955. Water‐soluble polymeric conditioners improved soil physical properties, thereby improving root penetration, infiltration, aeration, erosion resistance, and drainage. These physical improvements usually increased rooting volume and plant interception of nutrients and water, indirectly improving plant nutrition. The main strategy for water‐soluble polymeric soil conditioner use from the 1950s until the 1990s was application of suYcient conditioner material to physically modify soil properties to the depth of tillage. This mode of treatment usually entails multiple application operations, either as bulk solid materials or as sprayed liquids, solutions, or slurries at cumulative rates of 100 kg ha1 or more. Tillage is usually required following each application to incorporate material to a desired depth. Because the mass of soil to 150‐mm depth is typically 2 million kg ha1, it requires tonnes of organic or mineral amendments per hectare and hundreds of kilogram per hectare of water‐soluble polymeric amendments to alter physical and chemical properties in the entire mass of soil to the tillage or rooting depth. The most commonly used water‐soluble synthetic soil‐conditioning polymers since the 1950s included: hydrolyzed polyacrylonitrile (HPAN), isobutylene maleic acid (IBM), PAM, polyvinyl alcohol (PVA), sodium polyacrylate (SPA), and vinylacetate maleic acid (VAMA). Commercial formulations of these compounds often combined polymers and extenders or solubility enhancing agents. The most commercially successful water‐soluble soil‐conditioning polymer marketed before the 1990s was the powdered Monsanto product ‘‘Krilium’’ (Nelson, 1998; Quastel, 1953). It combined VAMA with a clay extender to improve application uniformity. Krilium and similar products
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typically cost $4–5 kg1 in the 1950s. The material cost and application amount limited use mainly to high‐value crops and specialized uses. After initial enthusiasm for these conditioners, most products were withdrawn from the agricultural market because of lack of demand. There has also been interest in super water‐absorbent polymers for use in soils (Akhter et al., 2004; Al-Darby, 1996; Al-Omran and Al‐Harbi, 1998; Austin and Bondari, 1992; Baasiri et al., 1986; Blodgett et al., 1993; Boatright et al., 1997; Bres and Weston 1993; Callaghan et al., 1988; Choudhary et al., 1995; Danneels and Van Cotthem, 1994; El‐Hady et al., 1981; Falatah and Al-Omran, 1995; Fonteno and Bilderback, 1993; Green et al., 2004; Hemyari and Nofziger, 1981; Ingram and Yeager, 1987; Johnson, 1984; Katchalsky et al., 1952; Miller, 1979; Orzolek, 1993; Rigas et al., 1999; Sabrah, 1994; Sivapalan, 2006; Taylor and Halfacre, 1986; Tripepi et al., 1991; Tu et al., 1985; Wang and Boogher, 1987; WoVord, 1991; Woodhouse and Johnson, 1991). These are not water‐soluble polymers, but rather are strongly hydrophilic gel‐ forming compounds that absorb up to 2000 times their weight in water. Cross‐ linked PAMs (gel‐forming PAMs) and hydrolyzed starch‐polyacrylonitrile graft polymers (H‐SPANs), patented by the USDA in 1975 using the product name ‘‘Super Slurper,’’ are the most common water‐absorbent polymers used as soil conditioners. These compounds are also the water‐absorbent polymers commonly used in such familiar products as disposable diapers. As soil conditioners, they improve the water retention of sandy soils, or around seeds, or roots of transplants or seedlings in situations where prolonged or untimely drought can occur, especially at planting. Spot placement of gel polymers can enhance emergence and seedling establishment without having to irrigate the entire soil profile. It should be noted that a perception exists that these gel polymers conserve water. But their mode of action is not one of water conservation but rather one of water storage enhancement. Optimal plant water requirements remain governed by principles of evapotranspiration. The polymers do not reduce the water demand or use, but can buVer the root zone against water loss in soils with low water retention properties (Letey et al., 1992). Despite the potential benefits if properly used, costs are usually too high to modify an entire field’s soil profile or its tillage zone or rooting depth, even at a cost of only $2–3 kg1. Thus their use is typically restricted to high‐value nursery or horticultural situations to reduce irrigation frequency, or lessen stress between irrigations, particularly where plant or crop quality and value are impaired by stress. Since the 1980s and early 1990s polymer purity and molecular size have increased, greatly improving the eYcacy, safety, and aVordability of environmental polymers. These changes, coupled with new application strategies that only target critical portions of the soil for treatment, and that do not require expensive application protocols, have renewed interest in polymers for a growing number of agricultural and environmental uses. The best example
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of this progress is PAM use for erosion control and infiltration management in irrigated agriculture. Although less extensively researched, PAM use is also increasing rapidly for construction site and road cut erosion protection, for ditch and canal sealing against seepage loss, and for dust suppression in military encampments, helicopter‐landing areas, and roadways.
IV. OVERVIEW OF CURRENT PAM USE Many field trials from the 1950s to the present investigated polymer amendment eVects on crop response and soil structural and hydraulic properties; these have been summarized in several reviews and monographs (Bouranis, 1998; Bouranis et al., 1995; De Boodt, 1990, 1992, 1993; Levy and Ben-Hur, 1998; Polyakova, 1976, 1978; Seybold, 1994; Stewart, 1975; Terry and Nelson, 1986; Wallace, 1998a,b; Wallace and Terry, 1998; Wallace and Wallace, 1986a,b, 1995). In the 1980s and 1990s there were many laboratory column and mini‐tray studies that investigated polymer eVects on soil structure, infiltration, hydraulic conductivity, and related phenomena, often focusing on surface sealing, runoV, and soil aggregate dispersion. With time, research on soil‐conditioning polymers focused on fewer polymers. Most polymer soil amendment research involved PAM, polysaccharides, or other biopolymer surrogates of PAM. Often the biopolymer surrogates of PAM have been grafted copolymers of PAM, developed in eVorts to lower cost, to use other raw materials, and/or to achieve faster decomposition. There have also been a few reports of eVective use of poly(diallyldimethylammonium chloride) (poly‐DADMAC) polymers for erosion control and improvement of soil physical properties (Bernas et al., 1995). The insight that reenergized research interest in PAM for large‐scale agricultural and environmental applications was its ability to prevent erosion when applied with surface irrigation water in very small amounts (1–2 kg ha1 per treated irrigation). This occurred at the same time that environmental concern about the impacts of sediment‐laden runoV from agriculture was emerging world wide as a major issue. Isolated reports hinted that very small amounts of PAM in irrigation water, flowing over soil in irrigation furrows, virtually eliminated detachment and transport of soil particles (Mitchell, 1986; Paganyas, 1975; Paganyas et al., 1972). Paganyas et al. (1972) and Paganyas (1975) did not adequately identify the polymer used, referring to it as a ‘‘K’’ compound; however, the observations of reduced erosion in furrow runoV at low per hectare polymer application rates are probably the first in the literature. Mitchell (1986) gave only anecdotal observations of erosion control and concentrated on infiltration eVects. Most of the literature of the 1990s to the present, which used PAM
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concentrations of 10 ppm or less in the water for erosion control, also reported infiltration increases on medium to fine‐textured soils. The concentration dependence of this phenomenon has become better understood in recent years and will be discussed more fully later in this chapter. Because of the ease of adapting PAM to furrow irrigation, that sector of irrigation received the most attention during the 1990s. Research showing eYcacy of PAM to reduce erosion, limit surface sealing, and improve infiltration with sprinkler irrigation, however, was also being reported (Ben-Hur et al., 1990) and has added to the momentum of this technology, although it was less recognized initially. The use of PAM for surface irrigation and sprinkler irrigation will be presented separately in this chapter. The first research reporting a potentially practical and economical PAM‐ based field approach to reducing furrow irrigation‐induced erosion was presented by Lentz et al. (1992). Related reports followed over the next several years (Ben-Hur et al., 1992a; Gal et al., 1992; Lentz and Sojka, 1994, 1996a,b,c, 2000; Lentz et al., 2000; Levy et al., 1995; Sojka and Lentz, 1994, 1996a,b,c, 1997; Sojka et al., 1998a,b; Trout et al., 1995). The success of this new research came from the realization that a better way to prevent furrow irrigation-induced erosion is to use the water to deliver the soil conditioner rather than modifying the soil to the depth of tillage. This delivery mode applies only minute amounts of PAM to a small fraction of the soil surface aVecting the interaction of the flowing water with the rest of the soil profile. Irrigation, in general, is well suited to this mode of application, and it is especially suited to furrow irrigation. In this mode of application, only 1–2 kg ha1 of PAM per treated irrigation were needed to halt an average of 94% of erosion from irrigation furrows (Lentz and Sojka, 1994). The soil treated in the irrigation furrow comprises only about 25% of the field surface area to a depth of a few millimeters. Inflows only need to be dosed as water crosses the field (the water advance). PAM application is halted at the initiation of runoV. To ensure environmental safety, this application method was developed around the use of a food‐grade class of PAM. These PAMs are anionic. They have a typical charge density of 18%, although the charge density can vary from only a few percent to 50% or more. The PAMs used for erosion control are regarded as moderately large molecules, having over 150,000‐chained monomer segments per molecule, resulting in typical molecular weights of 12–15 Mg mol1. The molecules are manufactured to a high purity, and are the same PAMs used for a variety of food‐processing uses and for drinking water treatment, with residual AMD monomer contents of <0.05%. The low AMD content and anionic nature of the molecule ensures safety for humans handling the PAM and for aquatic species, if any PAM is lost in runoV to surface waters. However, the anionic charge imparts the need for bridging cations in the solvating water to link the anionic polymer to the predominately
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anionic mineral and organic particulate surfaces. Waters and soils containing dissolved Ca enable better PAM eYcacy than low‐electrolyte (pure) water, and eYcacy is best when there is little or no Na present. The small hydrated radius of divalent Ca helps shrink the electrical double layer surrounding charged particles, promoting flocculation. The monovalent Na ion, by contrast, has a large hydrated radius that interferes with the flocculation process by preventing attracted surfaces from migrating close enough to one another to form floccules. PAM is so eVective at stabilizing surface structure, even at these small application amounts, that, in most medium to fine‐textured soils, infiltration is increased compared to nontreated water (Flanagan et al., 1997a; Lentz and Sojka, 1994; Lentz et al., 1992; Sojka et al., 1998a,b; Trout et al., 1995). Water without PAM, flowing over a soil surface, tends to disrupt aggregates and disperse them in the flow. As water containing the dispersed fines infiltrates, the fines are drawn into or over the pores, which plugs the pores and induces surface sealing. The eVect is intensified if water arrives at the soil surface via water droplets from sprinklers or rain, which have additional kinetic energy that adds to the disruption and dispersion of encountered aggregates. While initial uses of irrigation‐applied PAM were focused mainly on erosion control, farmers are often equally or more interested in using PAM for infiltration improvement where their particular soils or production systems are prone to slow infiltration. As technological barriers to PAM use in sprinkler irrigation are overcome, its growth in that segment of agriculture may be driven by eVorts to improve the uniformity and rate of infiltration (Aase et al., 1998; Bjorneberg and Aase, 2000; Bjorneberg et al., 2000a,b). When runoV occurs, water redistribution results in inadequate wetting of the centers of raised beds and ponding in low areas of the field. The ponding can induce poor aeration and disease problems and/or the leaching of nutrients or agrochemicals. With proper application strategies, PAM can be used to both increase infiltration and improve infiltration uniformity. With PAM in the water, soil structure is stabilized and surface sealing is reduced; water droplets enter the ground where they land rather than causing surface seals that induce runoV and, hence, redistribution of water. PAM use with irrigation for erosion control benefits water quality in a number of ways. By preventing erosion, it also reduces desorption opportunity for sorbed nutrients and pesticides, and limits dissolution of soil organic matter in runoV that elevates dissolved organic carbon (DOC) and biological oxygen demand (BOD) (Agassi et al., 1995; Bjorneberg et al., 2000b; Lentz et al., 1998, 2001a,b). PAM‐treated irrigation water has also proven highly eVective at reducing movement oV site of soilborne microorganisms and weed seed, greatly reducing the likelihood of downstream inoculation and, ultimately, reducing the need for pesticides (Entry and Sojka, 2000; Entry et al., 2002; Sojka and Entry, 2000; Sojka et al., 2000).
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Because PAM increases the viscosity of water flowing through soil pores (Letey, 1996; Malik and Letey, 1992), the eVects of PAM on infiltration are a balance of seal prevention (allowing greater infiltration) and increased viscosity (slowing the passage of water). Experiments are underway to use the viscosity eVects together with other application and management strategies for canal and pond sealing, for improved infiltration uniformity along long irrigation furrows, and for better water retention in coarse‐textured soils where infiltration is not a problem but poor water‐holding capacity and leaching are problems. The ability to use PAM to selectively increase infiltration or to reduce it is application and management dependent and is discussed in greater detail later in the chapter. Information on PAM use for erosion and pollution prevention and for better irrigation water management can be found at .
V.
PAM DEFINED AND DESCRIBED
The word polyacrylamide and the acronym ‘‘PAM’’ are generic chemistry terms, referring to a broad class of compounds. There are hundreds of specific PAM formulations. They vary in polymer chain length and number and kinds of functional group substitutions as well as molecular conformation, the most important conformation variation being linear or cross‐linked conformation. Cross‐linked PAMs are water absorbent but are not water soluble. Water‐soluble PAMs have little if any cross‐linking and the molecules, when dissolved in water, are nominally ‘‘linear,’’ although they may be coiled or curled to varying degree due to either substitutions along the chain or as a result of electrolytes in the solvating water. In PAMs used for erosion control and infiltration management, the PAM homopolymer is copolymerized. Some of the spliced chain segments replace PAM amide functional groups with groups containing Na ions or protons that freely dissociate in water, providing negative charge sites along the polymer chain (Fig. 1). Typically one in five chain segments provide a charged site in this manner. Barvenik (1994) described the common synthesis pathways for nonionic, cationic, and anionic PAM formulations. So‐called nonionic PAMs are actually slightly anionic homopolymeric formulations due to slight (1–2%) hydrolysis of some of the AMD units during manufacture (Halverson and Panzer, 1980). Cationic and anionic PAMs are produced by one of a variety of postpolymerization reaction sequences beginning with the AMD homopolymer or via copolymerization of AMD and a suitable cationic or anionic comonomer (Mortimer, 1991). Cationic PAMs are commonly produced via two general processes. One is copolymerization with
PAM IN AGRICULTURE AND ENVIRONMENTAL LAND H
H
H
H
C
C
C
C
H
C
H
C
O Figure 1
NH2 x
O
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Oy
Polyacrylamide polymer structure unit (Sojka et al., 2005).
acryloyloxyethyl‐trimethyl ammonium chloride or any of several other cationic comonomers, resulting in relatively random distribution of cationic units along the polymer (Lipp and Kozakiewicz, 1991). Another method subjects the AMD homopolymer to further reaction yielding a tertiary amine, which is then reacted with a quaternizing agent. This latter method is referred to as the Mannich pathway (Lipp and Kozakiewicz, 1991). Anionic PAMs are preferred for environmental applications because of their extremely low aquatic toxicity compared to nonionic or cationic forms. Anionic PAMs can also be produced by several reaction pathways. One pathway involves hydrolysis of nonionic PAM with a strong base. Using this pathway, the charge density is controlled by the quantity of base used. This pathway produces a copolymer of AMD and a salt of acrylic acid. The distribution of the anionic and nonionic units is controlled by varying the hydrolysis conditions (Lipp and Kozakiewicz, 1991). An alternative method produces anionic PAMs by hydrolysis of polyacrylonitrile (Halverson and Panzer, 1980). Common commercial anionic PAM formulations are produced by copolymerization of AMD and acrylic acid or one of its salts (Mortimer, 1991). The pH and ionic constituents of the formulation determine if the acrylic acid units are present as carboxylate ion or are paired with a counterion, which can be Hþ, NHþ, or Naþ. When anionic PAMs are dissolved in water and applied to the soil, the system pH controls the ionization. Above pH 6, the acrylic acid units tend to be anionic. Below pH 4, the anionic sites tend to be protonated, reducing the eVective molecular charge (Halverson and Panzer, 1980). Anionic PAMs with sulfonic acid groups that hold their charge at lower pH are better suited for acid environments (Halverson and Panzer, 1980; Mortimer, 1991). Natural gas has been the cheap abundant raw material from which the chemical building blocks used in PAM synthesis are derived. However, current supplies and economics may not reflect the future. The gradual increase in the cost of natural gas since 2000 has resulted in about a 30% increase in wholesale prices of PAM in that 6‐year period. Work using chitin and starch to develop new copolymers of PAM or biopolymer surrogates that perform comparable to PAM has proven promising, although results are yet to match those achievable currently with PAMs (Orts et al., 1999,
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2000, 2001). Potential use of these materials for synthesis of eVective flocculants and soil stabilizers carries the added benefit of using agricultural or other organic waste streams to produce value‐added products. PAM formulations for irrigated agriculture are water‐soluble (linear, not gel‐forming, not cross‐linked, not super water absorbent) anionic polymers. They have typical molecular weights of 12–15 Mg mol1 (over 150,000 monomer units per molecule). These PAMs are ‘‘oV the shelf’’ industrial flocculant polymers that are used extensively to accelerate separation of solids from aqueous suspensions. Lists of common industrial and food‐ processing uses of anionic PAMs have been presented by Wallace et al. (1986b) and Barvenik (1994). Some of the most important uses include sewage sludge dewatering, drilling muds, mine slurry conditioning and mineral separation processes, paper manufacture, clarification of refined sugar, fruit juices and drinking water, thickening agents in animal feed preparations, antiscaling water treatment in steam processes in contact with processed foods, and as a coating on paper used for food packaging. The surface chemistry of soils and the large physical‐chemical domain of PAM macromolecules make them useful compounds for management of soil processes governed by flocculation, aggregation, and structure stabilization. In water with suYcient electrolytes, coulombic and van der Waals forces attract soil particles to anionic PAM (Orts et al., 1999, 2000). These surface attractions stabilize structure by enhancing particle cohesion, thus increasing resistance to shear‐induced detachment and transport in runoV. The few particles that do detach are quickly flocculated by PAM, settling them out of the transport stream. Minute amounts of Ca2þ in the water shrink the electrical double layer surrounding soil particles and bridge the anionic surfaces of soil particles and anionic PAM molecules, enabling flocculation (Wallace and Wallace, 1996). Malik et al. (1991b) found that PAM applied via infiltrating water is irreversibly adsorbed in the top few millimeters of soil once dry. Lu and Wu (2003a) reported that PAM penetrated into organic matter‐free soil 20–30 mm. PAM delivery via furrow streams is very eYcient because it needs to only stabilize the thin veneer of soil directly active in the erosion process. In furrow irrigation PAM treats only about 25% of the field surface area to a 10‐ to 20‐mm depth, requiring only 1–2 kg ha1 of PAM per irrigation.
VI. PAM PROPERTIES AFFECTING EFFICACY Both molecular properties and product formulation or preparation can influence how easily PAMs are handled and applied, as well as their behavior during or following application (Callebaut et al., 1979). Barvenik (1994)
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noted that the most common commercial forms of PAM are as aqueous solutions, emulsions, and dry forms. Pure aqueous solutions of PAM become highly viscous at concentrations largely dependent on molecular weight. Lower molecular weight PAMs (30,000 g mol1) remain fluid even at 50% concentrations. However, the anionic PAMs commonly used for erosion control, infiltration management, and aggregate stabilization in soil since the early 1990s usually have molecular weights ranging from 12 to 15 Mg mol1 and some products are available near 20 Mg mol1. At these high molecular weights, aqueous solutions of 1–2% are too viscous for practical use in liquid injection systems or spray applicators. The viscosity can be reduced if high concentrations of salts are added to the solutions; however, the salts can either be problematic in and of themselves in the application environment or can influence the resulting conformation of the deployed molecule on dilution. In addition, there is the practical consideration of shipping weight and volume, and thus cost, associated with distributing PAMs for use as low‐concentration aqueous solutions. For industrial applications where liquid injection is desirable, inverse emulsion formulations are useful. These formulations consist of aqueous droplets containing polymer suspended in a petroleum distillate or other appropriate oil or lipid matrix; the polymer is stabilized by inclusion of a surfactant (Barvenik, 1994; Buchholz, 1992; Lipp and Kozakiewicz, 1991), while in the formulated emulsion the PAM concentration can be as high as 50% on a weight basis. On injection to an aqueous environment under the proper mixing regime, the carrier quickly disperses and the PAM is released for dissolution. This process is accelerated by the action of the surfactant. Injection of the emulsion must be into a rapidly flowing stream of water with enough turbulence to rapidly disperse and mix the emulsion into the water stream, and with a suYciently high flow rate to instantly dilute the PAM to low concentration. This prevents the rapid attainment of the high‐viscosity characteristic of high‐concentration aqueous PAM solutions. If adequate turbulence and flow are not provided, the partially hydrated PAM can form gel‐like ultrahigh viscous bodies that resemble latex rubber or soft plastic. Once in this configuration further PAM dissolution becomes dependent on water contact with the PAM in the viscous mass. Since the surface area available for further dissolution under these conditions is greatly restricted, the masses become stable semipermanent features in the aqueous environment and can cause severe problems with pumping equipment, injectors, piping, and spray nozzles. Protocols have been suggested for agricultural settings to avoid these problems (Sojka et al., 1998c), and emulsions have been utilized to a limited degree for injection of PAM in center pivots. The diYculty of maintaining adequate control in agricultural settings, however, has largely resulted in the abandoning of the use of emulsions for PAM application in irrigation.
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There is increased interest in local preparation of aqueous PAM solutions of a few percent concentration utilizing various fertilizer salts or Ca salts to reduce the viscosity. These solutions are injectable in irrigation systems without the drawbacks of emulsions. The approach is targeted primarily at use of PAM in sprinkler irrigation systems. While this approach avoids the clogging possibility of emulsions, it carries the drawback of preparation and handling of greater solution volumes. By far the greatest on‐farm use of PAM to date focuses on the use of dry granular PAM products that are either directly applied to irrigation furrows before the inflow or that are metered at head ditches with adequate turbulence and distance allotted to dissolve the PAM prior to water flowing onto the field. These approaches will be described in greater detail later in the chapter. A variety of PAM molecular properties and environmental properties interact to aVect PAM eYcacy. These interactions can be significant given the range of PAM use in the environment and the number of environmental and application factors that can vary. PAM uses in the environment include, but are not limited to, flocculation of suspended solids, soil stabilization, and infiltration enhancement. Soil stabilization helps resist erosion, and the suspended solids can be mineral, organic, or biotic material. Reduction of turbidity and mixing reduces opportunities for desorption of pesticides and nutrients or other soluble organics. PAM can be directly applied through irrigation water or indirectly on activation by water (irrigation or rain) when applied initially as dry powder or granules to the soil surface. Advances in theoretical chemistry have aided in the continued improvement of PAM performance through design of molecular conformations optimally suited to given industrial and environmental applications (Bicerano, 1994; Bouranis, 1998; Bouranis et al., 1995; Chamberlain and Cole, 1996). A good deal of information on the eVects of coagulants and flocculants comes from the wastewater treatment literature, although often from polymers and compounds other than PAM, or in the case of PAM, often from cationic or nonionic formulations. Nonetheless, many of the general principles are worth noting as a framework for understanding the behavior of anionic high molecular weight PAMs, the dominant class of polymers for erosion control and infiltration management in agriculture. At cool temperatures from 6 to 29 C, flocculation for a variety of inorganic and polymer compounds tended to be slower and flocs tend to be smaller than at higher temperatures (Fitzpatrick et al., 2004; Hanson and Cleasby, 1990). Furthermore, floc strength seems to vary with the shear conditions of the flow media in which flocs are formed. The larger flocs formed at higher temperatures are more easily disrupted and less capable of reformation than flocs formed at lower temperatures (Fitzpatrick et al., 2004; Yeung and Pelton, 1996; Yeung et al., 1997).
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The mechanisms by which flocculation of suspended sediments takes place in the presence of polyelectrolytes was studied by Ruehrwein and Ward (1952) who associated this eVect with stabilization of soil and resistance to dispersion. Ben-Hur and Letey (1989) and Ben-Hur et al. (1989) attributed this eVect as the mechanism that reduced particle dispersion when sprinkler irrigating with PAM, which in turn reduced surface sealing and slowed the reduction of infiltration rate. The adsorption of polymers from aqueous media onto mineral surfaces has been reviewed several times (Greenland, 1963, 1972; Harris et al., 1966; Theng, 1979, 1982) and has been the object of numerous investigations (Ben-Hur et al., 1992a,b; Gu and Doner, 1993; Haschke et al., 2002; Lakatos et al., 1981; Letey, 1994; Lu and Wu, 2003a; Lu et al., 2002a; Lurie and Rebhun, 1997; Malik and Letey, 1991; Mukhopadhyay et al., 1994). PAM conformation, charge type, and charge density influence their eYcacy for soil stabilization. Malik and Letey (1991) reasoned that longer, less‐coiled PAM molecules would be more strongly adsorbed to mineral surfaces. Michaels and Morelos (1955) reported that 20% hydrolysis of PAM provided the greatest degree of chain extension, facilitating adsorption. As PAM is drawn to mineral particle surfaces, surface‐adsorbed water is driven away because of the stronger attraction for the polymer (Parfitt and Greenland, 1970). Nonionic PAMs are attracted to solids mainly through H bonding of hydroxyl groups on the polymer attracted to oxygen atoms on the silicate mineral surfaces or via other charge‐dipole or dipole‐dipole interactions. Theng (1982) noted that cationic PAMs are adsorbed through the interaction of cationic sites on the polymer and negative charge sites on clay particles. Adsorption of anionic PAMs to mineral surfaces, which carry predominately negative charges, is aided by an abundance of Ca2þ in the aqueous system. The opposite occurs for cationic polymers, whose adsorption to anionic mineral surfaces is interfered with by an abundance of electrolytes in the suspending water (Shainberg and Levy, 1994). Aly and Letey (1988) found that adsorption of anionic PAMs and polysaccharides in water of electrical conductivity (EC) 0.7 dS m1 was greater than for water of EC 0.05 dS m1. Entropy change as water is displaced is an important actuating force in bringing about adsorption of negative and nonionic polymers to negatively charged clay surfaces (Lyklema and Fleer, 1987; Theng, 1982). In the case of nonionic polymers, the entropy change increases with polymer molecular weight. Malik and Letey (1991) found that, in general, the molecular size and conformation of polymers aVect adsorption with increasing molecular size and increasing chain extension leading to increasing adsorption. They also concluded that adsorbed polymers do not penetrate soil aggregates, but only coat and stabilize their surfaces. Ben-Hur et al. (1992b) found that adsorption of PAMs onto illite and montmorillonite clays was generally in
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the order cationic > nonionic > anionic regardless of the electrolyte content of the solvating water. Water that was more saline and sodic reduced adsorption of cationic and nonionic PAMs but increased adsorption of anionic PAMs. Janczuk et al. (1991) noted that PAMs aVected the surface free energy and, therefore, the wettability of soil. Wallace et al. (1986c) reported on the eVects of PAM on soil water relationships. Malik and Letey (1991) and Nadler and Letey (1989) used tritium‐labeled polymers to determine sorption isotherms of several types of polyanions on Arlington sandy loam (coarse‐loamy, mixed, thermic Haplic Durixeralfs). They interpreted their results as showing that polymer sorption was restricted to the external surfaces of soil aggregates. A similar conclusion was arrived at by El‐Hardy and Abd El-Hardy (1989) who saw only limited intrusion of high molecular weight PAMs into soil aggregates. Aly and Letey (1988) showed that the water quality of the solvating water used to apply the polymers also greatly influenced the degree of adsorption. Nadler et al. (1992) found that once adsorbed and dried there was subsequently little desorption of anionic PAM from soil. Lu et al. (2002a) found that PAM sorption isotherms on soil materials could be described well by the Langmuir equation and that soil texture, organic mater content, and dissolved salts all influenced the extent of PAM sorption. Soils with high clay or silt content and low organic matter content had high sorptive aYnity of anionic PAM and sorption increased as total dissolved salts increased with divalent cations 28 times as eVective in enhancing sorption as monovalent cations. Cation enhancement of sorption was more eVective in fine‐textured soils than in coarse‐textured soils and presence of greater amounts of organic matter tended to interfere with PAM sorption. Adsorption of PAM on soil and clay mineral surfaces has been demonstrated to be rapid and irreversible in several studies although the degree of adsorption is dependent on PAM conformation, soil or mineral properties, and soil solution characteristics (Hollander et al., 1981; Nabzar et al., 1984, 1988; Nadler et al., 1992; Pradip and Fuerstnau, 1980; Tanaka et al., 1990). Because of its high aYnity for clay mineral surfaces in soil, PAM becomes concentrated in the upper portions of soil profiles to which it is applied in irrigation water. PAM remained stable at the original application depth even 10 months after application and with 720 mm of additional water application (Nadler et al., 1994). The length of the polymer chain and large number of adsorption sites along the molecule contribute to PAM’s adsorption strength. Desorption is diYcult because some adsorption sites are nearly always attached to the adsorption surface, preventing removal of the molecule (Nadler and Letey, 1989). Because of the demonstrated high sorptive ability and low mobility (Malik and Letey, 1991; Malik et al., 1991b), PAM is generally regarded as incapable of penetrating soil more than a few centimeters from the soil surface. This assumption is probably restricted to
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high molecular weight PAMs. Shaviv et al. (1987a,b) demonstrated that low molecular weight PAM (<75,000 g mol1) could move to about the same depth as the wetting front. The molecular gyration radius of low to moderate molecular weight PAMs (Muller et al., 1979) is compatible with soil micropore size. This, considered in light of the relatively slow sorption kinetics of the large PAM molecule (Lu et al., 2002a), has led to recognition that the depth of PAM penetration depends on PAM properties, application method, and the soil and water properties present in the application scenario (Lu and Wu, 2003a). Using anionic PAMs of 10–15 Mg mol1, Lu and Wu (2003a) found that PAM penetration depth was about one‐eighth to one‐half of the water penetration depth, with a particularly high PAM retention in the top few centimeters of the soil. The PAM retained in the top 0–2 mm of soil ranged from 16% to 95% of the total applied. PAM retention was greater at shallow depths when solution contact with the soil was favored by pore arrangement, contact time, and drier soil conditions on addition of PAM solutions. Variations in PAM molecular properties and solution concentrations aVect PAM interaction with mineral surfaces influencing the eYcacy of PAM for erosion control and infiltration management when applied in the field. Lentz and Sojka (1996c) and Lentz et al. (1993, 2000) reported the eVects of PAM charge type, charge density, and molecular weight on infiltration and control of furrow irrigation‐induced erosion for Portneuf silt loam (Durinodic Xeric Haplocalcids) in Idaho production‐scale fields treated with 10 mg liter1 PAM during water advance in the furrow, followed by untreated water for an approximate PAM application of 1 kg ha1. Anionic and nonionic PAMs were twice as eVective as cationic PAMs for controlling sediment loss in new furrows, with erosion control in the order anionic > nonionic > cationic. Erosion control eYcacy increased with charge density from 8 to 19 to 35 mol% and with increasing molecular weight over the range of 4–17 Mg mol1. However, infiltration increased 14–19% when PAM molecular weight fell from 17 to 4 Mg mol1, and generally medium and high charge density anionic and nonionic PAM increased infiltration more eVectively than cationic PAMs. Nonionic PAMs produced the greatest season‐long infiltration gains compared to nontreated furrows (5% increase). Charged PAMs produced greater infiltration increases in the early season on newly formed furrows, but decreased infiltration on repeatedly irrigated furrows late in the season. Green et al. (2000) spray‐applied 288 mg liter1 PAM solutions to soils in small trays at an equivalent rate of 20 kg ha1, allowing them to air dry for 24 h before sprinkling them at 68 mm h1 for 1 h. They obtained mixed results for PAM charge densities and molecular weights, which they attributed to specific soil interactions. They concluded that charge density was the main factor aVecting infiltration, with a charge density of 30% optimal for the clay soil in their tests. On the sandy soil, however, molecular weight was the main factor aVecting infiltration with
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12 Mg mol1 optimal. Their results were somewhat similar to an earlier study with anionic PAMs (Levy and Agassi, 1995) using similar application rates sprinkled at 48 mm h1. In rainfall simulator studies on mine soils, Vacher et al. (2003) saw improved erosion control with higher molecular weight formulations, but no diVerences in infiltration. PAM performance in their study was enhanced on soils with higher clay contents. Levy and Agassi (1995) also noted the importance of viscosity in explaining the eVects of PAMs applied to soil surfaces for infiltration and erosion control. This aspect of PAM performance in soil was explained earlier by Malik and Letey (1992) and again by Letey (1996). In essence, the infiltration of PAM solutions is aVected by the increase in solution viscosity as concentration increases. As noted by Muller et al. (1979), the rotation radius of PAMs is aVected by their molecular size and conformation. As concentration increases this aVects the rate that viscosity increases. Molecular rotation is more constrained in smaller pores and, when polymers become attached to surfaces on the interior of pores, fluid movement in narrow pores is further restricted. This property of PAM and other large organic molecules was described by Malik and Letey (1992) and Letey (1996) as ‘‘pore size‐dependent apparent viscosity.’’ Using a Cannon‐Fenske style viscometer, Bjorneberg (1998) looked at PAM solution kinematic viscosity for performance in bulk flow situations and in pumping equipment and piping. The PAM evaluated was anionic with a molecular weight of 12–15 Mg mol1. PAM solution viscosity was not significantly aVected below 400 ppm and had only minor temperature eVects over the range of 10–40 C. In large diameter vessels and for pump performance, Bjorneberg (1998) found that PAM solutions above 400 ppm performed as a non‐Newtonian solution, meaning that viscosity changed with flow conditions. He also noted that large PAM molecules are subject to significant molecular shear when recirculated through pumping equipment (a common practice in agricultural solution tanks), resulting in loss of as much as half the kinematic viscosity with as little as five passes through a pump. This means that applying PAM as a liquid via standard agricultural mixing protocols can easily change PAM molecular conformation, reducing the average polymer chain length before PAM reaches the intended application target. MacWilliams (1978) and Tolstikh et al. (1992) associated a decrease in chain length or molecular weight with high‐speed agitation of PAM solutions or disruption via exposure to ultrasonic energy. High molecular weight PAM formulations are much more susceptible to shear. When exposed to a shear environment, high molecular weight formulations typically see an initial steep decline in viscosity which plateaus as the smaller remaining molecules flow around the source of shear more easily. Nagashiro et al. (1975) saw viscosity drop quickly as molecular weight fell from 5 to 3 Mg mol1. PAM solutions or PAM applied to targets can also experience gradual
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(e. g., weeks) reduction of molecular weight due to environmental exposure, particularly UV light, or through gradual unfolding of the macromolecule (Molyneux, 1983). As noted in the preceding paragraphs, viscosity can aVect eYcacy both for erosion control and infiltration management, as well as depth of PAM penetration into the soil. Ben-Hur and Keren (1997), using a rotary viscometer, found that a 10–15 Mg mol1 PAM formulation produced a rapid increase in viscosity beginning at a solution concentration of about 2 g liter1, whereas two other formulations with molecular weights of 0.1–0.2 and 0.2–2.0 Mg mol1 required about 50 and 4 g liter1, respectively, before rapid viscosity increases began occurring. The combination of the findings from Malik and Letey (1992), Letey (1996), Ben-Hur and Keren (1997), and Bjorneberg (1998) provides a theoretical starting point for one of the newer PAM uses, canal sealing, which will be described later in the chapter. PAM solutions usually promote flocculation of suspended solids at low concentrations but can have the opposite eVects at higher concentrations, where the large physical domain of the macromolecules themselves interfere with flocculation and actually stabilize dispersed suspensions as viscosity increases. The concentration at which the eVects reverse depends on several factors including molecular weight, charge and conformation of the molecule, size and chemistry of the dispersed solids, and chemistry of the water, particularly EC and SAR (Sato and Ruch, 1980). Experience on highly calcareous silt loam soils of the Pacific Northwest suggests that for use of anionic PAMs of 12–15 Mg mol1 with 18% charge density, applied in irrigation water, the concentration at which PAM ceases to aid flocculation and begins to act as a dispersant is in the range of 50–60 ppm, and the reversal occurs at even lower concentrations as soil SAR increases (Lentz, 2003). The actual value could be greater or lower, depending especially on EC and SAR of the irrigation water and pH and organic matter content of the soil. The role of Ca2þ in eYcacy of anionic PAMs in general, and for flocculation in particular, is discussed in greater detail in a later section.
VII. EARLY CONTRIBUTIONS The reduction of erosion and management of infiltration through improved uses of synthetic‐ and bio‐polymers has been identified by Natural Resource Conservation Service (NRCS) as one of the most dramatically eVective, agriculturally significant, and environmentally important advances in irrigated soil conservation management (Thomas SpoVord, NRCS National Irrigation Engineer, personal communication). Practical application of the same technology in rainfed agriculture has been more diYcult to
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achieve and less progress has been made. The advances in the irrigated sector from the late 1980s to the present have come about from the individual and collaborative eVorts of several research groups worldwide. It is diYcult to identify a single beginning for the acceleration in this field of research. We have attempted to cover the most noteworthy contributions. Several lines of investigation developed more or less naturally. Rainfed agriculture and irrigated agriculture pursued PAM use more or less independently. While some research focused more on erosion control than infiltration, and vice versa, it was soon apparent that the phenomena were interconnected. Furthermore, it eventually became clear that PAM could be used to selectively increase or decrease infiltration to meet spatial and temporal needs of a given situation. In irrigated agriculture, researchers found that furrow, sprinkler, drip, and flood irrigation each has specific and unique considerations that aVect how PAM technology needs to be deployed for those settings. Of necessity, we have segregated this topic into subsections. However, the reader should be aware of the arbitrariness of the separation. Research on PAM underscores how potently the properties of water aVect all soil processes. PAM is one of the few tools in soil management that does its work, in great respect, by changing the eVective properties of water and how water interacts with the soil. Changes in PAM conformation, charge, charge density, molecular weight, concentration, product formulation, application amount, flow properties of the water, mineral ion composition of the water and soil, soil mineralogy, soil texture, and soil antecedent water content all aVect the extent and ‘‘direction’’ of PAM eVects. Scientists seeking expertise in PAM technology for soil management and environmental protection must be aware of the many and sometimes seemingly contradictory nuances that result from the various combinations of these factors. Weeks and Colter (1952) and Bodman et al. (1958) recognized the potential for polymeric soil conditioners to reduce erosion and increase infiltration. Their obstacle was inability to identify an economical and practical application strategy. Investigations of a newer strategy of PAM and other polymer use, where the polymers were either only surface applied and/or applied via irrigation water began in the 1970s and early 1980s. Paganyas et al. (1972) and Paganyas (1975) treated only the surface of the soil with solutions of polymers prior to irrigation rather than attempting to modify the entire soil profile. Treating only the furrow bottoms in a light preirrigation with 15–20 kg ha1 polymer per irrigation, Paganyas (1975) found that soil was stabilized against erosion, reducing losses about 90% across polymer treatments. Net infiltration increased and the infiltrated water moved farther laterally. Aggregate stability of the treated furrows greatly increased and losses of N and P in runoV were greatly reduced. These findings were confirmed and quantified in greater detail with specifically identified PAM
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formulations in later work in Kimberly, Idaho before researchers were fully aware of the earlier Soviet experiments. The Soviet work did not specify the polymers they used, referring only to them as a series of ‘‘K’’ compounds. However, the description of the K compounds suggests they were PAMs or closely related polymers. Wallace and Wallace (1986a,b) noted the potential for use of very small application amounts of soil conditioners, especially PAM, to control erosion but reported only sparse observations. They also noted the potential for application of water‐soluble polymer conditioners via irrigation (Wallace and Wallace, 1987). Mitchell (1986) applied PAM in furrow irrigation water to alter the infiltration rate and noted, but did not quantify, a marked reduction in runoV turbidity. Work by Helalia and Letey (1988a,b) and Wood and Oster (1985) showed in rainfall simulator studies that soil dispersion was reduced and infiltration increased when PAM and other polymers were added to the water at rates as low as 10 ppm. This observation was bolstered by later studies (Ben-Hur et al., 1989; Helalia and Letey, 1989) in which aggregate stability was increased by low‐rate polymer additions to the water. Aly and Letey (1990) established that the direction and degree of these eVects was dependent on matching polymer and soil properties. Reduced dispersion, stabilization of aggregates, and increased infiltration are all factors that tend to reduce the potential for erosion.
VIII.
SURFACE IRRIGATION
Water‐soluble anionic PAM was identified in the 1990s as a highly eVective erosion‐preventing and infiltration‐enhancing polymer, when applied in furrow irrigation water at concentrations of 1–10 ppm for applications of 1–2 kg ha1 per treated irrigation (Lentz and Sojka, 1994; Lentz et al., 1992; McCutchan et al., 1993; Sojka and Lentz, 1997; Sojka et al., 1998a,b; Trout et al., 1995; Yonts et al., 2000). PAM achieves these results when applied to soil via the irrigation water at such low concentrations by stabilizing soil surface structure and pore continuity. Stabilized surface structure resists the shear forces of flowing water, thereby preventing detachment, transport, and dispersion of soil particles, thus eliminating detached solids for dispersal in flowing water that enter and block pores as water infiltrates downstream. In 1995 the US NRCS published an interim PAM‐ use conservation practice standard which was revised in 2001 (Anonymous, 1995, 2001; NRCS, 2001). The standard gives considerations and methodologies for PAM use. PAMs were first widely commercialized for erosion control in the United States in 1995. By 1999 about 400,000 ha were
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PAM-treated in the United States. The current estimate is about 1 million ha. The US market is expected to continue to grow as water quality improvements are mandated by State and Federal legislation and court action because PAM use is one of the most eVective, economical, and easily implemented technologies available that can achieve required water quality improvements. Overseas use of PAM for erosion control and infiltration management is also growing rapidly. PAM, used following NRCS guidelines (Anonymous, 1995, 2001), reduced sediment in runoV 94% in 3 years of furrow irrigation studies in Idaho (Lentz and Sojka, 1994). The 1995 NRCS standard called for dissolving 10‐ppm PAM in furrow inflow water as it first crosses a field (water advance‐typically the first 10–25% of an irrigation duration). PAM dosing is halted when runoV begins. The PAM applied during advance generally prevents erosion throughout a 24‐h irrigation. Application amounts under the NRCS standard usually work out to 1–2 kg ha1. PAM treatment is recommended whenever soil is disturbed (loose and highly erodible) before an irrigation. Following initial PAM treatment, erosion in later irrigations can usually be controlled with only 1‐ to 5‐ppm PAM if the soil was not disturbed between irrigations. Without reapplication of PAM, erosion control on reirrigation of previously treated furrows typically drops by nearly half (Lentz and Sojka, 1994; Lentz et al., 1992). Furrow irrigators often use a simple application strategy which they call the ‘‘patch method.’’ This involves spreading dry PAM granules onto the furrow bottom of the first 1‐ to 2‐m below the inflow point. The 2001 revision of the NRCS PAM standard recognized the patch method as an acceptable alternative to dosing furrow inflows with PAM predissolved in the irrigation water. The patch application method has become the most common mode of application for most furrow‐irrigated situations. In the patch method, the amount of PAM granules can be accurately determined on an area‐equivalent basis‐furrow spacing length at a 1 kg ha1 field application rate. Typical patch doses are 15–30 g per furrow (approximately half ounce to an ounce or teaspoon to tablespoon amounts). Several simple application devices are commercially available to lay down measured PAM granule doses on furrow bottoms near inflow points prior to irrigation. When water flows over a ‘‘patch’’ of dry granules, a thin slimy mat forms that slowly dissolves during the course of the irrigation. Erosion and infiltration eVects of the patch method are comparable to dosing the inflow at 10 ppm (Sojka et al., 1998b, 2003). Erosion control in subsequent nontreated irrigations is often better with patch application than where the initial PAM application was dissolved in the inflowing water supply. This is because bits of the patch are often still intact at the end of the initially treated irrigation, providing small amounts of PAM in later irrigations. Advantages and disadvantages of each application method depend on field conditions and
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system requirements (Sojka et al., 1998c). The patch method works well in most circumstances, but is less reliable on steep slopes (Lentz et al., 2003). Similarly, farmers have experienced problems with the patch if inflow rates are very high. These conditions can cause breakup and transport of the patch down the furrow, or burying of the patch by the sediment scoured at or near the inflow point. PAM predissolved in the advancing inflow performs more reliably at high water flow rates or on steep slopes. However, when soil is damp (from dew, or a light rainfall, or canopy shading) the patch method or use of a continuous low dosage seems to control erosion more reliably than the predissolved dosing only during advancing inflow; the reason for this is not fully understood. A possible explanation is that the initial surface soil wetness interferes with PAM adsorption (Lu and Wu, 2003a). Wetter soil also infiltrates less PAM‐bearing water. Thus, delivering a constant small dose of PAM may compensate for weaker initial stabilization of soil surfaces already damp prior to irrigation. In the US Pacific Northwest, farmers usually treat irrigation water with PAM only during the irrigation events that they perceive carry a high‐ erosion risk, or as required by conservation programs such as the Environmental Quality Incentives Program (EQIP). Farmers typically use 3–5 kg ha1 of PAM during an irrigation season depending on field conditions and crop, which dictate the number of cultivations and irrigations. Thus, although research has shown that 94% seasonal erosion control is achievable, results from commercial farming situations are more commonly in the range of 80% because usually fewer irrigations are treated (Clair Prestwich, NRCS Water and Climate Center, Portland, OR, personal communication). Much has been learned about controlling erosion and infiltration with PAM in furrow irrigation since the reports in the early 1990s. Trout et al. (1995) noted that there was a relationship between the eVectiveness of erosion reduction and the resulting infiltration during an irrigation event (Fig. 2). Most of the erosion and depositional seal formation that drives this relationship occurs within the first minutes or hours of an irrigation set. Much of the eYcacy of PAM in controlling furrow erosion and the related enhancing of infiltration is because, when properly applied, the initial detachment, aggregate dispersion, transport, and resulting deposition do not occur. This is because the furrow is immediately stabilized as water advances, delivering PAM simultaneously. Sojka et al. (1998b) suggested that a shallow subsurface impermeable layer (e.g., in a wheel track furrow) could also strongly aVect infiltration and erosion relationships. Lentz and Sojka (1999, 2000) noted the importance of achieving full dissolution of PAM when employing the strategy of predissolving PAM to a desired concentration prior to delivering the water to the furrow. They also noted the relative eVectiveness of several dosing strategies. For freshly formed furrows, eVectiveness of applying PAM at a uniformly dosed inflow
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A Cumulative infiltration rate (l m−1)
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B
y = a + bx c r 2 = 0.555 150
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Final infiltration rate (l m−1 h−1)
0 y = a + bx c r 2 = 0.59 25 20 15 10 5 0 −5
0
20 30 40 50 10 First hour's sediment concentration (g liter−1)
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Figure 2 (A) EVect of sediment concentration in furrow water on cumulative infiltration into a Portneuf silt loam as measured for 8 h using a recirculating infiltrometer. (B) EVect of the first hour’s sediment concentration on final (8 h) infiltration rate into a Portneuf silt loam, measured using a recirculating infiltrometer (Sojka et al., 1998a).
concentration varied with inflow rate, PAM concentration, duration of furrow exposure, and amount of PAM applied. Erosion control with PAM on 1–2% slopes was similar for three application methods: (1) the NRCS 10 ppm standard, (2) application of 5 ppm during advance, followed by 5–10 min of 5‐ppm reapplication every few hours, or (3) continuous application of 1–2 ppm. Constant application of 0.25 ppm controlled erosion about one‐third less eVectively. McCutchan et al. (1993) described a PAM‐ dosing strategy using predissolved stock solutions metered into furrow flows to achieve a constant 2.5‐ppm PAM concentration. This proved highly eVective at reducing sediment loss in runoV and was comparable to the
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third treatment described by Lentz and Sojka (1999, 2000). One disadvantage of maintaining a constant dose in this concentration range is the use of two to three times the total PAM per hectare in a typical 24‐h irrigation set (vs 10 ppm dosing of the advance only) and some increased risk of eventual PAM loss in the runoV water. Sojka and Lentz (1997) discussed general technical and practical considerations for controlling erosion in furrow irrigation with PAM. Among these was the importance of assuring that the first water to flow down the furrow already contains PAM. Adding PAM to an established flow reduces turbidity and erosion, but can be less durable and usually loses the infiltration‐enhancing eVect because PAM stabilizes the soil surface structure it encounters, but cannot create structure (other than the formation of flocs as PAM clarifies initially turbid water). A common erosion control practice in some regions is placement of straw in irrigation furrows. Although highly eVective at controlling erosion and increasing infiltration, the presence of straw in furrows often creates other problems. Straw can migrate downslope, leading to damming of the furrow and washing of water into adjacent furrows. This in turn damages the planted beds and results in some furrows being under‐irrigated below the washout and others being over‐irrigated. Shock and Shock (1998) and Lentz and Bjorneberg (2001, 2003) examined the relative eVectiveness of straw and PAM as erosion prevention methods for furrow irrigation. Shock and Shock (1998) saw similar eVectiveness of straw mulching and PAM use for both erosion control and infiltration, with slight advantages in their trials for straw mulching. They also reported a cost advantage for straw mulching at $140 ha1 versus $169 ha1 for PAM. Water and PAM application anomalies between treatments and costing assumptions, however, prompt some questions regarding the final analysis. PAM application was varied among irrigations resulting in erratic PAM performance, and irrigation duration was longer for PAM treatments than straw treatments. PAM cost was put at $9.90 kg1 (for a seasonal material total cost of $106 ha1) with an additional estimate of $55 ha1 for delivery and mixing services. Competitive PAM prices currently remain well below this in most markets, and most farmers use application technologies that carry little or no delivery, preparation, or direct application cost. Furthermore, fuel costs for straw application since the study was published have doubled. Aside from the monetary aspects, a major barrier to adoption of straw mulching remains the intrusion of the practice itself (an inconvenient additional operation at a busy time of the year) and complaint by many farmers that bringing straw from other fields risks introducing weeds, in addition to frequent problems associated with straw migration and furrow damming. Lentz and Bjorneberg (2001, 2003) evaluated the relative eVectiveness of two straw rates and PAM plus the two straw rates compared to untreated
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furrows. Treatment eVects varied among irrigations, particularly whether irrigating freshly irrigated furrows or reirrigating undisturbed furrows. In general, PAM in combination with straw gave erosion control of nearly 100% versus about 80% for straw alone and slightly added to the infiltration increase of straw alone. The eVects of all treatments, and particularly the PAM enhancement, were greater in the early irrigations or when soil was disturbed. Two noteworthy observations in this study were reduction of residue migration down furrow and (because sediment moved less in the presence of PAM) the reduced tendency for transported soil to form dams when encountering clumps of transported straw. Straw remained better‐ anchored to the furrow, and what little sediment did move along the furrow passed under or through the anchored straw because straw accumulations were less restrictive at any one point along the furrow (Fig. 3). These two eVects together prevented PAM plus straw furrows from overflowing into
Figure 3 View upstream from the outflow end of furrows showing characteristic amounts of sediment and straw residue transported from upstream reaches and deposited there for control (A), low straw (B), high straw (C), PAM treated with low straw (D), and PAM treated with high straw (E). Photos were taken after the first irrigation (Lentz and Bjorneberg, 2003).
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adjacent furrows. Somewhat similar results were reported by King et al. (1996). This observation is encouraging since farmer reluctance to adopt reduced tillage in furrow‐irrigated agriculture is due largely to residue migration, furrow damming, and downstream irrigation uniformity problems. Leib et al. (2005) found reduction in sediment loss with PAM‐treated furrow irrigation, but reported the need to combine the practice with grassed return flow ditches to achieve desired return flow standards for the lower Yakima River Basin of Washington. Some farmers were reluctant to adopt PAM for erosion control because of concerns over increased advance time. Sojka et al. (1998b) examined the eVects of high (45 liter min1) and low (23 liter min1) furrow inflow rates and wheel track or nonwheeled furrows on erosion control, advance time, and infiltration. The application of PAM at 23 liter min1 increased advance time 33% and reduced runoV soil loss 88% compared to controls. PAM applied at 45 liter min1 reduced advance time 8% and soil loss 75% compared with controls irrigated at 23 liter min1, whereas untreated 45 liter min1 inflows cut advance time 42% but raised soil loss 158%. PAM reduced erosion in all furrows, but in wheel track furrows had no eVect on advance time and little infiltration eVect after two or three irrigations. The authors also noted that the soil in treated furrows saw a 2‐year mean increase of 23% in water‐stable aggregates and that the soil on the furrow bottom had soil strength reduced from 1.7 to 1.1 MPa. This study showed that the potent erosion‐reducing ability of PAM could allow farmers to use higher inflow rates and still greatly reduce erosion. At the same time infiltration opportunity time diVerence from the upper to lower end of the field was greatly reduced, allowing for improved infiltration uniformity in the field. In this study ‘‘cut back’’ irrigation was used, whereby inflows from all treatments were reduced to a low flow‐sustaining rate of 19 liter min1 once runoV began. Another irrigation practice often used to reduce advance time is surge irrigation, sometimes called interrupted flow irrigation. The advantage of surge irrigation is that an intentional depositional seal is laid down in the furrow during the first surge of water across the field, and the brief interruption of flow stabilizes the seal through matric potential eVects (Kemper et al., 1988; Trout, 1991). The high flow rate used to form the initial depositional seal, however, is often at the expense of excessive erosion in the upper reaches of the furrow. Sirjacobs et al. (2000) studied the eVects of combining PAM treatments with flow interruption on 0.5‐m long 0.05‐m wide 0.12‐m deep miniflumes on a silt loam Alfisol and a clay Vertisol. They found that PAM reduced soil loss in all instances, but that changes in infiltration rate varied with the soil, irrigation, and PAM treatment. PAM reduced infiltration rate in the Alfisol and increased it in the Vertisol. In the Alfisol, interrupted flow with PAM reduced infiltration rate by 37% versus 18%
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without PAM. In the Vertisol, PAM had no eVect and interrupted flow had only slight eVects. They explained the reduced infiltration rate with PAM in the interrupted flow treatment as due to pore blocking by the PAM, leading to greater suction and compaction of the soil surface. Because surge irrigation is a process very much dependent on large‐scale field dynamics (diVering eVects in the upper scoured area of the field vs lower depositional areas), and since clay cracking eVects cannot be well represented spatially in such small flumes, it is not certain how well these results reflect actual field irrigation. Kornecki et al. (2005) used PAM applied to sugarcane field ‘‘quarter drains’’ in Louisiana to control erosion and transport of sediments to riparian waters following heavy rainfall. Water entering the quarter drains from sugarcane fields is somewhat like the inflow of water into furrows in furrow‐irrigated fields, except the quarter drains would be wet by the time runoV began entering them. They spray‐applied PAM at 18 kg ha1 following formation of the quarter drains and monitored erosion over the course of six rainfall events totaling 368 mm. The treatment was highly successful, reducing erosion by 88% in the first three rainfall events (161 mm cumulative total) and by 76% for the six events. Significant advantage was also seen in elimination of the need to reshape the drains following severe rainfall events. Similarly, Peterson et al. (2003) looked at the eVects of PAM treatment on erosion in concentrated flow channels prior to vegetation. PAM‐treated channels had 93–98% reduction in erosion compared to untreated channels. Channel incision depth with PAM treatment was not significantly diVerent from controls but incision width was significantly greater, and the rate of headcut movement varied from less than 0.6 m h1 in PAM‐treated channels to over 17.8 m h1 in the controls. Research showing the prevegetative stabilization of concentrated flow areas with PAM is further encouragement to research showing that PAM stabilization of soil can enhance grass emergence (Rubio et al., 1989, 1990, 1992). Sojka et al. (2003) showed that loss of weed seed from irrigated furrows was greatly reduced by PAM use; this is a further indication that PAM can benefit seed retention during the establishment phase of hydroseeded surface drainage control features. Others have shown increased emergence of fragile seedlings where PAM has been applied to reduce crusting eVects (Chan and Sivapalan, 1996; Helalia and Letey, 1989).
IX. SPRINKLER IRRIGATION Although the interest in PAM use for furrow irrigation has been phenomenal and dominated the initial technology development and practice adoption, there is as much and perhaps more interest in developing the
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technology for use in sprinkler irrigation. Farmers who irrigate with sprinklers are familiar with concepts of chemigation and generally have greater expectations for precision application of inputs, including water application. Because PAM reduces structural degradation caused by droplet impact, potential exists for reduced depositional sealing, with less runoV or runon, and hence more uniform infiltration. There is also the potential for improved stand establishment through reduction in crusting and reduced ponding. RunoV and erosion increase with increasing water drop energy. PAM, however, limits aggregate disruption caused by water drop impacts. Smith et al. (1990) and Levin et al. (1991) reported that the relative eVect of PAM on aggregate stability increased with increasing kinetic energy of the water drops. This may be related to the finding that only the outer surfaces of aggregates are stabilized by PAM treatment due to the relatively shallow penetration of the macromolecule (Ben-Hur and Keren, 1997; Malik and Letey, 1991). Thus, the value of PAM for erosion control in sprinkler irrigation is greater in systems that have high application rates, which typically means larger droplet sizes and greater droplet energy. Very few papers have been published that report results of PAM application through the sprinkler system itself, especially in field scale studies. Most reports involve pretreatment of soil with PAM sprays or powders, and in this respect are closely related to rainfall simulator work targeted to rainfed situations. Shainberg et al. (1990, 1992) applied three rates of PAM on dry soil in a small‐ tray laboratory study prior to sprinkling in a rainfall simulator and reported that 20 kg ha1 PAM best maintained high‐infiltration rates. Smith et al. (1990) and Levin et al. (1991), in similar studies, found that 20 kg ha1 of PAM increased infiltration and greatly reduced runoV and erosion. Peterson et al. (2001, 2002) also found in small‐tray studies that PAM sprayed on soil prior to simulated rainfall reduced runoV and erosion. Cochrane et al. (2005) using a rainfall simulator on steep (8–12% slopes) coarse‐textured tropical Alfisols found an average of 90% erosion reduction and 35% runoV reduction using a series of PAM‐based soil amendment treatments in which PAM was sprayed before rainfall at 20 kg ha1. Kim et al. (2001) spray‐applied PAM at 20 or 40 kg ha1on plots in vegetable fields in Korea which were then watered at 80 mm h1 with a rainfall simulator three times for 30 min each at diVerent times in the season. PAM reduced runoV and erosion significantly during the growing season but was no longer eVective after harvest, probably as a result of soil disturbance during harvest. Ben-Hur et al. (1989) concluded from a small‐tray laboratory study that applying 5 kg ha1 PAM with simulated irrigation water was more eVective in maintaining high infiltration rates than was spraying the polymer on the dry soil surface prior to simulated irrigation. Levy et al. (1992) found that applying PAM at 10 mg liter1 to irrigation water in a small‐tray laboratory study gave optimal eVect on final infiltration rate and cumulative infiltration
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as well as on reducing erosion. Flanagan et al. (1997a,b) applied 10 mg liter1 of PAM to tap water in simulated rainfall studies and found increased water infiltration, which they attributed to reduced surface sealing in the PAM treatment. They reported increased sediment concentrations from the PAM‐treated runs compared to controls, but it was not clear how total sediment loss (concentration times runoV volume) was aVected. Surface sealing, crusting, runoV, and erosion have been reduced in field plot studies by spraying PAM on dry soil prior to sprinkler irrigation (Ben-Hur, 1994; Levy et al., 1991; Zhang and Miller, 1996; Zhang et al., 1998). Levy et al. (1991) and Ben-Hur (1994) reported the eVects of 20 kg ha1 of PAM spray‐ applied to small field plots that were subsequently irrigated with a moving sprinkler irrigation system. They saw reduced runoV, reduced erosion, and increased soil water profile uniformity. EVects were greatest on bare soil and greatly diminished after three or four irrigations. Stern et al. (1991, 1992) sprayed dry soil with 20 kg ha1 PAM before sprinkler irrigation and saw increased wheat (Triticum aestivum L.) yields where PAM had been applied. They attributed the yield increases to improved water distribution and increased irrigation water use eYciency. Ben-Hur (2001) reported results from a field study with 3 20 m2 treatment plots and 3.5 m2 runoV plots on which a series of emitters and PAM application rates were compared. The PAM used was a relatively low molecular weight nonionic formulation. In that experiment PAM improved infiltration and reduced runoV and erosion with increased eYcacy for application rates up to 10 kg ha1, but showed no eVect on potato (Solanum tuberosum L.) yield. As in other studies, as the season progressed, the eVectiveness of the initial PAM application declined. While pretreating soil with PAM solutions or powder mixtures can be eVective for erosion control and runoV reduction, they are not always easily executed on a field scale or production agricultural basis because of the bulk and viscosity of solutions and the diYculty of routinely achieving even surface distribution of granular‐ or powder‐based PAM amendments. Sprinkler irrigation, particularly center‐pivot irrigation, has the potential to overcome these obstacles through chemigation. Ben-Hur et al. (1989) found that the equivalent of 5 kg PAM ha1 applied via water during rainfall simulations on small trays of soil prevented crust formation better than spraying the same amount of PAM on the soil surface. Flanagan et al. (1997a) reported that applying PAM at 10 ppm with simulated rainfall increased final infiltration rate compared to the untreated control, whereas 20 kg PAM ha1 applied directly to the soil did not. Levy et al. (1992) applied 3, 6, or 12 kg PAM ha1 with irrigation water for three consecutive irrigations on small trays. PAM increased final infiltration rate during treated irrigations, but final infiltration rates decreased to control values after irrigating twice more with only water.
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A series of experiments (Aase et al., 1998; Bjorneberg and Aase, 2000; Bjorneberg et al., 2000a,b) were conducted in Kimberly, Idaho in a sprinkler irrigation simulator using soil boxes 1.5‐m long 1.2‐m wide 0.2‐m deep on a 6.5% slope. The soil used was a Rad silt loam (Durinodic Xeric Haplocambid). In these experiments PAM was injected in the irrigation water using application scenarios that center‐pivot farmers already familiar with chemigation procedures had indicated might be reasonable for use in commercial operations. In the initial study, PAM rates of 2–4 kg ha1 applied only on the initial irrigation reduced runoV 70% and soil loss 75% compared to controls (Aase et al., 1998). By the third irrigation runoV reduction in the PAM treatments was only 20% and soil loss reduction 40%, indicating that, as has been seen in other studies, gradual eVects of droplet impacts began disrupting surface structure and forming a seal. The gradual seal formation in PAM treatments was dependent on PAM application protocol. The presence of seals was determined through measurement of infiltration under 40 and 100 mm of tension. When the total PAM application was applied in the first 8 of 20 mm sprinkled water, seal diVerences were detectable when measured following the third irrigation. However, if the same amount of PAM was applied at greater dilution in the full 20 mm of sprinkled water, there was no longer a detectable seal diVerence between PAM treatments and controls. This indicated that treatment eVects on runoV and erosion were predominately in the initial irrigation but did not persist in subsequent irrigations. Using the 8‐mm PAM application protocol, the infiltration under 40‐mm tension increased from 14 mm h1 on the check treatment to 29 mm h1 for the 4 kg ha1 application rate and increased from 9 to 17 mm h1 under 100‐mm tension, when measured following the third 20‐mm irrigation. Flanagan et al. (1997a,b) reported increased infiltration when rainfall simulator water contained 10‐ppm PAM; they also attributed this to reduced surface sealing although direct measurements of seal properties were not made to verify the inference. The role of PAM in stabilizing surface structure and seal prevention was further verified by Aase et al. (1998). Aggregate stability measurements across treatments showed a statistically significant 23% increase in aggregate stability with PAM treatment in the initial irrigation, but no statistical diVerence in aggregate stability in the third irrigation. Ben-Hur and Keren (1997), Levin et al. (1991), Aase et al. (1998), and Smith et al. (1990) all reported improved aggregate stability from sprinkler‐applied PAM, leading to decreased runoV and erosion. Although soil loss reductions in the Aase et al. (1998) sprinkler experiment were slightly less than is achievable for PAM use in furrow irrigation, it should be noted that properly engineered and managed sprinkler irrigation is already an eVective erosion‐limiting practice compared to most types of
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surface irrigation, and a 75% reduction of soil loss in this instance represents a conservation gain beyond that already expected from a sprinkler system. The results also underscore the observations that eVectiveness of sprinkler‐ applied PAM is more variable than for furrow irrigation because of application strategies and system variables that aVect water drop energy, the rate of water and PAM delivery, and possible application‐timing scenarios (Aase et al., 1998; Levin et al., 1991; Smith et al., 1990). In a second experiment, Bjorneberg and Aase (2000) noted that greater erosion control was achieved by applying PAM over several sprinkler irrigations rather than applying all the PAM in the initial irrigation. Water was applied at 80 mm h1 for 10 min (13‐mm application) for four irrigations. PAM was either applied at 3 kg ha1 in the irrigation water once in the first irrigation or at 1 kg ha1 in each of the first three irrigations. Both PAM treatment scenarios were followed by a fourth nontreated 13‐mm irrigation and were compared with controls of four nontreated 13‐mm irrigations. All PAM treatments significantly reduced runoV and erosion compared to the controls for all four irrigations. The multiple PAM treatment, however, reduced runoV 30% more than the single application during the last two irrigations. The single PAM application reduced cumulative soil loss 60% compared to the control, whereas the multiple PAM application reduced cumulative soil loss 80% compared to the control. Splitting the PAM application increased both the eVectiveness and the duration of the erosion‐ reducing and infiltration‐enhancing eVect. At the end of the irrigation series, the percentages of stable aggregates of the single and multiple PAM application treatments were 80% and 85%, respectively, compared to 66% in the controls. A column study by Gardiner and Sun (2002) saw similar results for splitting PAM applications. A third study (Bjorneberg et al., 2000a,b) evaluated the relative eVectiveness of PAM and straw cover compared to untreated bare soil for erosion control with sprinkler irrigation. Where 4 kg ha1 PAM was applied via sprinkler irrigation only in the first of three 20‐mm irrigations (80 mm h1 rate at 25 J kg1), a 30% straw cover treatment matched PAM treatment erosion and runoV eVects. A 70% straw cover treatment had slightly greater erosion reduction and infiltration increase. Erosion, runoV, and P loss were controlled as well or slightly better than any separate PAM or straw treatment if PAM and straw were combined. If PAM application was split among irrigations, erosion and infiltration eVects of PAM were much more persistent compared to a single application in the first irrigation. In summary, PAM eVects under sprinkler irrigation can be more transitory and less predictable than under furrow irrigation, depending on application amounts and protocols. Slightly higher seasonal field application totals for eYcacy may be required in sprinkler application of PAM compared to furrow application in some cases. Sprinklers must stabilize two to
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three times more surface area than furrow streams, especially early in the season, and must also protect against water drop energy eVects. Despite higher rates and technical challenges for provision of user‐friendly PAM formulations allowing controlled injection, farmers with problems such as damping oV or N loss, stemming from sprinkler infiltration nonuniformity, leading to runoV or runon, have begun to apply PAM with sprinklers. These problems are common on variable or steep slopes or in high‐application areas at outer reaches of center pivots. Because of the larger scale necessary to run reasonable agronomic comparisons of PAM treatments under center pivots and other forms of production sprinkler irrigation, where PAM is injected via the system, some uncertainty remains as to seasonal costs and benefits for specific crops. This is particularly true given diVerences in season length and canopy eVects. Many farmers have noted that, with PAM use, yield often improves from zero or near‐zero on upslope field areas or in areas with localized steep slopes, to near‐normal yields when using PAM in either furrow‐ or sprinkler‐irrigated systems. This is because in steep areas untreated water runs oV so readily that insuYcient infiltration occurs to allow plant growth and production (Shock et al., 1988). This kind of production result and anecdotal observation is diYcult to quantify systematically in normal field plot studies but is a very real eVect that often greatly outweighs the more moderate eVects reported from controlled studies in uniform experimental plots. A farmer invests inputs in these field areas, but receives no return from them. When PAM allows infiltration to occur in these areas the eVect on farmer economic return can be substantial. The more extensive the occurrence of such areas in a farmer’s field the greater the benefit from PAM use.
X. INFILTRATION As noted in the prior section on erosion, many papers reporting erosion control with PAM application also reported increases in infiltration. We focus here on papers that were seminal in this area, that produced specific new insights on the infiltration process as aVected by PAM application, or that were from field scale studies, which may provide better insight to real world deployment of the technology. The papers that first investigated infiltration eVects in which PAM application was through the irrigation system were Paganyas et al. (1972), Paganyas (1975), and Mitchell (1986). The reports by Paganyas and coworkers covered a range of application concentrations and amounts, and as stated previously, may have been a polymer other than PAM. Nonetheless, they provided coarse estimates of runoV reduction and infiltration increases,
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as well as yield improvement of several crops. Mitchell reported an initial increase in infiltration with PAM treatment which did not persist to the end of an irrigation. The PAM rates used were 6.6, 13.3, and 32.2 kg ha1 applied as liquid concentrations of 25, 50, and 150 mg liter1 in irrigation flows. Mitchell estimated the material application rates were three times the calculated per hectare calculations since application was only in the actual furrow, and nonfurrow areas were not treated. PAM applied as an evenly dispersed powder at 42 kg ha1 had no eVect on infiltration. Mitchell speculated that the swell of wet subsoil overrode any PAM infiltration‐enhancing properties. Shainberg and Levy (1994) provided an excellent review of the concept of hydraulic conductivity‐reducing surface seals, their role governing infiltration, and the history of polymer amendment of seals (particularly with PAMs or polysaccharides) through the late 1980s. Cultivated soils are structurally unstable and form a seal at the soil surface when exposed to rain or flowing water. The formation of a seal limits the water infiltration rate and influences runoV and erosion. A seal is a thin layer of oriented soil particles with low porosity, often high in clay, located at the soil’s surface. Numerous early papers reported increased infiltration with dilute or low‐rate PAM application to the soil surface, whether applied as a pretreatment dusting, low‐volume aqueous spray, or in the irrigation water, or simulated rain water (Agassi and Ben-Hur, 1992; Agassi et al., 1981; Ben-Hur and Letey, 1989; Ben-Hur et al., 1989; Bryan, 1992; Fox and Bryan, 1992; Gabriels, 1990; Gabriels et al., 1973; Helalia and Letey, 1988a,b; Levin et al., 1991; Levy et al., 1992; Shainberg et al., 1990; Shaviv et al., 1986, 1987; Smith et al., 1990; Wallace and Wallace, 1986a,b). Soluble PAM was identified as a highly eVective erosion‐preventing and infiltration‐enhancing polymer when applied in irrigation water at the rate of 1–10 ppm (Lentz and Sojka, 1994). PAM achieves this result by stabilizing soil surface structure and pore continuity against the eVects of rapid wetting and flow shear that otherwise promoted detachment and transport of soil, leading to erosion and formation of infiltration‐reducing seals. As noted in much of the literature already reviewed in this chapter, very low application rates or dilute solutions of PAM stabilize soil surface structure, preventing or limiting formation of surface seals that reduce surface hydraulic conductivity. On irrigation or rainfall, this phenomenon results in better maintenance of initial infiltration rates compared to untreated soils (often reported as ‘‘increased’’ infiltration). The eVect is concentration and application rate dependent. It is also aVected by soil properties, including the structural condition of the soil at the time PAM is applied as well as being aVected by PAM formulation, molecular configuration, concentration, and application methodology (Lentz, 2003; Sojka et al., 1998a). The upper limit of ‘‘low application rate’’ and ‘‘dilute’’ cannot be precisely set; however, there is a large body of literature that reports infiltration
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increases for PAM concentrations in irrigation water of 20 ppm or less. Similarly, where PAM is applied other than through irrigation water, PAM application rates associated with infiltration increases are usually 20 kg ha1 or less. The choice of these limits is supported by recent research (Lentz, 2003). PAM maintains higher infiltration rates in treated soils primarily by stabilizing soil structure, preventing the breakdown of soil aggregates, and reducing dispersion of waterborne soil particles, leading to formation of infiltration‐inhibiting surface seals. If the soil structure has already been destroyed or if the soil is sandy (thus composed mainly of primary particles and lacks soil structure resulting from the aggregation of fines), PAM treatment may reduce rather than increase infiltration. Since soil structure degradation has little influence on sealing under these conditions, PAM’s tendency to increase viscosity of the infiltrating water becomes the dominant phenomenon. Increasing soil solution viscosity results in reduced infiltration (Malik and Letey, 1992). Lentz (2003) demonstrated that when 1‐ to 20‐ppm PAM was dissolved in the first irrigation water applied to structured soils containing dispersible fines, it increased infiltration rates relative to untreated soils but at higher concentrations it decreased infiltration. Whereas, if the soils were pretreated with PAM solutions and allowed to dry before irrigation, PAM treatment did not inhibit infiltration into silt loam soils until the PAM concentration in the pretreatment solutions exceeded 500 ppm. Furrow irrigation stream advance is usually slower when using PAM for erosion control, especially for the first irrigation on newly formed or cultivated furrows (Lentz et al., 1992; Sojka et al., 1998a,b; Trout et al., 1995). Advance rate depends on inflow rate, slope, and infiltration rate. Advance of PAM-treated furrows slows because of the eVect of PAM on soil structure, which in turn aVects infiltration rate. Surface seals form on untreated furrow bottoms due to the destruction of soil aggregates with rapid wetting, and the detachment, transport, and redeposition of fine sediments in the furrow stream. This seal formation process blocks most of the pores at the soil surface, reducing the infiltration rate. For equal inflows, net infiltration on freshly formed PAM-treated furrows in silt loam soils is typically 15% more, compared to untreated water. Net infiltration usually increases more with PAM-treated irrigation on finer textured soils because the relative eVects of sealing are greater with a greater abundance of clay available for dispersion, transport, and redeposition into pores. Pore continuity is maintained when aggregates are stabilized by PAM. Ross et al. (1996) and Sojka et al. (1998a) reported that infiltration at 40‐mm tension varied among irrigations over the range 12.9–31.8 mm h1 for controls and 26.7–52.2 mm h1 for PAM‐treated furrows, and that infiltration at 100‐mm tension varied from 12.3 to 29.1 mm h1 for controls and 22.3 to 42.4 mm h1 for PAM‐treated furrows.
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PAM infiltration eVects are a balance between prevention of surface sealing and apparent viscosity increases in soil pores. Bjorneberg (1998) reported that in tube diameters >10 mm, PAM solution eVects on viscosity are negligible at 15 and 30 C. Macropore viscosity rose sharply only after PAM exceeded 400 ppm. In small soil pores, ‘‘apparent viscosity’’ increases greatly, however, even at the dilute PAM concentrations used for erosion control (Malik and Letey, 1992). The more significant eVect in medium to fine‐textured soils is the maintenance of pore continuity achieved by aggregate stabilization and prevention of surface sealing. In coarse‐textured soils (sands), where little pore continuity enhancement is achieved with PAM, there have been reports of no infiltration eVect or even slight infiltration decreases, particularly at concentrations above 20 ppm (Sojka et al., 1998a; Trout and Ajwa, 2001). For furrows formed on wheel tracks, the increase of infiltration often seen with PAM did not last as long as on nontraYcked furrows (Sojka et al., 1998b). They postulated that in wheel traYcked furrows reduced surface sealing with PAM improves infiltration only until repeated wetting and drying begins to disrupt subsurface aggregates and/or deliver enough surface‐derived fines to seal the few remaining subsurface pores that are already partially reduced by compaction. On further consideration, it may have been that the repeated PAM applications in undisturbed wheel track furrows, which had greater pore restrictions to begin with due to compaction, eventually had some pore blockage from the PAM itself. Gardiner and Sun (2002) used PAM rates of 0, 10, 25, and 40 ppm in deionized tap water or in water from a municipal wastewater treatment facility in a column study that tracked saturated hydraulic conductivity for a single PAM application over seven irrigations versus alternate PAM no‐PAM irrigations. In all single‐application PAM treatments, soil hydraulic conductivity was increased for the initial 2–3 weeks of column irrigation but fell to control values thereafter, whereas treating with PAM in alternate irrigations maintained higher hydraulic conductivities throughout the study, regardless of water quality. Gardner and Sun (2002) noted the reduction of infiltration rate at higher PAM concentrations and expressed concern over the cumulative eVect of high application rates on long‐term infiltration rate and hydraulic conductivity. Trout and Ajwa (2001) saw an absence of response or minor reductions in infiltration rate with PAM use in a 2‐year furrow irrigation study on a Hanford sandy loam (Typic Durixeralf), a coarse‐textured San Joaquin Valley soil. Their conclusion was that if there is not an abundance of disruptable aggregates in a soil, then the balance of PAM eVects shifts to greater expression of the ‘‘apparent viscosity’’ eVect proposed by Malik and Letey (1992) and Letey (1996). Falatah et al. (1999) also saw reduced infiltration with
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use of PAM on a 90% sand soil in Saudi Arabia, which related to increased PAM concentration and measurements of PAM viscosity. Because PAM prevents erosion of furrow bottoms and sealing of the wetted perimeter, lateral water movement increases about 25% in silt loam soils compared to nontreated furrows (Lentz and Sojka, 1994; Lentz et al., 1992). This increased lateral movement of water results from several factors. One reason is simply because the furrow is not eroding, thus the water level in the furrow maintains a higher elevation. To the extent that the wetting pattern has shoulders, these shoulders are nearer the soil surface. The pores along the side of the furrow (upper portions of the wetted perimeter) are stabilized, just as the pores at the bottom of the furrow, thus the restriction to water movement along potential gradients in the lateral direction are less if PAM is present to preserve pore continuity. Also, to the extent that apparent viscosity is a factor in PAM movement through soil pores, the downward movement of water may be slightly more impeded, again, tending to broaden the shoulders of the wetting pattern. Increased lateral wetting can be a significant water‐conserving eVect for early irrigations, where the intent is only to germinate seeds or to provide water to seedlings that are still evapotranspiring smaller daily amounts of water. Since most farmers only irrigate alternate furrows, the early irrigations need only apply enough water for the wetting front to reach seeds or seedling roots; they do not need to thoroughly wet the entire rooting depth or wet beyond the row middle for water storage. These goals are achieved with less total water per hectare if lateral movement of water is favored. Falatah et al. (1999) used concentrations of 10–50 ppm of three diVerent water‐soluble PAMs. They saw decreased depth of wetting and greater lateral movement of water that corresponded to the increases in concentration and resultant viscosity changes of the polymers they tested. The calcareous soil (Typic Torripsamment) contained 90% sand, 5% silt, and 5% clay. The eVects also occurred with decreases in infiltration rate in the PAM‐ treated soils. The authors concluded that the PAMs could help prevent deep percolation losses in this sandy soil, but were concerned that the reduced infiltration reduced the usefulness of water‐soluble PAMs for water conservation. However, in other areas of the world, this specific pairing of properties has been interpreted as a way in sandy soils (where infiltration rate is rarely a problem) to slow the loss of water, making it and the nutrients carried in the water available for a longer time by the crop root system (Shane Phillips, Adelaide University, Australia, personal communication). In Australia, water‐soluble PAMs are being marketed for use in center pivots on coarse sands as a water and nutrient conservation tool. Levy and Rapp (1999) reported that water‐soluble PAM, applied to the surface of a silt loam soil, reduced the loss of water during the drying process.
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With further verification this could be an important new direction for PAM research in coarse‐textured soils and other water‐limiting situations. PAM use in both furrow and sprinkler irrigation increased infiltration and reduced erosion in Portugal (Bjorneberg et al., 2003; Santos and Serralheiro, 2000; Santos et al., 2003). Furrow infiltration was characterized in 100‐m furrows with recirculating infiltrometers, double‐ring infiltrometers for saturated infiltration, as well as tension infiltrometers. Net infiltration improved 20% and 14% for continuous application of 1 and 10 ppm advance‐only application. Tension infiltration of controls was 19.4 and 15.8 mm min1 for 40‐ and 100‐mm tension values, respectively, compared to 36.6 and 30.1 mm min1 for the continuous 1‐ppm PAM treatment and 28.0 and 20.2 mm min1 for the 10‐ppm advance‐only treatment. Double‐ring infiltration rates were 53% greater than controls on the final irrigation with the continuous 1‐ppm PAM treatment and 60% greater with the advance‐ only treatment. Surface hydraulic conductivities of the control treatment were 14.4, 8.8, and 11.8 cm h1 for 0‐, 40‐, and 100‐mm tension values, respectively, compared to 67.5, 24.1, and 42.4 cm h1 for the continuous 1‐ppm PAM treatment and 52.4, 15.3, and 32.4 cm h1 for the 10‐ppm PAM at advance‐only treatment. PAM’s erosion prevention properties can permit farmers who furrow irrigate to improve field infiltration uniformity through altered inflow management. This can be done by increasing inflow rates two‐ to threefolds (compared to normal practices), thereby reducing infiltration opportunity time diVerences between inflow and outflow ends of furrows (Sojka and Lentz, 1997; Sojka et al., 1998b). When runoV begins, the higher initial inflow must be reduced to a flow rate that just sustains the furrow stream at the outflow end of the field. Initial field observations suggest that coupling PAM with surge flow irrigation can be a beneficial practice (Bjorneberg and Sojka, unpublished data). With PAM in the water, there is still enough reconsolidation of the furrow surface for surges to accelerate advance. However, the upper‐field scouring associated with doubled flows (as is common when surge valves are used) does not occur. It is generally accepted that lower molecular weight polymers (0.1–5.0 Mg mol1) can be more eVective at stabilizing infiltration than higher molecular weight polymers because they penetrate soil pores more readily and provide structural stabilization to a greater depth, better preserving soil hydraulic properties. However, lower molecular weight polymers are usually less eVective for erosion control. Thus the choice of polymer can be a compromise dependent on which management factor is of greater concern. Because anionic PAMs of 12–15 Mg mol1 have so many safety and eYcacy attributes favoring them for multiple environmental uses, this class of PAMs has become the standard and the center of discussion for PAM use in erosion and infiltration control since the early 1990s.
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As noted in the opening discussion about infiltration, PAM’s eVect on structure stabilization is a key element in surface seal prevention and increasing infiltration. However, it should be emphasized that PAM does not improve soil structure. It merely stabilizes the existing structure it encounters during application. Several researchers have noted that application of PAM for erosion control works best when applied to soil with newly prepared, aggregated soil surfaces (Cook and Nelson, 1986; Lentz and Sojka, 1994; Shaviv et al., 1987a,b; Sojka and Lentz, 1997). In this manner the PAM applied stabilizes structure that is both conducive to infiltration and preserves surface roughness to resist shear in the presence of flowing water. Sojka et al. (1998b) found increased aggregate stability within the entire wetted perimeter of PAM-treated irrigation furrows in all PAM treatments, which also had increased infiltration and reduced erosion. Several papers have been published indicating that PAM adsorption is relatively shallow in soil and that perhaps only the outsides of aggregates adsorb PAM, so that the applied PAM does not reach aggregate interiors (Malik and Letey, 1991; Malik et al., 1991b; Nadler and Letey, 1989; Nadler et al., 1994, 1996). Work by Miller et al. (1998) and Levy and Miller (1999) indicate, however, that this may not be universally the case. Their work with Worsham sandy loam (Typic Ochraquults) and Cecil sandy clay (Typic Hapludults), soils with predominately 1:1 clay minerals, concluded that PAM penetrated and stabilized the interiors of even relatively large aggregates (6–8 mm) and increased the percentage of large stable aggregates. They concluded that PAM was a more eVective structure stabilizer in light‐ to medium‐textured soils, where the addition of 10–20 kg ha1 tripled the fraction of water‐stable aggregates. As with many other aspects of PAM performance, the evidence suggests that PAM properties and solution concentrations may interact diVerently with varying soil properties, aVecting the degree of PAM penetration into aggregates. Although many papers have reported that PAM application ‘‘prevents’’ seal formation, it is probably more accurate to state that PAM ‘‘changes’’ seal formation. Extensive work from Kimberly, Idaho, applying PAM through both furrow‐ and sprinkler‐irrigated systems, has repeatedly noted that soil surface structure changes with irrigation, both with and without the application of PAM in the irrigation water. Surface seals form in both cases. The diVerence is that in the PAM‐treated systems, the seals formed are typically more porous and thinner than the seals formed by untreated water (Fig. 4). Soil in PAM‐treated furrows is held in place against shear forces of flowing water by the thin web of polymer that coats particle surfaces at the soil water interface, maintaining better pore continuity to the soil below (Fig. 5). Finally, although we state rather unequivocally that PAM does not create structure, but rather it only stabilizes the structure it encounters on application, there is a minor exception. PAM does create
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Figure 4 EVects of furrow irrigation water treatment on soil surface structure in a Portneuf silt loam soil. Top left: Turbid water carrying sediment flowing in an untreated furrow. Top right: Slick impervious surface seal left after flow ceases in untreated control furrows. Bottom left: Clear sediment free water flowing in a PAM‐treated furrow. Bottom right: Rough porous surface left after flow ceases in a PAM‐treated furrow.
structure to the extent that, in turbid water, it creates floccules which settle out of suspension onto the furrow’s (or other water body’s) soil surface. These floccules tend to be loose open structures that do not restrict water
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Figure 5 Scanning electron micrographs of untreated soil particles (left) and PAM‐treated soil particles (right) showing strandlike PAM filaments coating and binding soil particles (Sojka et al., 2005).
entry, and which do not seal surfaces in the manner of dispersed clays or other unconsolidated fines.
XI.
PAM SAFETY, FIELD RETENTION, AND ENVIRONMENTAL IMPACTS
At this time PAM is not regulated under the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), however, it is regarded as a macropollutant with low toxicity and side eVects. As noted earlier, PAM has been used for many decades ubiquitously in a number of food, environmental, and other sensitive applications, often involving significant disposal or release to the environment. Some safety and environmental cautions are warranted, but the low toxicity of PAMs in general, especially large molecular weight anionic PAMs, means that if used according to prescribed guidelines risks to human and environmental health are small. The greatest concern surrounding PAM use is generally a concern stemming not from PAM itself, but rather from the presence of unreacted residual AMD as a product contaminant. AMD is a neurotoxin and a suspected carcinogen in humans and animals (Garland and Patterson, 1967; WHO, 1985). It has not been shown to cause mutations in bacteria but has been shown to cause chromosome damage to
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mammalian cells in vitro and in vivo (Bull et al., 1984; Shiraishi, 1978; WHO, 1985). AMD is readily absorbed by ingestion, inhalation, or dermal contact and is then moved freely through body fluids. Unlike PAM, AMD can cross membrane barriers. However, AMD is also easily metabolized and is largely excreted as metabolites in urine and bile, although fecal excretion is minimal in hours to a few days (Miller et al., 1982). AMD exposures have resulted in isolated human fatalities and temporary injury or impairment with ingestion or extensive exposure to concentrations of over 400‐ppm AMD (Garland and Patterson, 1967; Igisu et al., 1975). However, the exposure levels required to cause neurotoxic or carcinogenic eVects in humans are several orders of magnitude above those conceivable from exposures resulting from current environmental applications. National Institute of Occupational Safety and Health (NIOSH) guidelines recommended an exposure limit 0.03 mg m3 which is equivalent to 0.004 mg kg1day1 for an 8‐h work day (NIOSH, 1992), and for a 100‐kg human that would equal 0.4‐mg AMD, which is 80% of the allowable unreacted AMD in a kilogram of the anionic PAMs used for erosion control. PAM’s potent eYcacy for reducing runoV and erosion from treatment sites translates into substantial additional benefits oV site as well. Irrigation runoV ultimately reaches riparian waters in most instances. These waters are important ecologically, as drinking water sources and for recreational use, with potential for human exposure to or ingestion of contaminants. A substantial body of research documents water quality benefits of PAM use beyond reduction of runoV sediment per se. Among the components of water quality impairment that PAM use significantly mitigates are sediment, BOD, mineral nutrients, pesticides, weed seed, and pathogenic microorganisms. Furthermore, rigorous tests of PAM concentration downstream from PAM application sites have shown that, properly applied, PAM poses no serious risk of PAM loss. Where minor PAM losses occur, its strong surface attractive properties result in rapid PAM removal from water bodies through adsorption and flocculation of suspended solids within a few hundred meters of transport from an application site (Lentz et al., 2002). In most instances, the latter aspect is actually a continuation of PAM’s water quality improvement benefit occurring downstream when small losses from the application site occur. An important environmental and applicator safety consideration is the need to use PAMs that contain <0.05% AMD. AMD is a neurotoxin, but PAMs below these AMD contents are safe, when used as directed at low concentrations. In soil, PAM degrades at rates of at least 10% per year as a result of physical, chemical, biological, and photochemical processes and reactions (Azzam et al., 1983). Because PAM is highly susceptible to UV degradation, its breakdown rate when applied at the soil surface for erosion control may be faster than the 10% per year reported rate, which was for
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biological degradation of PAM mixed into a large soil volume. Possible indirect evidence of the accelerated breakdown of surface‐applied PAM is the gradual loss of treatment eVectiveness between irrigations on furrows receiving no additional PAM (Lentz et al., 1992). PAM does not revert to AMD on degradation (MacWilliams, 1978). Furthermore, AMD is easily metabolized by microorganisms in soil and biologically active waters, with a half‐life in tens of hours (Lande et al., 1979; Shanker et al., 1990). Bologna et al. (1999) and Barvenik et al. (1996) showed that AMD is not absorbed by plant tissues and apparently breaks down rapidly when exposed to living plant tissue. While anionic PAMs are safe if used as directed, prolonged overexposure can result in skin irritation and inflammation of mucus membranes. Users should read label cautions and take reasonable care not to breathe PAM dust and to avoid exposure to eyes and other mucus membranes. Another caution is that PAM spills become very slippery if wet. PAM application onto roadways should be avoided, and PAM spills should be thoroughly cleaned with a dry absorbent and removed before attempting to wash down with water. Practical user considerations are numerous. Labels, website information, and available extension information should be consulted before embarking on large‐scale use of PAM. Used at prescribed rates, anionic PAMs are environmentally safe. Although cationic and neutral PAMs have toxicities warranting caution or preclusion from sensitive environmental uses, NRCS specifies anionic PAMs for controlling irrigation‐induced erosion. Negative impacts have not been documented for aquatic macrofauna, edaphic microorganisms, or crop species for the anionic PAMs used for erosion control when applied at recommended concentrations and rates. Several studies examined the fate of PAM when applied to furrow irrigation inflows at 10 ppm during furrow advance. PAM was applied to inflowing irrigation water as an aqueous solution or granular material either by injection into a gated pipe (Lentz and Sojka, 1996a,b; Lentz et al., 2002) or soil‐lined water supply ditch (Stieber and Chapman‐Supkis, 1996) at the head of the field. The researchers measured dissolved PAM flux in treated inflows, furrow streams, and in tailwater ditches and calculated cumulative PAM losses. They determined that PAM concentrations in furrow streams decreased to undetectable levels within 30 min of stopping PAM applications and, because of PAM’s high aYnity for suspended sediments and soil in waste ditch streams, only 1–5% of the PAM applied left fields in runoV. Furthermore, for those early season, highly erosive irrigations most commonly treated with polymers, any PAM leaving the field in waste ditches only traveled 100–500 m before being completely adsorbed on sediments in the flow or onto ditch surfaces (Lentz et al., 2002). Ferguson (1997) reported on a watershed scale test of PAM, where over 1600 ha were irrigated using
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PAM‐treated water for a 2‐week period. On any given day, about half of the 40 farms in the study were contributing runoV to the watershed’s drainage, which collected in Conway Gulch, a tributary of the Boise River. Waste water from the fields and the drain was analyzed for P, sediment, and PAM. About half of the water in the drain was field runoV. PAM was not found detrimental to the drain’s water quality and was detected in drain water samples only twice (<0.8 ppm) during monitoring. PAM was found to be an eVective sediment control practice that was well adopted by farmers and did not negatively impact the drain. PAM water quality protection begins with the choice of an appropriate PAM formulation and product. Barvenik (1994) and Deskin (1996) summarized the safety considerations for use of PAMs in environmentally sensitive applications and as impacts human safety for exposure during material handling. Their summaries note that the broad class of PAM chemicals in general exhibit a low order of toxicity to mammals, with high acute LD50 by oral and dermal routes (>5 g kg1). They noted there were no significant adverse eVects in chronic oral toxicity studies, no compound‐ related reproductive lesions in a three‐generation study in rats, and only slight dermal and ocular irritation at high doses (Stephens, 1991). Human epidemiologic studies saw no association between unintentional occupational exposure to PAMs and tumors, which support the findings from chronic animal studies. Furthermore, the molecular size of these PAMs is too large to allow absorption via the gastrointestinal tract since the dimensions of the macromolecules preclude movement across biological membranes (Stephens, 1991). While nonionic and especially cationic PAM formulations pose some risk to aquatic organisms at low concentration (Biesinger and Stokes, 1986; Hamilton et al., 1994), the anionic formulations do not. Environmental toxicities of PAM and AMD have been published in a number of reports (King and Noss, 1989; Krautter et al., 1986; McCollister et al., 1964, 1965; Petersen et al., 1987; Shanker and Seth, 1986; Walker, 1991). Cationic PAMs have LC50 values of 0.3–10 ppm. Cationic PAMs bind to sites rich in hemoglobin such as fish gills, posing a barrier to oxygen diVusion rather than causing a systemic toxicity. However, the class of anionic PAMs specified by NRCS for use in soil erosion and infiltration management shows no measurable toxicity at concentrations up to 100 pm (i.e., LC50 > 100 ppm in deionized water). It is well established that values of PAM toxicity determined in deionized water indicate lower LC50 values than actually occur in natural waters because of the interferences of suspended sediments, humic substances, and other dissolved organic compounds normally present in natural waters (Buchholz, 1992; Goodrich et al., 1991; Hall and Mirenda, 1991). Dissolved humic substances have been shown to raise LC50 measurements for test
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organisms by an order of magnitude for as little as 5 ppm (Goodrich et al., 1991) and as much as two orders of magnitude for 60 ppm (Hall and Mirenda, 1991). Carey et al. (1987) and Biesinger et al. (1976) showed that the addition of organic C and bentonite clay also reduced polymer toxicity to test species. It should also be noted that the absence of a measurable LC50 for PAM at 100 ppm represents an order of magnitude safety margin for the highest concentration of PAM present during initial application on agricultural fields (10 ppm) following the NRCS application standard (most aquatic organisms carried in irrigation water onto a cropped field will not survive regardless of PAM concentration). Furthermore, since only 1–5% of applied PAM leaves field application sites, and is only active for a few 100 m from field tail ditches (Lentz et al., 2002), there are an additional two to three orders of magnitude concentration protection even if field runoV were to flow directly into a riparian body. Just as for human exposure, concern regarding the aquatic environment is not simply for PAM exposure but also for the more toxic AMD monomer, which is present in very small quantities in PAM formulations. US Environmental Protection Agency (USEPA, 1994) reported AMD LC50 values for several aquatic species. The 24‐, 48‐, and 96‐h flow‐through LC50 values for harlequin fish (Rasbora heteromorpha) were 460, 250, and 130 ppm, respectively. The 24‐ and 96‐h static LC50 values for goldfish (Carassius auratus) were 460 and 160 ppm, respectively. The 7‐day LC50 value for guppy (Poecilia reticulata) was 35 ppm. The 24‐h LC50 for water flea (Daphnia magna) was 230 ppm. These concentrations and exposure values are all several orders of magnitude above any conceivable exposure scenarios to AMD derived from PAM application for erosion of infiltration management, and probably for any other responsible and aVordable agricultural or environmental use of PAM. PAM used for erosion control has been shown in numerous studies to benefit runoV water quality not only by limiting the loss of sediment itself, but also nutrients carried on and released from eroded sediment. Lentz et al. (1996b) reported results from a study that applied 0.25–0.50 ppm of a nonionic PAM to furrow inflows during a single 24‐h irrigation. RunoV samples taken at 4 and 9 h were analyzed for P and showed little eVect of PAM treatment on ortho‐P but about a 25% reduction in total P. Lentz et al. (1998) compared treating furrow advance flow (only) with 10‐ppm PAM or continuously treating with 1‐ppm PAM throughout the irrigation. Significant water quality improvement compared to controls was seen in both cases. Sediment was reduced 89% and 92%, respectively, for 1 ppm continuous and 10 ppm advance dosing. Dissolved reactive phosphorus (DRP) and total P concentrations in control tailwaters were five to seven times that of PAM treatments and chemical oxygen demand (COD similar to BOD) of
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controls were four times those measured in the PAM treatments. Results in several reports from Kimberly, Idaho have confirmed the ability of PAM used in irrigation water to reduce the nutrient enrichment and general cation concentration of runoV water (Bjorneberg et al., 2000b; Entry and Sojka, 2003; Lentz et al., 2001a; Sojka et al., 2005). Vanotti et al. (1996) showed that PAM was also highly eVective at removing solids and nutrients from swine wastewaters. A common practice in many furrow‐irrigated areas where erosion is a problem is the use of settling ponds to remove sediment, nutrients, and other agrochemicals from tailwater. In the Lower Boise River Pollution Trading Project in southwest Idaho, the question arose whether PAM use and settling ponds gave an additive eVect on tailwater quality protection. Bjorneberg and Lentz (2005) found that in 3 years of study either PAM use or sediment ponds reduced sediment 86% and total P loss 66% but the combined eVect of PAM and sediment pond treatments reduced mass transport of sediment 95–99% and total P 86–98%. Neither PAM nor settling ponds had any appreciable eVect on DRP retention. The Imperial Valley of California is one of the most intensively farmed and economically important surface irrigated areas in the United States. In recent years, public concern has mounted for the impacts of return flows on algae growth and eutrophication in the Salton Sea. Goodson et al. (2006) conducted a series of tests using continuous application of 1‐, 5‐, or 10‐ppm PAM to irrigation water. The 1‐ppm PAM treatment reduced turbidity 74% and total suspended solids 82%. The loss of particulate borne P was reduced 48% but there was no reduction in the soluble fraction. The higher PAM applications improved both suspended solid retention and particulate borne P retention but resulted in some measurable PAM loss, which was seen as an unnecessary risk for the relatively small gains and the significant increased costs of dosing. Public concerns for water quality are probably more emphatic regarding pesticides than any other component of water pollution. When detached sediments are transported in runoV, their agitation and mixing while flowing in the runoV stream increases the potential for desorption of nutrients and pesticides. Reducing erosion helps prevent contamination of receiving waters with pesticides much as it helps prevent nutrient enrichment. Agassi et al. (1995) used miniature furrows in the laboratory to study the loss of the nonionic herbicide napropamide from Hanford sandy loam soil (Typic Xerorthent) in runoV. Irrigation flow rates were controlled among treatments and were either distilled or treated with 10‐ppm anionic PAM. Napropamide loss varied in direct proportion to sediment loss regardless of water treatment. Agassi et al. (1995) expressed caution in extrapolating the minifurrow results to field results, noting that in field situations, most of the water running along a furrow infiltrates to irrigate the field, and deposition
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of some of the fines occurs before loss in the tailwater. Thus, the proportional relationship between sediment loss and napropamide loss in the minifurrow might not be the same relationship that occurs in an actual field. Nonetheless, the study pointed toward the potential for PAM to help with pesticide sequestration. Singh et al. (1996) conducted a field study on a Capay clay soil (Typic Haploxererts) to study the eVects of PAM‐treated furrow irrigation on loss of the miticide Kelthane. Kelthane is regarded as slightly water soluble with a high soil sorption aYnity. The two irrigation treatments were a control and dosing with at 10‐ppm anionic PAM. PAM greatly reduced sediment loss and increased infiltration, with amounts varying with sampling times and irrigation dates. Kelthane loss was proportional to sediment loss. In two posttreatment irrigations, Kelthane loss was double in the untreated controls compared to the furrows that had only residual PAM eVects. Several studies in Idaho have shown reductions in pesticides in PAM‐ treated runoV. PAM‐treated furrow irrigation runoV was compared to controls for all forms of N, total and ortho‐P, and the pesticides terbufos, cycloate, EPTC, bromoxinil, chlorpyrifos, oxyfluorfen, trifluralin, and pendimethalin in two production sugarbeet fields and one onion field (Bahr and Steiber, 1996; Bahr et al., 1996). PAM was predissolved at 10 ppm in the inflow and applied only during inflow advance across the field. The PAM treatment reduced sediment losses up to 99% and N and P concentrations were reduced up to 86% and 79%, respectively. For sites where pesticides were detected in control runoV, PAM treatment greatly reduced the pesticide losses. Pesticide reductions were more variable than the nutrient reductions, and were related both to compound attributes and pesticide application protocols and timing. Nonetheless, the eVectiveness of PAM in reducing the losses was substantial. Watwood and Kay-Shoemake (2000) investigated the impact of PAM on the sorptive dynamics and degradation of 2,4‐dichlorophenoxyacetic acid (2,4‐D) and atrazine in soils where PAM had been used for erosion control for 5 years. In their study sorption of atrazine and 2,4‐D in soil was unaVected by PAM treatment, as was atrazine desorption. However, 2,4‐D desorbed slightly faster in PAM‐treated soil. Decarboxylation of the 2,4‐D carbolic acid side chain was significantly reduced in the PAM‐treated soil. Degradation of atrazine to CO2 or bound residue components was also reduced in PAM‐treated soils. The authors concluded that the modifications in fate of these two pesticides were not ‘‘dramatic’’ and that the interpretation could be seen as positive from a herbicide eYcacy perspective, but drew attention to an aspect of PAM–herbicide interaction that might warrant further research over a broad spectrum of additional herbicides. There has been great concern in Australia over runoV containing residues of the pesticide endosulfan. A series of on‐farm and laboratory studies were
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conducted in the late 1990s to determine the relative eVectiveness of conservation tillage practices and use of PAM for erosion control and prevention of endosulfan contamination of runoV (Hugo et al., 2000; Waters et al., 1999a,b). In surface irrigated situations PAM treatment was equally eVective as conservation tillage for controlling soil loss and endosulfan loss in runoV, on the order of 70%. Part of the benefit in both instances was recognized to be the infiltration enhancement of either practice. Where fields were subject to intermittent rainfall, residual PAM eVects (from the PAM applied during irrigation) did not always withstand the additional erosive force of raindrop impact. PAM was recognized as a potent tool for irrigated control of erosion control with a need recognized for either a combination approach or further research to improve the PAM methodology for better performance in intermittent rain situations. Oliver and Kookana (2006a,b) investigated the eVect of using PAM‐treated irrigation water in the Ord River irrigated region of Australia on pesticide losses. PAM reduced the loss of endosulfan, chlorothalonil, and bupirimate 54%, 49%, and 38%, respectively. The bupirimate loss was not statistically significant at the 5% level. Endosulfan and chlorothalonil are relatively insoluble whereas bupirimate is more soluble. The reduced eVectiveness for PAM at reducing soluble pollutants is not uncommon. Lu et al. (2002b,c) studied the eVects of anionic and cationic PAM on picloram and napropamide sorption and anionic PAM eVects on sorption and desorption of metolachlor, atrazine, 2,4‐D, and picloram. The authors concluded that, as might be expected, the charge make up of the polymers and the pesticides influenced the degree to which aYnity for sorption to soil was aVected. However, the degree of change in sorption was very minor and was mitigated by the presence of salts in soil and by the increased infiltration and reduced loss of sediment that occurred with PAM treatment. In view of the small amounts of polymers added to soil and the far greater impact on soil loss, the authors did not identify use of PAM as contributing meaningfully to field loss of pesticides. By far the most significant environmental eVect of PAM use for erosion reduction is its improvement of surface water quality. This is achieved through reduction of sediment and nutrient losses, and decrease of organic and agrochemical contaminants released from sediment in runoV.
XII. PAM EFFECT ON ORGANISMS IN RUNOFF AND SOIL As human population and the numbers of animals reared to feed the human population continue to increase, land disposal of animal and human wastes is becoming more widespread. This trend is also encouraged by the
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interest in some sectors for organically produced crops. When runoV occurs from fields amended with animal manures or municipal waste there is a significantly increased risk of pathogen contamination of receiving waters. Even in the general context of agricultural production, the movement of soilborne disease organisms in runoV is an important vector for the spread of crop diseases. PAM has proven to be as eVective at sequestering microorganisms in runoV as it is in sequestering sediment. Broad categories of microorganisms carried across and among furrow‐ irrigated fields by furrow streams, runoV, and return flows are reduced by PAM in irrigation water (Entry and Sojka, 1999; Sojka and Entry, 1999, 2000). Similar reductions occur for weed seed in runoV (Sojka et al., 2003). These findings point to potential improved management that may ultimately reduce pesticide use. Sojka and Entry (2000) examined runoV from furrow‐irrigated plots fed by a storage reservoir that frequently experienced algal blooms when the runoV‐enriched water warmed in summer months. Plots were either controls or treated with PAM patch application in the first meters of the furrow. Samples taken 40‐m down furrow from the PAM patch on three dates at three diVerent flow rates saw significant reduction in numbers of algae, numbers of active and total bacteria, active and total fungal length, and total bacterial and fungal biomass compared to the control treatment (Entry and Sojka, 2000; Entry et al., 2003). Reductions varied with flow rate, furrow‐sampling position, and organism and ranged from 0% to 20% for total bacteria sampled 1 m below the patch at the lowest flow rate to 100% removal of active and total fungi at the 40‐m sampling point and greatest flow rate. Higher flow rates and greater flow distance generally favored organism removal indicating that mixing and opportunity for exposure to PAM and furrow adsorption sites favored organism removal. Common removal rates for all organisms, flow rates, and sampling positions ranged from 50% to 90%. The results have immediate implications for phytosanitation and soilborne phytopathogen epidemiology. There are additional implications that can be extrapolated from the finding. If disease spread is reduced, then pesticide use can likely be reduced as well. The Sojka and Entry (2000) findings prompted immediate follow‐up to determine the potential for reducing migration of coliform bacteria from furrow‐irrigated fields with recent manure application. Entry and Sojka (2000, 2001) found that after water flowed over three manure sources and then PAM, PAM þ Al2(SO4)3, or PAM þ CaO in furrows, total coliform bacteria, fecal coliform bacteria, and fecal streptococci were reduced up to 99.9% in water flowing 1‐ and 27‐m downstream of the treatments compared to the control treatment. A similar study conducted in Australia confirmed the organism sequestration, findings but underscored the need for adequate exposure to PAM in the furrow system for optimal performance
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(Entry et al., 2003). For grassed systems, where the manure source at the head of the furrow was the only dosing point for organisms and PAM, 100 m of uncontaminated grassed furrow below the dosing point was as eVective at removing coliform organisms as PAM (Spackman et al., 2003). The latter study suggests that if coliform losses are a problem in runoV from heavily stocked pastures, the PAM application would likely be more eVective in the lower reaches of the pasture. As with microorganisms, the spread of weed seeds in fields can also occur in furrow irrigation. Where runoV is collected for reuse, the water can be a potent vector for inoculation of other fields. Sojka et al. (2003) conducted a 2‐year furrow‐irrigated field experiment that compared the eVects of predissolved or patch‐applied PAM on weed seed loss and weed growth dynamics in a corn (Zea mays L.) crop. As in previous studies erosion was greatly reduced and infiltration was increased with PAM use. PAM also reduced runoV loss of weed seeds 62–90%, including barnyard grass (Echinochloa crus‐galli), kochia (Kochia scoparia L. Schrad.), redroot pigweed (Amaranthus retroflexus L.), common lambsquarters (Chenopodium album L.), and hairy nightshade (Solanum sarrachoides L. Sendtner). Where PAM was used, the reduction in furrow erosion slightly favored the establishment of in‐furrow weed emergence, although weed vigor was greatly reduced by the herbicides. Nonetheless, the result suggested the need for greater attention to weed control at lay‐by when using PAM for erosion control in furrow irrigation, since the scouring of emerging weed seedlings, which normally occurs in eroding furrows, is prevented with PAM. The results were also seen as evidence that PAM used for erosion control as an additive to hydromulching can be expected to favor seed retention and seedling establishment. Corn yield was slightly increased in 1 year of the study with PAM use, which the authors attributed to the increased infiltration with PAM. Reports on the eVects of PAM on bacterial biomass in soils and waters have been mixed and sometimes conflicting (Kay-Shoemake et al., 1998a,b; Mourato and Gehr, 1983; Nadler and Steinberger, 1993; Steinberger and West, 1991; Steinberger et al., 1993). Larger populations of culturable heterotrophic bacteria were found by Kay-Shoemake et al. (1998a) in PAM‐treated soils planted to potatoes, but not if planted to beans. These observations and other from studies showing either increased or decreased bacterial numbers for PAM‐treated soils suggest that the eVects are site, season, and cultural practice specific and may interact with other important variables such as nutrient levels, crop cover type, or herbicide regimes. Bacterial enrichment cultures, derived from PAM‐treated field soils, were capable of growth with PAM as a sole N source but not sole C source, whereas AMD served as either a sole N or C source for bacterial growth (Kay-Shoemake et al., 1998b). Work by Grula et al. (1994) showed that PAMs are an N source for bacteria and stimulate the growth of a number of Pseudomonas sp.;
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only cationic formulations were toxic to the organisms they cultured using PAM concentrations under 0.2%. Kay-Shoemake et al. (1998a,c) described a unique PAM‐specific amidase that is apparently induced by PAM in soils. It breaks amide linkages found in PAM, releasing N4þ which is rapidly assimilated by bacteria during growth. In laboratory incubations, 20% of the N in the added PAM was removed within 120 h (Kay-Shoemake et al., 1998b). PAM‐specific amidase activity was documented in laboratory cultures and in field soil samples following exposure to PAM (Kay-Shoemake et al., 1998b). The enzyme seems to have a broad substrate range and exhibits activity against formamide and propionamide, but does not impact degradation rates of carbaryl, diphenamid, or naphthalene acetamide in PAM‐treated soils (Kay-Shoemake et al., 2000a,b). Intra‐ and extracellular activities were noted, and production and secretion of the enzyme seemed dependent on C availability, as cells cannot derive C rapidly enough from PAM as the sole C source to sustain cultures. Nitrification of added urea appeared somewhat accelerated (approximately 10% over 2 weeks) in PAM‐treated microcosm soils (Kay-Shoemake et al., 2000b), but no other significant impacts of PAM application on fertilizer fate were noted. Sorptive dynamics of the common pesticides, 2,4‐D and atrazine, were not dramatically altered by PAM treatment of field soil samples, but some slight changes in desorption and degradation rates were reported (Watwood and Kay-Shoemake, 2000). Kay-Shoemake and Watwood (1996) and Kay-Shoemake et al. (1998a,b, 2000a) reported that although PAM additions to field soils correlate with detectable changes in microbial C utilization, the eVects are masked by the influences of other field variables such as crop cover type or nutrient status. Sojka et al. (2006) reported the eVects of ultrahigh PAM application rates to irrigated soils. Over a 6‐year period 1000 kg ha1 year1 of a commercial anionic PAM product was added to the soil. New plots were established each year to give a range of 0–6 years for eVects to materialize and allowing a sixfold range of high application rates for analysis. Analysis at the end of the study concentrated on plots receiving either 2691 or 5382 kg active ingredient PAM ha1. Active bacterial, fungal, and microbial biomass were not consistently aVected by high PAM additions; eVects were moderate (considering the massive PAM rates) and were driven more by date of sampling eVects than by PAM treatment eVects. In June and August, active bacterial biomass in soil was 20–30% greater in the control treatment than where soil was treated with 2691 or 5382 kg PAM ha1, but there were no significant diVerences in July (Table I). There were no diVerences in soil active bacterial biomass between the 2691 or 5382 kg PAM ha1 treatments regardless of sampling time. Control treatment active fungal biomass was 30–50% greater than soil treated with 2691 or 5382 kg PAM ha1 in June and July, but not in August. There was no diVerence in soil active fungal biomass between the
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Table I Active Bacterial, Fungal, and Microbial Biomass in Soils Treated with 2691 and 5382 kg Active Ingredients (a.i.) PAM ha1 (Adapted from Sojka et al., 2006) Month June
July
Aug
Treatment
ABB mg C g1 soil
AFB
AMB
Control 2691 kg PAM ha1 5382 kg PAM ha1 Control 2691 kg PAM ha1 5382 kg PAM ha1 Control 2691 kg PAM ha1 5382 kg PAM ha1
9.04 a 7.20 b 7.32 b 5.31 b 4.86 b 5.39 b 9.13 a 7.20 b 6.33 b
10.16 a 6.77 b 7.24 b 10.64 a 6.64 b 5.32 b 6.28 b 6.93 b 4.70 b
19.21 a 13.97 b 14.56 ab 15.95 a 11.51 b 10.71 b 15.42 a 12.54 b 11.03 b
ABB, active bacterial biomass; AFB, active fungal biomass; AMB, active microbial biomass. In each column values followed by the same letter are not significantly diVerent as determined by the least square means test (p 0.05; n ¼ 27).
2691 or 5382 kg PAM ha1 on any sampling date. Active microbial biomass in soil was 27–48% greater in the control treatment than soil treated with 2691 or 5382 kg PAM ha1 except in June for the 5382 kg PAM ha1 treatment. Nutritional characteristic analysis using Biolog GN plates suggested a separation of the nonamended control soils from the high PAM treatment for the June sampling, but not for July or August. Whole‐soil fatty acid profiles showed no change in the soil microbial community due to any PAM application rate on any sampling date. In contrast, both the fatty acid and Biolog analyses indicated that the microbial communities present at the June sampling (in all plots) were diVerent from those sampled in July and August, both taxonomically and metabolically independent of PAM treatment. The Sojka et al. (2006) study was important because despite large cumulative additions of PAM over a 6‐year period, there was little eVect on soil microbial biomass or metabolic potential as measured by gram‐negative Biolog microtiter plates or whole‐soil fatty acid methyl ester (FAME). Although measurable, the eVects on soil microbial population size were inconsistent and moderate, considering the massive PAM amounts added. Their results from massive PAM application rates and prolonged exposure times suggest that concerns about eVects of PAM on soil microorganism population size and function, applied at more typical 5–10 kg ha1 year1 application rates, are not warranted. To the extent they saw soil microbial biomass shifts, interpretation, especially in light of earlier literature, suggests this could be due to N enrichment resulting from the PAM addition rather than a direct eVect of the PAM polymer chemistry itself. This has been seen in other studies where high soil N concentration reduced microbial biomass and mineralization of cellulose, lignin, and herbicides (Entry, 1999, 2000;
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Entry and Backman, 1995; Entry et al., 1993). Thus if large amounts of PAM are applied to soil, it is reasonable to expect the additional N contained in PAM may slightly reduce microbial biomass. In addition to the N enrichment from the amide groups in PAM, some commercial formulations also contain a few percent additional N, commonly as urea, to promote dissolution on hydration. Environmentally, the small shifts in soil microbial biomass and metabolic potential in the Sojka et al. (2006) study were insignificant when weighed against the significant erosion prevention and runoV water quality protection aVorded by the use of 5–15 kg PAM ha1 per season in normal PAM use scenarios, compared to the 2691 and 5382 kg PAM ha1 applied in their study. In order to apply the amounts of PAM added in their study, even assuming no PAM degradation annually, farmers would need to apply 15‐kg active ingredient PAM ha1 for 180 years to accumulate 2691 kg ha1 and 360 years to accumulate 5382 kg ha1. In a separate study, Spackman et al. (2003) reported that PAM applied at 16 kg ha1 active ingredient did not aVect survival of total bacteria or coliform bacteria in soil. Wallace et al. (1986b) also reported on the eVects of ultrahigh rates of PAM application to soil in a small pot study in the greenhouse. They compared the eVect of adding 1% and 5% by weight of anionic PAM to soils with controls. Additional details of the PAM formulation were not given. The 1% PAM rate increased vegetative growth of wheat (T. aestivum L., cv. INIA66R) and tomato (Lycopersicon esculentum Mill., cv. Tropic). The 5% rate produced growth results equivalent to controls. Increased Na concentrations in plant tissues were attributed to Na associated with the PAM formulation used. Other minor reductions in mineral nutrients occurred, but were regarded as largely inconsequential in view of the vegetative yields and because of the exceedingly high PAM application rates. The overwhelming interest in PAM use on farm fields is for erosion control and/or infiltration management. However, the increase of interest in PAM in the last decade has prompted exploration of other novel uses of PAM. In a completely diVerent strategy for PAM use, Entry et al. (2000, 2005) found that PAM could be used in mixtures with wood chips or other organic materials to provide a protective physical barrier to verticillium wilt infection for potato seed pieces. In this case PAM, enriched with beneficial organisms, forms the immediate seed environment, making competition and infection from the disease organisms more diYcult.
XIII. PAM DEGRADATION Very few experiments have been conducted to quantify PAM degradation, especially for PAM applied to soil. Significant problems surround the ability to remove PAM from soil once applied. The use of radiolabeled PAM
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would provide the best option for studying PAM decomposition; however, the diYculty and cost of labeling the appropriate C bond to follow chain fragmentation are significant. These diYculties increase when attempting to label the large molecules in current use. Thus what is known about PAM degradation is drawn from sparse reports, often using smaller molecular weight PAMs, radiolabeling of the molecule’s H atoms, or from indirect measurements of decomposition. PAM degradation occurs slowly in soils as a result of several processes including chemical, photo, biological, and even mechanical processes (such as tillage abrasion, freezing and thawing, and so on) because of the enormous molecular size. Abiotic processes break the polymer molecule into progressively shorter segments over time (Hayashi et al., 1993). When polymer segments are 6 or 7 monomer units long, they are consumed by soil microorganisms (Hayashi et al., 1993). Both temperature and soil salt content are thought to influence degradation processes (Tolstikh et al., 1992; Wallace et al., 1986b). Azzam et al. (1983) estimated these rates at around 10% per year, but it is not certain how well their experimental conditions reflect the common mode of PAM use where PAM is mainly added to the soil surface and exposed to far harsher environmental extremes, than when mixed into the soil volume. Soil microcosm studies examining biodegradation rates of cross‐ linked PAM copolymer indicate that polymer molecules may be mineralized at rates as high as 7% per 80 days (Stahl et al., 2000). PAM cannot reasonably be expected to degrade in such a way as to release AMD because of the high‐ temperature requirement (MacWilliams, 1978). Johnson (1985) followed degradation of a cross‐linked PAM over an extended period in sandy desert conditions and found no AMD degradation products. Some controversy arose over reports by Smith et al. (1996, 1997) suggesting that PAM could degrade to AMD due to thermal and photolytic eVects occurring in a natural environment in the presence of the herbicide glyphosate. Their paper reported a slow release of AMD over the course of a 6‐week study, which they attributed to natural degradation of the PAM macromolecule to release AMD. They concluded that PAM degrades to AMD via a free radical process initiated by sunlight. In their study the PAM was added as an emulsion requiring inversion to dissolve into stock solutions and they reported that solutions were initially milky in appearance but became clear over the 6 weeks. A subsequent study by Ver Vers (1999) disputed the Smith et al. (1996, 1997) results, pointing to problems in proper dilution of the emulsions (possibly accounting for the initial milky appearance of solutions) and attributing the detection of small amounts of AMD over time to the gradual release of unreacted AMD contaminant from the incompletely dissolved emulsion. Ver Vers (1999) concluded that PAM does not degrade to AMD in the presence of glyphosate or sunlight or any combination of the two in a natural environment and that glyphosate
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influences the solubility of PAM requiring that added care must be used when combining the two. Photodegradation of PAM has been described by Decker (1989). The C–C, C–H, and C–N bonds in PAM have bond strengths of 340, 420, and 414 kJ mol1, which can be cleaved by wavelengths of 325, 288, and 250 nm, respectively (Crosby, 1976). However, most of the UV radiation in sunlight at wavelengths below 300 nm is absorbed by atmospheric ozone before reaching the earth’s surface (DiVey, 1991). A study by Caulfield et al. (2003a,b) reported that strong UV radiation at 254 nm released AMD from solutions of a nonionic water‐soluble PAM. However, the release of AMD was generally <50 ppm repeat monomer units in the polymer. They also noted a drop in solution viscosity which indicated that the AMD released was the result of chain scission, not an ‘‘unzipping of the polymer chain.’’ Their study reported that PAM was stable under fluorescent lights and did not release detectable amounts of AMD in hot aqueous solutions at 95 C. Suzuki et al. (1978, 1979) also reported a number of low molecular weight PAM decomposition products when degraded using ozone or strong UV irradiation in the presence of ozone, but AMD was not among them. These results together with those of Ver Vers (1999) and MacWilliams (1978) indicate that there is no basis for assuming that PAM degrades to AMD in the natural environment. In soils acclimated to PAM amendment, it has been shown that microbes attack the amide functional group on the polymer, utilizing it as an N source without degradation of the molecule’s C spine (Kay-Shoemake et al., 1998a,b). Thus even if there existed a small probablility for the AMD production from polymer chain scission, it would decrease drastically with time. Other research shows that any AMD present in microbiologically active soil environments is rapidly metabolized as an N source by several soil microorganisms including Nocardia rhodochrous, Bacillus sphaericus, Rhodococcus sp., Arthrobacter sp., and Pseudomonas putrefaciens (Abdelmagid and Tabatabai, 1982; Arai et al., 1981; Brown et al., 1980; Croll et al., 1974; Lande et al., 1979; Shanker et al., 1990; USEPA, 1985). Wallace et al. (1986b) noted that the end products resulting from PAM decomposition would not be AMD, even if the first step of PAM decomposition yielded the monomer. AMD is rapidly decomposed to propionamide and propionic acid, and propionamide rapidly hydrolyzes to propionic acid as well. They noted that propionate is a fatty acid which is metabolized by plants (Mahler and Cordes, 1971). Propionates are the nontoxic mold inhibitors commonly used in the baking industry (Goodman and Gilman, 1965) and in perfume formulations (Morrison and Boyd, 1966). The ultimate breakdown products of AMD are CO2, NH3, and H2O. Kay-Shoemake and Watwood (1996) and Kay-Shoemake et al. (1998a,b, 2000a) reported that biodegradation of anionic PAMs are likely limited to enzymatic hydrolysis and the release of NH3. Hayashi et al. (1994) suggested that the simple
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compound polyacrylic acid might be another product of PAM degradation under some conditions, which can also be metabolized by microorganisms. Kay-Shoemake and Watwood (1996) and Alexander (1994) suggested that to the extent that PAM molecules remain intact in the soil, even if somewhat reduced in chain length they are likely incorporated into the soil organic fraction. Degradation of the AMD is fairly rapid (Kay-Shoemake et al., 1998a; Lande et al., 1979; Shanker et al., 1990). AMD was completely degraded within 5 days after applying 500 kg PAM kg1 garden soil (Shanker et al., 1990). Lande et al. (1979) applied 25 kg PAM kg1 soil and reported that the half‐life of AMD in agricultural soils was 18–45 h. Degradation may be slower in cooler more sterile waters, in sandy soils, or soils with low respiration rates because of temperature, soil water content, or other factors slowing microbial metabolism (Brown et al., 1980, 1982; Conway et al., 1979; Croll et al., 1974; Davis et al., 1976). Metcalf et al. (1973) and Neely et al. (1974) concluded that because of the ease with which AMD is metabolized by biological organisms and otherwise degraded, it is not likely that it can bioaccumulate to any extent in the food chain. Reports by Tareke et al. (2002) and others (Ahn et al., 2002; Andrzejewski et al., 2004; Bacalski et al., 2003; Konings et al., 2003; Palevitz, 2002; Roach et al., 2003; Rosen and Hellenas, 2002; Svensson et al., 2003; Zyzak et al., 2003) have drawn new attention to health concerns related to AMD. Their papers (and many others) reported the AMD content of a wide range of cooked, baked, and fried foods. The range of AMD found in all categories of food tested by Svensson et al. (2003) was from 25‐ to 2300‐ppb AMD. Baked and deep fried starchy foods proved most problematic. The mean values for some popular food items were: potato chips (1360 ppb), French fries (540 ppb), bread crisps (300 ppb), cookies (300 ppb), tortilla chips (150 ppb), popcorn (500 ppb), and breakfast cereals (220 ppb). Various meat products ranged from 30 to 64 ppb. The Food and Agricultural Organization and World Health Organization concluded that food makes a significant contribution to total exposure of the general public to AMD, with average intake rates in the range of 0.3–0.8 mg of AMD intake per kilogram of body weight per day. AMD concentrations in these commonly eaten foods are 5–460 times greater than the maximum residual AMD concentrations expected in irrigation water treated with 10 ppm of the food‐grade anionic PAM products containing no more than 0.05% AMD. Yet, no neurotoxic eVects are expected from the AMD concentrations ingested in diets that include these foods. The ubiquitous human exposure to AMD from common food provides a quantitative contrast for considering risk from exposure to AMD from environmental uses of recommended PAM products at recommended application rates.
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Very few studies have analyzed for fate of PAM applied in irrigation water or for other environmental uses. This is largely because PAM cannot be eVectively desorbed (extracted) for analysis once it has been adsorbed on mineral or other solid surfaces. Lentz et al. (1996a) developed an indirect assay that measured light transmittance in a spectrophotometer as aVected by flocculation and settling of a known concentration of kaolinitic clay. The technique showed sensitivity at or slightly below the range of 0.1 ppm. However, the technique was only applicable for PAM still in solution in irrigation water and required minor adjustments for salinity‐wide variation in runoV water sediment content, possibly due to the influence of other dissolved organic materials. The method was a significant improvement over methods previously compiled (Daughton, 1988), both in sensitivity and because earlier methods required use of significant amounts of toxic reagents. Several recent papers have explored other new techniques for PAM analysis and techniques for removal of PAM adsorbed to soil (Lu and Wu, 2001, 2002, 2003b; Lu et al., 2003). These PAM analytical techniques use size exclusion chromatography. They can be slightly more sensitive than the earlier analytical techniques but require more sophisticated analytical capacity than the Lentz et al. (1996a) method. Although PAM can be removed from soil with vigorous chemical stripping, some questions remain about the thoroughness of the removal and the accuracy of the determination because of the eVects on analyte molecular conformation and influence of other organics that might mask or otherwise interfere with determination of the analyte. Nonetheless, the contribution of new techniques for PAM analysis has greatly widened the scope of potential PAM fate studies. This potential has yet to be fully realized because of the considerable additional time and expense involved in making these analyses.
XIV. PAM AND Ca Anionic PAMs bond to mineral surfaces only if there is suYcient electrolyte present to overcome the repulsion of the polymer anionic sites and mineral anionic sites to allow weaker van der Waals forces, H bonding, or dipole attractions to be eVective; this eVect is enhanced if polycations such as Ca2þ are present to ‘‘bridge’’ between the negative charge sites of the polymer and mineral surfaces (Laird, 1997; O’Gorman and Kitchener, 1974; Orts et al., 2001; PeVerkorn, 1999; Theng, 1982; Wallace and Wallace, 1996). El-Morsy et al. (1991a,b) noted that water quality interacted with PAM treatment, aVecting clay migration and infiltration and that these eVects
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were magnified with increased sodicity but moderated with increased salinity (EC). Lecourtier et al. (1990) found that a critical salt concentration exists for adsorption of anionic PAMs to overcome electrostatic repulsion from charged mineral surfaces. Anionic polymers may also adsorb to the broken edges of minerals, where positive charges from aluminum ions or other isomorphously substituted crystal lattice elements may be exposed (Greenland, 1972). Lurie and Rebhun (1997) noted that the adsorption phenomena of PAMs and other polymers with suspended organic solids and in waters with high concentration of dissolved organic compounds is a complex process that is impacted by molecular mass as well as charge and molecular conformation. Larger polymer chains behave diVerently than smaller polymer chains and the interaction can vary with the properties of the other dissolved organics in solution. For cationic PAMs Edwards et al. (1994) found that the ability of dissolved organic matter to interact with cationic polymers through precipitation increased with increasing molecular mass and decreasing anionic functional group content of the soluble organics. Haschke et al. (2002) measured the sorbtive strength of a single PAM molecule to a mica surface to be 200 pN. H bonding (Kohl and Taylor, 1961; Nabzar et al., 1984, 1988) and ligand exchange (Theng, 1982) are often sighted as primary bonding mechanisms. H bonding may occur between polymer amide groups and free hydroxyl groups of the adsorbent surface that are not already bonded with other close hydroxyls (Griot and Kitchener, 1965; PeVerkorn et al., 1990). Theng (1982) suggested that ligand‐exchange bonding results when the carboxylic group of the PAM enters the inner coordination layer of edge Al, thereby forming a coordination complex. However, both these sorption mechanisms are unlikely in the normal soil pH ranges of 5–9 because of electrostatic repulsion, which can only be overcome by presence of additional electrolytes in the soil solution (Lecourtier et al., 1990). Rengasamy and Sumner (1998) presented the relative flocculation power of cations as Naþ ¼ 1, Kþ ¼ 1.8, Mg2þ ¼ 27, and Ca2þ ¼ 45. Lu et al. (2002a) working with a range of soils, water qualities, and polymer concentrations found that on average divalent cations were 28 times more eVective in enhancing PAM sorption than monovalent cations. Wallace and Wallace (1996) and Orts et al. (2001) noted the need for Ca electrolytes in irrigation water when using anionic PAM for infiltration and erosion control. Ca has a double charge and small hydrated radius which favors flocculation. Na, on the other hand, has a large hydrated radius which generally prevents ion bridging, generally leading to dispersion rather than flocculation of solids. Lentz and Sojka (1996b) noted that when irrigation water SAR was increased from 0.7 to 9.0 [m molc liter1]0.5 that PAM’s infiltration enhancement over control water was greatly diminished. Water low in electrolytes or
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with high SAR can be amended relatively easily through addition of gypsum (CaSo4) or Ca(NO3)2 fertilizer. PAM has been used in conjunction with gypsum to accelerate leaching of sodic soils, by reducing surface sealing (Malik et al., 1991a; Zahow and Amrhein, 1992). When surface seals are prevented and when near surface structure is stabilized infiltration and throughput of water are increased and the added Ca applied via gypsum is more eVectively delivered to deeper in the profile. A similar response was noted for use of PAM with soybean in Australia (Sivapalan, 2003). Gypsum used with PAM aided soil management of soils irrigated with high Na waste water (Gardiner, 1996). Wallace et al. (2001) reported synergistic eVects of gypsum and PAM in limiting erosion on southern Brazilian soils. In rice (Oryza sativa) in Australia, water management involves balancing several considerations which often are in conflict with one another. Turbidity is a problem for rice establishment and early growth, but growers prefer to restrict infiltration to conserve water. Sivapalan (2005) found that with sodic waters used for rice irrigation on Vertisols in Australia, the dispersive action of Na could be overcome by adding gypsum to the inflows and following a split PAM application as proposed by Sojka and Surapaneni (2000). Yu et al. (2003) found that in small‐tray studies that applying PAM alone at the surface of two coarse‐textured soils reduced erosion from simulated rainfall but not runoV; adding gypsum alone decreased runoV but not erosion. However, spreading dry PAM at the equivalent of 20 kg ha1 mixed with gypsum at 4 Mg ha1 increased the final infiltration rate by a factor of 4 and reduced erosion 70% compared to controls.
XV. PAM FOR CONSTRUCTION SITES AND OTHER DISTURBED LANDS Ironically, interest in the use of polymer soil amendments was first prompted by their use in road and runway construction during World War II (Wilson and Crisp, 1975). Interest spread rapidly to the agricultural sector from whence emanated the majority of soil‐conditioning research on polymers and soil applications for most of the years since then. Polymers have, however, been used in construction for a variety of applications, including grouting, drilling muds, dust suppression, roadway stabilization, and a variety of other, largely high‐rate applications, in much the original manner identified originally. PAM and other polymers were introduced as drilling mud additives in about 1949 and rapidly became an important tool in the drilling industry that has continued to the present (Barvenik, 1994; Scanley, 1959). DeBoot (1975) reported that during the construction of Belgium’s
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superhighway system, in 1975, PAM was widely used to stabilize more than 3000 ha of exposed soil in road cuts. More recently, however, the dramatic successes of low‐rate PAM application strategies in irrigated agriculture and the improved eYcacy of new higher molecular weight environmentally friendly anionic PAMs have spawned renewed interest in PAM for construction site and road cut protection at low rate of application, and hence at low cost. Traditional techniques involving rock, metal or plastic armoring, straw bales, fiber batting, filter fences, settling ponds, and so on commonly cost $2000–$10,000 ha1 depending on site conditions and the requirements of applicable laws governing water quality protection in the particular state, county, or municipality. Reduction of sediment from fiber mats, straw bales, and so on in well‐ designed sites can be as high as 80–90% on a mass basis (Barnett et al., 1967; Benik et al., 2003a,b; Grace, 1999; Jennings and Jarrett, 1985). Nonetheless, Minton and Benedict (1999) noted that the turbidity from these best management practices (BMPs) often still range in value from hundreds to even thousands of nephelometric turbidity units (NTUs). Turbidity is generally aVected more by suspended clay‐sized fractions than coarser particulates. Thus, for many soils runoV NTUs can remain high using conventional BMPs despite great reductions in sediment mass. Brown et al. (1981) showed that the sediment loss from holding ponds is almost entirely in the clay fraction, which carries most of the soil nutrient and chemical load responsible for surface water quality impairment. Depending on design adequacy and eYcacy of individual or combinations of treatments, many traditional techniques are still incapable of meeting sediment retention requirements, which in recent years are commonly prescribed in terms of maximum NTU thresholds (Tobiason et al., 2001). Turbidity is generally raised mostly as a result of clay‐sized sediments in suspension, which traditional techniques are least eVective at removing from runoV. PAM, on the other hand, is particularly eVective at stabilizing soil against detachment and transport of fines as well as flocculating and removing fines from runoV, especially if a runoV pond can briefly provide quiescent storage. Even in the absence of retention ponds, flowing water, as the experiences with furrow irrigation have shown, can also be very eVectively clarified, although in nonagricultural settings, limitations of flow volume, rate, and capacity can challenge dosing strategies. In performing economic assessments of the cost eVectiveness of traditional versus PAM‐based erosion control at construction sites, an additional factor is important to the analysis that does not come into play in agricultural scenarios. That factor is the magnitude of fines that are often incurred if the contractor fails to prevent runoV quality deterioration beyond prescribed limits (Tobiason et al., 2001). A single failure at a development site in 1996 in Washington state resulted in a $65,000 fine (Bremerton Sun, 1996). This factor has increased the interest in PAM use because of its ability to
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ensure against failure of traditional erosion protection techniques, which may be more eVective against massive point failures, but which cannot adequately meet turbidity thresholds in and of themselves. In their study, Tobiason et al. (2001) found that wet PAM applications of as little as 90 g ha1 applied in a 10‐ppm spray reduced runoV turbidity considerably for as long as 6 weeks. Optimal PAM application doses were 40–80 ppm. Dry granular applications were also eVective but required 10 times the material to yield the same degree of eVectiveness as spray‐applied PAM. Their study used a cationic polymer from Calgon identified as Catfloc 2953 and was described as a polyaluminum chloride‐based PAM. Tests of aquatic organism survival in treated runoV using daphnia in this series of studies showed no mortality under these conditions. The series of tests found turbidity reductions with PAM from 80% to 100% where influent turbidities were as high as 1000 NTUs or more, with all PAM‐treated discharges meeting Washington’s strict guidelines. The tests were conducted over a 5‐month period with rainfall totally 1010 mm. Roa-Espinosa (1996) and Roa-Espinosa et al. (1999) found that several diVerent PAM formulations provided excellent eYcacy for erosion control. In these experiments anionic formulations with 15% or greater charge density were generally among the most eVective PAMs. Overall treatment strategies for the field tests were best when the PAM was mixed with grass seed and used, essentially, as a hydroseeding matrix. In these treatments, seed germination was improved because the PAM prevented seed from washing away and promoted better seedling emergence and sward establishment, an observation which was further substantiated in work by Sojka et al. (2003) who saw greater weed establishment in PAM‐treated furrows. Teo et al. (2001, 2006) compared a number of PAM formulations for use in reducing erosion in a variety of Hawaiian soil management situations, including from construction sites where sediment‐laden runoV posed significant risks to reef flora and fauna. Their findings indicated that although there were occasional minor advantages of matching specific polymer formulations to specific soils, generally good performance was achieved with anionic PAMs of the type specified by the NRCS PAM standard (Anonymous, 2001). Flanagan et al. (2002a,b) reported good successes applying PAM for erosion control on steep slopes. Soupir et al. (2004) investigated a number of PAM‐ and mulch‐based treatments for erosion control on construction sites in Virginia. Her results parallel others in the literature including those of Roa‐Espinosa et al. (1999) that optimal overall eVectiveness is obtained by combining PAM with a mulch or hydroseeding mix. Another aspect of the study by Soupir et al. (2004) worth noting is the diYculty that can occur if the PAM application solution is too concentrated, resulting in restriction of infiltration, thereby generating more runoV and preventing maximum erosion control eYcacy or stand
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establishment. This result is in line with the finding of Lentz (2003) and indicates that for best results, especially in hydroseeding a careful control of PAM concentration is important, as well as provision of enough total solution volume to assure some penetration of the PAM into the surface few millimeters of soil. Similar results were obtained in a study of PAM for erosion control on road cuts in North Carolina (Hayes et al., 2005). Their study saw less than optimal PAM performance when applying PAM alone at concentrations of 76 and 468 ppm of product formulation (active ingredient concentrations not given). While PAM mixed with mulch saw sediment reductions of 95%, the high PAM concentration applications only showed 20–40% reductions. As noted by Lentz (2003), unless PAM in these high‐concentration ranges are allowed to achieve complete dry‐down before being irrigated or rained on, they will restrict infiltration. As noted earlier in the chapter, a significant body of literature has noted the desirability, when applying PAM in solution form, to hold concentrations below 20 ppm for optimal infiltration and erosion control in furrow irrigation; it could be that similar concentration‐dependent eVects influenced the outcomes of both the Soupir et al. (2004) and Hayes et al. (2005) studies. Open pit and strip mining can pose significant environmental risks due to erosion and runoV. They can also pose significant challenges to revegetation because of poor infiltration and, when rainfall is seasonal, due to sheet erosion of seeded areas before stand establishment has had a chance to occur. Vacher et al. (2003) studied the use of PAM in large erosion plots using soil from three Australian mine sites. A range of PAM materials and application rates and strategies were studied in replicated tests under a rainfall simulator. Application rates of an anionic PAM meeting US NRCS recommendations (Anonymous, 2001) were applied at rates of 5, 10, 20, and 40 kg ha1. All materials were applied as liquid solutions diluted to allow application with hand sprayers, applying 57 liter m2 total solution to rainfall plots and 25 liter m2 to overland flow plots. In addition to PAM treatments, the study included a 2.75 t ha1 barley straw plot and one straw plus PAM plot. Overland flow plots also applied 5 t ha1 gypsum. Plots were allowed to dry 12 h before erosion and runoV tests were conducted. All PAM treatment infiltration rates were significantly improved compared to controls but did not match the straw or straw plus PAM treatment. Most PAM treatments significantly reduced erosion, with the numerically best treatment being the straw plus PAM treatment, statistically equal to the straw‐only treatment. These two treatments typically performed five‐ to tenfold better than the PAM‐only treatments for erosion control and about twofold better for infiltration improvement. In a side study, PAM performance was enhanced in some instances by adding small amounts of suspended clay material to the coarser textured soils. Higher molecular weight formulations outperformed
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lower molecular weight formulations for erosion control, but no diVerences were seen for infiltration. Another important development in PAM use has been its deployment for dust control in helicopter‐landing zones and high‐traYc areas of military encampments (Mikel, 2003; Orts et al., 2006; The Furrow, 2004). Again, this represents a full circle return to uses originally developed in the late 1930s and early 1940s. However, better PAM formulations and five decades of additional scientific insight and experience with application techniques have improved the eVectiveness and longevity of application. Over two dozen military PAM application rigs have been deployed in Iraq and Afghanistan to improve the safety of landing zones and the hygiene and living conditions of bivouac areas and have been attributed with preventing costly dust‐related helicopter repairs as well as preventing landing accidents that have often been fatal in the past. Use of PAM for wind erosion reduction in agricultural settings has not been thoroughly researched. A few studies (Armbrust, 1999; Armbrust and Dickerson, 1971; Armbrust and Lyles, 1975) demonstrated that PAM and other polymer materials can be eVective at reducing detachment, but did not eVectively resist the erosive eVects of saltation from adjacent unprotected areas. Since few studies have been conducted to explore the range of new polymer materials and potential application strategies available with sprinklers and other forms of irrigation and so on, there may yet be room for optimism for development of polymers for economical wind erosion control for agriculture.
XVI. CANAL AND POND SEALING Water conservation and eYcient transfer of precious water resources in arid zones from water source to point of use via unlined canals is becoming increasingly important. Seepage losses from unlined canals can be significant, typically 20–30% of the volume conveyed (Tanji and Kielen, 2002). In a world with ever increasing water shortages and water demands, prevention of unwanted seepage loss could be of staggering importance. In many arid areas, seepage also results in the mobilization of Se from soils and underlying strata, which can accumulate in seeps and wetland areas. Excessive Se has proven to be toxic to waterfowl. Conventional canal lining methods using concrete or various types of membranes can eVectively reduce seepage losses but are costly. Development of lower cost seepage control technologies can increase benefit to cost ratios and may provide a better investment (Kahlown and Kemper, 2005). Polymer applications are being investigated as a potentially more cost eVective means
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for controlling irrigation‐related seepage losses. In the late 1950s, the US Bureau of Reclamation evaluated the use of a proprietary emulsion made with resinous polymers and diesel oil. When added to the irrigation water in canals the chemical penetrated the soil and altered its hygroscopic properties. The product reduced seepage losses in nonreplicated tests, but was toxic to fish (Cron, 1959). Better polymers have been developed since then, and greater sensitivity to and regulation of environmental eVects have resulted in more sophisticated exploration of the potential for use of polymers as seepage‐inhibiting sealants. More recently, a demonstration project in Colorado evaluated two canal seepage control treatments (Valliant, 2000). In one treatement, water‐ soluble granular PAM (18 kg per season) was metered intermittently into canal water. In the second treatment, cross‐linked PAM was applied to the canal perimeter (630 kg ha 1) prior to filling the canal. The tests monitored the seepage and sediment eVects of treating a number of canal lateral segments with granular anionic water‐soluble PAM applied into flowing water in two 2.3‐kg applications separated by an hour between each application. The treated canal lateral sections were 70 m in length. Depending on the individual canal segment and date, flows varied from approximately 10,000 to 40,000 liter min1. The cross‐linked PAM treatement proved ineVective. However, seepage loss was reduced from 40% to 70% for the water‐soluble PAM treatment, although results were variable. Sediment in the delivered water was also reduced with PAM application. Variations in results were attributed to flow rate, sediment load, and the peculiarities of the individual lateral segments monitored in each test. Strict replication is not possible in these kinds of experiments, however, the trends among the compared control and PAM‐treated lateral segments were consistent. Other demonstration projects are evaluating treatments that apply water‐ soluble PAM directly to the canal perimeter before water fills the canal in spring (Marc Catlin, personal communication, 2000). One of the interesting properties of PAM, noted earlier in this chapter (Lentz, 2003), is that although at low concentrations it can stabilize soil pores and enhance infiltration, at higher concentrations viscosity eVects eventually reduce infiltration. Lentz (2003) conducted a systematic soil column and miniflume study that quantified the infiltration‐inhibiting eVect of water‐soluble PAM treatments for diVerent soils under ponded and flow conditions. Surface applications of 250‐ to 1000‐ppm PAM solutions to silt loam and clay loam soil reduced water infiltration 50–90%. Lentz (2001) also evaluated cross‐linked PAM treatments which amended a thin layer of soil with 5‐ to 10‐g PAM hydrogel per kilogram soil. These treatments reduced eVective conductivity of soils 87–94%. The seepage reduction was greatest in soils with balanced particle size distributions and least in soils with high sand fractions or higher clay and organic carbon contents. Preliminary field investigations have shown that
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these treatments eVectively reduce seepage losses in irrigation ponds and canals (Lentz and Kincaid, 2004). With respect to water‐soluble PAM treatments, it is uncertain whether the viscosity is the sole or primary mechanism responsible for seepage reduction. The PAM may decrease infiltration simply by increasing sediment deposition in conducting pores, or PAM’s ability to attract suspended fines from flowing water may contribute to establishment of a thin sediment layer of low permeability along the channel bottom that is held in place by the PAM. Because of their ability to restrict infiltration at suYciently high concentration, either alone or in combination with clays, there has been interest in the use of PAM and other polymers for pond and landfill sealing since the 1970s. Use of cationic polymers in conjunction with bentonite clays were shown to help stabilize the desired low hydraulic conductivity of clay liners against fluctuations in water content and the adverse eVects of leachate constituents from landfills, often lowering hydraulic conductivity more than an order of magnitude (Ashmawy et al., 2002; Bart et al., 1979; Elhajji et al., 2001; Petrov et al., 1997; Pezerat and Vallet, 1973; Shackleford et al., 2000). Experiments are continuing among United States Bureau of Reclamation (USBR), Agricultural Research Service (ARS), and Desert Research Institute (DRI) to quantify the magnitude of the PAM sealant eVect, the influence of a variety of application strategies, the influence of site conditions and soil properties, its durability, and eVects, if any, on introduction of AMD from product residual AMD. NRCS has issued an Interim Conservation Practice Standard (NRCS, 2005) to define the practice and provide guidelines during the time of further testing and development.
XVII. BIOPOLYMERS Farmers, environmentalists, the polymer industry, and other industries producing recalcitrant organic waste streams have shown interest in the possible development of biopolymer surrogates of PAM for a variety of reasons. PAM is inexpensive because the raw material currently used most commonly to synthesize the molecular building blocks of PAM is natural gas. Natural gas prices have risen greatly in recent years, resulting in about a 30% increase in PAM wholesale costs since 2000. Because so many industrial and food‐ processing activities depend on PAM‐like polymers there is interest in guaranteeing the future availability of suitable polymers. Development of biopolymers may help assure future availability of suitable polymers. A class of biopolymers commonly explored in soil‐conditioning research has been polysaccharides (Ben-Hur and Letey, 1989; Fuller et al., 1995; He and Horikawa, 1996;
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Malik and Letey, 1991; Malik et al., 1991b; Nadler et al., 1992; Parfitt and Greenland, 1970; Singh et al., 2000a,b; Wallace et al., 1986a). Also, there is a perception among some that biopolymers represent a more sustainable and environmentally friendly basis for industrial and environmental technology. Research is underway to develop biopolymers synthesized from organic by‐products of crop agriculture and shell fish food processing. Biopolymers may be substitutes for PAM in uses where easier biodegradability is desired or where bio‐based chemistry is seen as an environmental benefit (Orts et al., 1999, 2000, 2001, 2002; Sojka et al., 2005). Orts and colleagues tested biopolymers for furrow irrigation erosion control and infiltration enhancement in laboratory soil bins and in field plots. They showed that biopolymers are feasible, although current compounds are less eVective and more expensive than PAM. Figure 6 shows the relative eYcacy of PAM surrogates based on starch xanthate and/or microfibril suspensions in laboratory tests. Degree of substitution (ds) is the number of hydroxyls per glucose molecule (maximum of 3) replaced with a xanthate (CS2) group. While several biopolymers reduced erosion significantly compared to controls, PAM was still five to six times more eVective at a much lower concentration. Similar results were obtained for field and laboratory tests of chitosan‐based polymers, although they showed eYcacy at much lower concentrations (Fig. 7). These data also show the diYculty of drawing conclusions solely based on laboraory results. Earlier studies with polysaccharides and cheese whey for erosion control in furrow irrigation were also promising, fueling optimism that commercially viable biopolymer compounds may eventually be developed (Brown et al., 1998; Robbins and Lehrsch, 1997; Shainberg and Levy, 1994; Singh et al., 2000a,b).
Figure 6 Relative eYcacies of several biopolymer surrogates of PAM as determined from benchtop miniflume studies (Sojka et al., 2005).
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Control (tap water) Chitosan (20 ppm) Chitosan (10 ppm) Field PAM (10 ppm)
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Figure 7 Degree of correspondence of benchtop miniflume results for actual field results using Portneuf silt loam soil in comparing a chitosan‐based biopolymer with PAM (Sojka et al., 2005).
XVIII. CONCLUSIONS The advancement of PAM‐based agricultural and environmental management technologies since the early 1990s has been rapid, dramatic, and expansive. PAM is an extraordinarily versatile polymer. The variety of its eVects on the properties of water itself and the surface interactions of solids it sorbs with allow a wide range of potential management scenarios for the protection of the environment and the improved productivity of managed lands, especially in irrigated agriculture. The compound is very safe and very inexpensive in view of its remarkable potency to influence physicochemical processes. Coupled with the ingenuity and creativity of soil and water researchers, PAM, related synthetic polymers, and potential future biopolymers hold significant potential for aVordable environmental protection and improved eYciencies and economies of environmental, agricultural, and industrial processes dependent on the management of soil structure, water behavior, and control of suspended solids.
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Wallace, A. (Ed.) (1995). ‘‘Soil Conditioner and Amendment Technologies,’’ Vol. 1, p. 340. Wallace Laboratories, El Segundo, CA. Wallace, A. (Ed.) (1997). ‘‘Soil Conditioner and Amendment Technologies,’’ Vol. 2, p. 466. Wallace Laboratories, El Segundo, CA. Wallace, A. (1998a). Use of water-soluble polyacrylamide for control of furrow irrigationinduced soil erosion. In ‘‘Handbook of Soil Conditioners, Substances That Enhance the Physical Properties of Soil’’ (A. Wallace and R. E. Terry, Eds.), Marcel Dekker, New York, NY. Wallace, G. A. (1998b). Use of soil conditioners in landscape soil preparation. In ‘‘Handbook of Soil Conditioners, Substances That Enhance the Physical Properties of Soil’’ (A. Wallace and R. E. Terry, Eds.), Marcel Dekker, New York, NY. Wallace, R. E., and Terry, R. E. (Eds.) (1998). ‘‘Handbook of Soil Conditioners, Substances That Enhance the Physical Properties of Soil.’’ Marcel Dekker, New York, NY. Wallace, A., and Wallace, G. A. (1986a). EVects of very low rates of synthetic soil conditioners on soils. Soil Sci. 141, 324–327. Wallace, G. A., and Wallace, A. (1986b). Control of soil erosion by polymeric soil conditioners. Soil Sci. 141, 363–367. Wallace, A., and Wallace, G. A. (1987). Conditionerigation: New process proves successful. Irrig. J. 37, 12–15. Wallace, A., and Wallace, G. A. (1995). Over 40 years of literature concerning synthetic polymer soil conditioners for land improvement. In ‘‘Soil Conditioner and Amendment Technologies’’ (A. Wallace, Ed.), Vol. 1, pp. 217–276. Wallace Laboratories, Los Angeles, CA. Wallace, A., and Wallace, G. A. (1996). Need for solution or exchangeable calcium and/or critical EC level for flocculation of clay by polyacrylamides. In ‘‘Proceedings of the Managing Irrigation-Induced Erosion and Infiltration with Polyacrylamide’’ (R. E. Sojka and R. D. Lentz, Eds.), May 6–8, 1996, College of Southern Idaho, Twin Falls, ID. University of Idaho Misc. Pub. 101‐96, pp. 59–63. Wallace, A., AbouZamzam, A. M., and Cha, J. W. (1986a). Interactions between a polyacrylamide and a polysaccharide as soil conditioners when applied simultaneously. Soil Sci. 141, 374–376. Wallace, A., Wallace, G. A., and AbouZamzam, A. M. (1986b). EVects of excess levels of polymer as a soil conditioner on yields and mineral nutrition of plants. Soil Sci. 141, 377–380. Wallace, A., Wallace, G. A., and AbouZamzam, A. M. (1986c). EVects of soil conditioners on water relationships in soils. Soil Sci. 141, 346–351. Wallace, B. H., Reichert, J. M., Eltz, L. F., and Norton, L. D. (2001). Conserving topsoil in Southern Brazil with polyacrylamide and gypsum. In ‘‘Proceedings of the International Symposium: Soil Erosion Research for the 21st Century,’’ January 3–5, 2001, ASAE publication 701P0007, Honolulu, Hawaii, pp. 183–187. Wang, Y., and Boogher, C. A. (1987). EVect of a medium-incorporated hydrogel on plant growth and water use of two foliage species. J. Environ. Hort. 5, 127–130. Waters, D., Drysdale, R., and Kimber, S. (1999a). Benefits of planting into wheat stubble. The Australian Cotton Grower Magazine 20, 8–13. Waters, D., Drysdale, R., and Kimber, S. (1999b). Reducing oV-site movement of sediment and nutrients in a cotton production system. In ‘‘Proceedings of the NPIRD Nutrient Conference. June 1999.’’ Brisbane, Qld. Watwood, M. E., and Kay‐Shoemake, J. L. (2000). Impact of polyacrylamide treatment on sorptive dynamics and degradation of 2,4-D and atrazine in agricultural soil. J. Soil Contam. 9, 133–147. Weeks, L. E., and Colter, W. G. (1952). EVect of synthetic soil conditions on erosion control. Soil Sci. 73, 473–484.
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NUTRIENTS IN AGROECOSYSTEMS: RETHINKING THE MANAGEMENT PARADIGM L. E. Drinkwater1 and S. S. Snapp2 1
2
Department of Horticulture, Cornell University, Ithaca, New York 14853 Department of Crop and Soil Science and W. K. Kellogg Biological Station, Michigan State University, East Lansing, Michigan 48824
I. II. III. IV.
Introduction Nutrient Management in Agriculture Internal Biogeochemical Processes in Agroecosystems Toward an Ecosystem‐Based Approach to Improving Nutrient Use EYciency A. Using Plant Diversity to Restore Ecosystem Functions B. Restoration of Ecosystem Function Through Plant–Microbial Interactions C. Microbially Mediated Processes V. Plant Adaptation to Ecosystem‐Based Nutrient Management VI. Conclusions Acknowledgments References
Agricultural intensification has greatly increased the productive capacity of agroecosystems, but has had unintended environmental consequences including degradation of soil and water resources, and alteration of biogeochemical cycles. Current nutrient management strategies aim to deliver soluble inorganic nutrients directly to crops and have uncoupled carbon, nitrogen, and phosphorus cycles in space and time. As a result, agricultural ecosystems are maintained in a state of nutrient saturation and are inherently leaky because chronic surplus additions of nitrogen and phosphorus are required to meet yield goals. Significant reductions of nutrient surpluses can only be achieved by managing a variety of intrinsic ecosystem processes at multiple scales to recouple elemental cycles. Rather than focusing solely on soluble, inorganic plant‐available pools, an ecosystem‐based approach would seek to optimize organic and mineral reservoirs with longer mean residence times that can be accessed through microbially and plant‐mediated processes. Strategic use of varied nutrient sources, including inorganic fertilizers, combined with increases in plant diversity aimed at expanding the functional roles of plants in agroecosystems will help restore desired agroecosystem functions. 163 Advances in Agronomy, Volume 92 Copyright 2007, Elsevier Inc. All rights reserved. 0065-2113/07 $35.00 DOI: 10.1016/S0065-2113(04)92003-2
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L. E. DRINKWATER AND S. S. SNAPP To develop crops that can thrive in this environment, selection of cultivars and their associated microorganisms that are able to access a range of nutrient pools will be critical. Integrated management of biogeochemical processes that regulate the cycling of nutrients and carbon combined with increased reservoirs more readily retained in the soil will greatly reduce the need for # 2007, Elsevier Inc. surplus nutrient additions in agriculture.
I. INTRODUCTION The unintended consequences of modern agriculture extend well beyond agricultural landscapes themselves (Altieri, 1989; Hambridge, 1938; Matson et al., 1997; Pimentel et al., 1991). Biogeochemical cycles have been profoundly altered at multiple scales (Carpenter et al., 1998; Vitousek et al., 1997) and the rate of soil loss still exceeds soil formation (Pimentel et al., 1991). Nitrogen (N) and phosphorus (P) are the two most important nutrients limiting biological production (Chapin et al., 1986; Tyrrell, 1999) and are the most extensively applied nutrients in managed terrestrial systems, mainly as soluble inorganic fertilizers (Fi). Agriculture accounts for 60% of the biologically active N from anthropogenic sources (Vitousek et al., 1997). Available P in the biosphere has also increased in the last 50 years, largely as a result of P applications to agricultural lands. P flux to coastal oceans has nearly tripled, from 8 106 Mg year1 to the current rate of 22 106 Mg year1 (Howarth et al., 1995). Nutrient enrichment has complex, often detrimental eVects in natural ecosystems (Carpenter and Cottingham, 1997; Galloway, 2000; Vitousek et al., 1997). Global N and P fluxes are projected to increase substantially as developing countries increase Fi production capacity (Galloway, 2000; Tilman, 1999). A new approach to nutrient supply in intensively managed ecosystems is required to reverse this process of global eutrophication. In this chapter, we briefly consider the origin and consequences of the current soil fertility management paradigm. We then present an ecosystem‐ based conceptual framework that can serve as a basis for nutrient management in agriculture and assess the potential contributions from a wide range of ecosystem processes. Our aim is to critically evaluate the potential for intrinsic ecosystem processes to improve nutrient use eYciency and nutrient balance at the ecosystem scale while maintaining productivity.
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Since the late 1800s, research eVorts to improve soil fertility have been based on the premise that agricultural production must continue to increase in order to keep pace with population growth (Bear, 1926; Crookes, 1899). Initially, Fi were viewed as supplemental nutrient sources. During the first half of the twentieth century, agriculturalists emphasized the importance of long‐term experimentation, temporal plant diversity, maintenance of soil organic matter (SOM) and the need to understand and manage soil biological processes (Andrews, 1947; Hopkins, 1945; Waksman, 1936). A divergent view, that manufactured Fi could support high yields without additions of organic amendments and diverse rotations, gained support through the 1940s and 1950s to become the dominant paradigm (Hopkins, 1945; Tisdale et al., 1993). This transition represented a fundamental paradigm shift. The focus of soil fertility management became the relatively small, ephemeral, soluble inorganic N and P pools rather than nutrient reservoirs with longer mean residence times (MRTs). As purchased Fi became more widespread, agronomists sought to minimize fertilizer costs while maximizing yields and profitability through the development of soil tests for plant‐available nutrients combined with fertilizer trials conducted for a single growing season (Schreiner and Anderson, 1938). Dependable soil tests for available P were developed in the late 1940s and have been in common use since the 1960s (Bray and Kurtz, 1945; Olsen et al., 1954). Predicting plant‐available soil N has proven to be more challenging, and eVorts to develop a soil test for N continue. Currently, the pre‐sidedress nitrate test is the most widely recommended soil N test used to assess plant‐available N just before the exponential growth phase of the crop (MagdoV et al., 1984). The impacts of nutrient losses from agricultural lands on aquatic ecosystems became apparent in the 1970s prompting a debate about how to best achieve yields without harming the environment (Carpenter et al., 1998). As a result, the aim of soil fertility management has broadened to encompass both economic and environmental goals and fertilizer recommendations have undergone substantial refinement and become quite complex (Fig. 1). Meanwhile, the ‘‘ecological’’ agriculture paradigm also emerged in the 1980s advocating the application of ecological principles to develop food production systems based on internal, biologically driven processes in an eVort to reduce external inputs while achieving adequate yields (Lowrance et al., 1984). The application of ecology in agricultural pest and weed control has become embedded in US agricultural policy, resulting in management options that blend biologically and chemically based strategies (Ehler and Bottrell, 2000; Liebman and Gallandt, 1997). In contrast, an integrated
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Figure 1 Refinement of N fertilizer recommendations for New York during the past 50 years by 5‐year increments. Initially, recommended amounts of N fertilizer were based simply on whether animal manure would be applied. By the 1980s fertilizer recommendation tables included history of animal manure application, cropping history, and soil type resulting in >100 diVerent fertilizer rates (i.e., in 1985, there were 5 soil types10 cropping histories3 manure rates). Data from Cornell Cooperative Extension, 1950–2003.
nutrient management strategy based on ecological concepts has yet to be broadly applied. Instead, the current paradigm guiding nutrient management amounts to a technologically advanced version of the approach developed before the ecosystem concept became a guiding paradigm in ecology. The problem of improved Fi use eYciency has been viewed mainly as a consequence of temporal asynchrony and spatial separation between applied nutrients and the crop (Stevenson and Baldwin, 1969; Welch et al., 1971). As a result, nutrient management research continues to emphasize improved delivery of Fi to the root zone during the period of crop uptake through modifications such as banding, fertigation, and split fertilizer applications (Bolland and Gilkes, 1998; Cassman et al., 2002). Fertilizer use eYciency is evaluated using metrics that reflect crop uptake of fertilizer added in the current growing season (Cassman et al., 2002). To increase crop access to N fertilizer, a variety of additives have been developed that inhibit nitrification and denitrification (Wolt, 2004). This approach has been extremely successful in terms of maximizing yields; however, attempts to reduce nutrient losses have met with limited success (Cassman et al., 2002). Despite more than 30 years of concentrated eVort, mass balances indicate annual N and P inputs consistently exceed harvested exports by 40 to 60% resulting in substantial losses of these nutrients to the environment (Bolland and Gilkes, 1998; David and Gentry, 2000; Galloway and Cowling, 2002; Van der Molen et al., 1998).
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III. INTERNAL BIOGEOCHEMICAL PROCESSES IN AGROECOSYSTEMS The reliance on Fi combined with use of chemical weed controls has led to a series of management changes that have restructured agroecosystems and uncoupled N, P, and carbon (C) cycles. Simplified rotations became possible when these technologies made it unnecessary to grow cover crops and forages in sequences that alternated with cash crops (Auclair, 1976). These crops have little or no cash value per se but help to maintain internal cycling capacity through a variety of mechanisms. Specifically, the preferential removal of winter annuals from large expanses of agricultural lands has increased the prevalence of bare fallows. This reduction in the time frame of living plant cover and C fixation combined with tillage increases soil erosion and depletion of SOM stocks (Aref and Wander, 1997; Campbell and Zentner, 1993) and increases the susceptibility of these ecosystems to nutrient saturation and nutrient losses (Fenn et al., 1998; McCracken et al., 1994; Tonitto et al., 2006). In these simplified rotations where bare fallow is maintained for 4–8 months, microbial assimilation is the only other major route for biologically mediated retention of added Fi. The arrangement of roots, soil aggregates, and pores creates tremendous microscale spatial heterogeneity. As a result, environmental conditions favoring either aerobic or anaerobic processes and N‐ or C‐limiting conditions frequently co‐occur within the soil matrix (van Elsas and van Overbeek, 1996). While C limitation rarely occurs in the rhizosphere (Cheng et al., 1996), decomposers in bulk soil are usually C limited (Koch et al., 2001) and less numerous (Rouatt et al., 1960). Under the prevailing conditions in agricultural soils, microbial assimilation of N and P occurs primarily in the rhizosphere, exactly the same location as plant uptake, while processes contributing to nutrient losses predominate in bulk soil (Smith and Tiedje, 1979). Under conditions of surplus N additions, increased denitrification can also occur in the rhizosphere (Smith and Tiedje, 1979). The reduction of plant‐driven sinks in space and time combined with the emphasis on supplying soluble, inorganic nutrients creates a ‘‘fertilizer treadmill’’ that promotes the requirement for chronic surplus additions of Fi. In essence, it is the management framework that resulted from the transition to Fi that has created agricultural systems which inadvertantly maximize nutrient saturation in space and time. The concept of ecosystem‐ scale N saturation was originally applied to forests receiving anthropogenic N deposition when the N additions exceed the capacity of the ecosystem to cycle or store N internally (Aber et al., 1989; Agren and Bosatta, 1988; Fenn et al., 1998). For the purposes of our discussion of agroecosystems, we expand the concept to apply to both N and P when availability exceeds the
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capacity of the ecosystem to cycle or store the nutrients in internal reservoirs that can be accessed by plants or microorganisms. It is not surprising that the degree of nutrient saturation is greatest in intensive annual crop production systems. In these systems, NO3 leaching is strongly correlated to N additions (Fig. 2A) indicating that other potential sinks have reached steady state conditions. Mesocosm studies following the fate of 15N from organic versus inorganic sources support the idea that microbial assimilation of Fi in bulk soil is C limited (Azam et al., 1985; Hodge et al., 1999). The fate of soluble P is also influenced by C abundance (Kouno et al., 2002) because geochemical processes leading to the adsorption and precipitation of P into occluded pools are reduced when C is available to drive microbial assimilation of P. Agronomic 15N studies indicate that strategies targeting improved crop uptake of Fi do increase the proportion of Fi assimilated by the cash crop while permitting farmers to reduce application rates (Mackown and Sutton, 1997; Tran et al., 1997). However, examination of the fate of 15N fertilizer in these improved management regimes raises questions about whether this strategy will lead to production systems that approach a balanced steady state. For example, in a recent study, split applications of 15N fertilizer to wheat achieved comparable yields with less applied N and reduced N fertilizer losses (Matson et al., 1998). However, since the proportion of fertilizer exported in the wheat increased while proportional N losses remained the same, the amount of fertilizer remaining in the soil and returned as wheat stubble/roots decreased with reduced N additions (Fig. 2B). Reductions in the quantity of fertilizer N entering the internal N cycle would lead to declines in organic soil N reservoirs and could ultimately reduce yields since the wheat crop obtained about the same amount of N from the soil in both treatments.
IV. TOWARD AN ECOSYSTEM‐BASED APPROACH TO IMPROVING NUTRIENT USE EFFICIENCY Restoration of ecosystem function and the recoupling of C, N, and P can best be accomplished by managing ecosystem processes at a variety of temporal and spatial scales to reduce the need for chronic additions of surplus nutrients. Landscape level strategies that integrate wetlands and other types of riparian buVers into agricultural landscapes are very eVective in protecting sensitive natural ecosystems and are a key strategy in ecosystem‐ scale nutrient management (Lowrance, 1992; Mitsch et al., 2001). Incorporation
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Figure 2 Fate of N relative to N additions across a range of ecosystems. (A) NO 3 leaching losses relative to N additions in annual cropping systems, forests, and a pasture (Brandi‐Dohrn et al., 1997; Fenn et al., 1998; Jemison and Fox, 1994; Kanwar et al., 1997; McCracken et al., 1994; Randall and Iragavarapu, 1995; Randall et al., 1997; Staver and Brinsfield, 1998; Steinheimer et al., 1998). Linear regression and 95% confidence intervals for annual rotations without cover crops is shown. The graph includes leaching studies that met the following criteria: (1) studies conducted in North America in annual rotations that included corn, (2) at least 2 years of data from treatments that had been in place for at least 1 year before data collection began, (3) year‐ round collection of leachate from below the root zone, and (4) medium soil textures (clay loam or silt loam to silty‐clay loam). (B) Fate of fertilizer‐derived N applied to wheat as a single application of 250 kg ha1 compared to split applications totaling 180 kg ha1. Modified after Matson et al. (1998) with additional data provided by the authors to L.E.D.
of strategically located plant communities that can act as nutrient sinks in managed landscapes can eVectively capture particulate and soluble nutrients before they reach adjacent waterways or aquatic ecosystems (Mitsch et al., 2001).
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Conceptual framework for an ecological approach to nutrient management
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To complement these strategies, nutrient use eYciency within agroecosystems themselves must also be improved. Our conceptual framework for an ecosystem‐based approach is outlined in Fig. 3. A key feature of this conceptual model is the overarching goal of developing cropping systems that approach steady states where yields and soil reservoirs are maintained with nutrient inputs that are approximately equal to harvested exports (Fig. 3). This will require management of a wider array of ecosystem processes that govern internal cycling capacity such as decomposition, microbial assimilation, biologically mediated weathering, microbially mediated N and P transformations, and soil aggregate formation. Recoupling C, N, and P cycles will reduce the need for chronic additions of surplus nutrients by increasing the sink strength of retention pathways. As a first step, the full range of organic and inorganic nutrient reservoirs must be considered, with the goal of enhancing those with longer MRTs that can be accessed by microorganisms and plants (Fig. 3). Diversification of N and P inputs is an important means of building these pools, through greater use of recycled organic residues, biological N fixation (BNF), and mineral forms of P such as apatite. Soluble Fi should be managed to enhance assimilation of N and P in biologically regulated sinks through both plant‐driven and microbially driven processes. This framework expands the focus of nutrient management to include a variety of sinks in addition to crop uptake and to explicitly target internal as well as external sources. Enhancing biologically mediated N and P reservoirs will have long‐term and cascading impacts on the internal cycling capacity agroecosystems. For example, organic P reservoirs are challenging to measure and not generally considered to be significant in supplying P to crops. As a result, organic P is not measured during routine estimates of soil P availability, although total organic P is sometimes calculated as the diVerence between total soil P and extractable P. However, if we manipulate the relative abundance of C and P in the soil, the small organic pools with a rapid turnover rate will
Figure 3 Conceptual model for the current nutrient management strategy (blue background) compared to our proposed ecosystem‐based framework (green background). Arrow colors indicate dominant elemental fluxes as follows: N (blue), P (green), N and P (blue–green), and C (gray). (A) Under current practices, N and P are added primarily as soluble, plant‐available forms. The majority of these inputs are lost either through leaching (mainly N, some P), gaseous losses (N), P fixation into occluded pools, and erosion (N and P). Soluble pools are relatively large while microbial biomass and SOM pools are reduced. (B) Shifting the focus to management of pools with longer MRTs including the range of SOM pools and sparingly soluble P would reduce standing pools of soluble inorganic N and P, increase microbially mediated assimilation and mineralization and reduce nutrient losses. Nutrient sources are diversified (BNF, organic residues, sparingly soluble rock PO 4 and reduced amounts of soluble Fi). In both cases, labile C is exchanged to access soil reservoirs, however, this mechanism is actively promoted in our ecologically driven framework.
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compete with geochemical pathways of P adsorption and absorption that lead to P occlusion. As a result, more P would remain in organic reservoirs which can be accessed by plants either directly or through collaboration with microbes. The few published studies that have examined agroecosystem‐scale functions in diversified cropping systems designed to promote linkages between C, N, and P cycles are promising. In these systems, productivity was maintained while nutrient balances were improved and internal nutrient reserves increased (Blake et al., 2000; Clark et al., 1998; Drinkwater et al., 1998; Gregorich et al., 2001). A greater proportion of total N inputs was accounted for, either as harvested exports or as N stored in the soil, in diversified rotations using low C:N residues as N sources compared to simplified rotations managed with Fi (Clark et al., 1998; Drinkwater et al., 1998). Long‐term studies also indicate that P use by plants was much more eYcient if P was applied in balance with C availability (Blake et al., 2000) or when diversified rotations were used to increase the proportion of fertilizer P in biologically mediated pools (Bunemann et al., 2004b). When nutrient sources are manipulated, a smaller proportion of the nutrients added as organic residues or apatite is taken up by the crop, however, a greater proportion is retained in various soil pools and is available to the crop in subsequent years (Azam et al., 1985; Bundy et al., 2001; Hodge et al., 1999; Ladd and Amato, 1986). These studies suggest that a nutrient management strategy based on a broader range of ecosystem processes is worth further investigation. They also demonstrate that nutrient mass balance and storage in various soil reservoirs must be considered in conjunction with the current approach of estimating only the proportion of Fi harvested as crop yield to accurately judge the eYcacy of added nutrients. While this kind of information is limited, there are numerous examples of research that targets a single process either in natural or managed ecosystems that could contribute to an integrated strategy for nutrient management. Our discussion will focus on three areas with the greatest potential for contributing to this approach: (1) increased plant biodiversity, (2) plant–microbial interactions, and (3) microbially mediated processes.
A. USING PLANT DIVERSITY
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EYcient use of plant diversity to restore ecosystem functions will entail a more sophisticated approach than simply reinstating traditional rotations. Plants and their associated microbes regulate myriad processes which ultimately control ecosystem fluxes of C, N, and P (Eviner and Chapin, 2001; Fierer et al., 2001; Hooper and Vitousek, 1997; Wedin and Tilman, 1990). Intentional management of plant diversity based on the capacity of a species to contribute to ecosystem processes will help restore desired agroecosystem
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functions and can increase yields in systems where fertilizers are currently under applied (Drinkwater, 1999; Snapp and Silim, 2002; Vance et al., 2003). Indeed, the potential for a single plant species to significantly influence ecosystem function is large in agroecosystems since single species eVects tend to be more pronounced in ecosystems with limited biodiversity (Chapin et al., 2000; Hector et al., 1999), particularly when a missing functional group is added (Naeem and Li, 1997). Plant species diversity can be increased either by introducing additional cash crops or noncash crops, such as cover crops or intercrops, selected to serve specific ecosystem functions (noncash crops will hereafter be referred to as accessory crops). The most commonly identified functional roles used in classifying accessory crops are those relating to phenology (summer/winter annuals, perennial), productive potential, plant architecture, and the nature of symbiont requirements (N‐fixing, non‐N‐fixing). Plant species characteristics, such as litter biochemistry, root exudate composition, fine root turnover, and the characteristics of the rhizosphere environment, also influence a variety of processes that control C, N, and P cycling. Significant plant species eVects have been documented for decomposition dynamics and net mineralization of N and P (Fierer et al., 2001; Wedin and Tilman, 1990), aggregate formation (Angers and Mehuys, 1989; Haynes and Beare, 1997), ability to access nutrients such as Ca, Mg, and P from mineral sources (Johnson et al., 1997; Kamh et al., 1999; Marschner and Dell, 1994), and microbial community composition (Burke et al., 2002; Kennedy, 1999; Kent and Triplett, 2002) and function (Cheng et al., 2003). Replacing bare fallows with appropriate cover crops should be a top priority of nutrient management programs. A meta‐analysis of the literature showed that cover cropping reduced NO 3 leaching by an average of 70% without incurring any sacrifice in yield compared to conventional rotations where gaps between crops were maintained as bare fallows (Tonitto et al., 2006). While the potential for reducing NO 3 leaching has been determined under a variety of environments, other impacts have received limited attention. Bunemann et al. (2004a) showed that including crotelaria (Crotalaria grahamiana) in rotation with maize shifts 50% more P fertilizer into the microbial biomass compared to continuous maize. Some recent work has screened cover crop species based on root architecture and root growth rates to identify species with the greatest potential for scavenging NO 3 that had leached below the cash crop root zone (Thorup‐Kristensen, 2001). Depending on the biochemical composition, the additional litter from these plants remains in the ecosystem and enhances internal N and P supply through additions to labile SOM pools, such as particulate organic matter, which are decomposed during subsequent growing seasons (Ladd and Amato, 1986; Puget and Drinkwater, 2001) while also contributing to humified pools with much longer turnover times. Altering the timing of labile C inputs from root
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exudation may have cascading eVects on the microorganisms that regulate internal cycling processes and increase the conversion of C to microbial biomass (Anderson and Domsch, 1990; Jans‐Hammermeister et al., 1998). Selecting plant species for their ability to contribute to P availability is an exciting possibility that is being studied in tropical systems where P is often the major limiting nutrient. In pot experiments aimed at screening a large number of legume species for P cycling attributes lupin (Lupinus albus) and pigeonpea (Cajanus cajan) were the most eVective legumes at (1) excreting organic acids and (2) enhancing P bioavailability for subsequent maize crops (Kamh et al., 1999). Long‐term studies comparing grass versus legume‐grass pasture systems also show larger reservoirs of labile organic P (Oberson et al., 2001). To assess the full value of accessory crops, contributions to other ecosystem functions, such as enhanced disease suppression (Abawi and Widmer, 2000), reduced weed competition and herbicide requirements (Gallandt et al., 1999), and pesticide reductions due to beneficial arthropod communities (Lewis et al., 1997), need to be considered.
B. RESTORATION OF ECOSYSTEM FUNCTION THROUGH PLANT–MICROBIAL INTERACTIONS Agriculture has a long history of research aimed at understanding how to improve the eVectiveness of root symbionts such as rhizobia and mycorrhizae (Kiers et al., 2002). Plant–mycorrhizal associations are the major mechanism for P uptake in over 80% of plant species. In low‐fertility soils they also þ enhance uptake of NO 3 and NH4 (Marschner and Dell, 1994). Ectomycorrhizal symbiosis in perennial horticultural systems and rhizobia–legume symbioses are routinely promoted (Graham and Eissenstat, 1994; Peoples et al., 1995) yet in general, agricultural production practices appear to have inadvertently reduced diversity, function, and eYciency in these symbioses, shifting a mutalistic, association to a parasitic relationship in some instances (Daniell et al., 1998; Denison et al., 2003; Johnson et al., 1997). A promising approach based on understanding how natural selection regulates changes in mutualistic interactions has been proposed (Denison et al., 2003; Johnson et al., 1997; Kiers et al., 2002). Knowledge of basic evolutionary processes could be used to develop agricultural management practices that favor the most eVective symbionts. Management of the exchange of C from primary producers to decomposers in return for nutrients has not been attempted in agroecosystems, despite the opportunity aVorded by the rhizosphere as the site of this mutual codependency between decomposers and plants (Naeem et al., 2000; Wall and Moore, 1999). Plants can stimulate decomposition of organic substrates
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by supplying labile C to decomposers in the rhizosphere (Cheng et al., 2003; Clarholm, 1985; Hamilton and Frank, 2001). The identity of the SOM pools accessed through this mechanism remains unknown, however, decomposition of chemically recalcitrant substrates is accelerated in the rhizosphere (Siciliano et al., 2003). The rate of decomposition and N mineralization varies with plant species (Cheng et al., 2003), rhizosphere community composition (Chen and Ferris, 1999; Clarholm, 1985; Ferris et al., 1998), and nutrient availability (Liljeroth et al., 1994; Tate et al., 1991). Net mineralization does not simply depend on a surplus of nutrients relative to C during decomposition but is enhanced by the involvement of secondary consumers feeding on the primary decomposers due to diVerences in the stoichiometry between the two trophic levels (Chen and Ferris, 1999; Clarholm, 1985; Ferris et al., 1998). This trophic cascade provides a mechanism for the primary producers to influence nutrient mineralization analogous to the so‐called ‘‘microbial loop’’ in aquatic ecosystems where primary producers often increase excretion of soluble C under nutrient‐limiting conditions (Berman and Dubinsky, 1999; Elser and Urabe, 1999). Under these conditions, food web structure is a significant regulator of nutrient availability and can determine whether the primary producers are N or P limited (Elser and Urabe, 1999). There is growing evidence that plants can influence the rate of net N mineralization through this mechanism, based on their need for nutrients by modifying the amount of soluble C excreted into the rhizosphere (Hamilton and Frank, 2001). Greater reliance on plant‐mediated mineralization for nutrient acquisition in agroecosystems would reduce the potential for nutrient losses due to the tight coupling between net mineralization of N and P and plant uptake in the rhizosphere. Inorganic nutrient pools can be extremely small while high rates of net primary productivity (NPP) are maintained if N mineralization and plant assimilation are spatially and temporally connected in this manner (cf. Jackson et al., 1988). To eVectively manage this process, many questions remain to be answered. In particular, understanding which SOM pools are being accessed by plant‐mediated decomposition will be key as will the development of strategies to manage agroecosystems to increase these reservoirs while minimizing net mineralization in the absence of plants. Other aspects such as food web structure could also be influenced by management to optimize this process.
C.
MICROBIALLY MEDIATED PROCESSES
Microorganisms represent a substantial portion of the standing biomass in terrestrial ecosystems and contribute to the regulation of C sequestration, N availability and losses, and P dynamics. The amount of N and P in soil
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prokaryotes is nearly equal to the amount in terrestrial plants (Whitman et al., 1998). For cultivated systems, the N and P in soil prokaryotes in the top meter of soil is estimated to average 630 and 60 kg ha1, respectively (Whitman et al., 1998). Microbial biomass P turnover is rapid, approximately twice as fast as C (Kouno et al., 2002), suggesting the potential for microbial P pools to support plant P requirements may have been markedly underestimated. The size and physiological state of the standing microbial biomass are influenced by management practices, including rotational diversity (Anderson and Domsch, 1990), tillage (Holland and Coleman, 1987), and the quality and quantity of C inputs to the soil (Fliessbach and Mader, 2000; Lundquist et al., 1999; Wander and Traina, 1996). We see exciting possibilities for influencing microbially mediated processes. Increased knowledge of the environmental physiology of soil microbes would greatly enhance our understanding of the relationship between management and microbial community function and support intentional manipulation of microbial functional groups in favor of desired outcomes. During decomposition, microbial community composition and metabolic status determine the balance between C respired and C assimilated into biomass. Management strategies such as reduced tillage foster increased abundance of fungal decomposers and can lead to increased C retention (Holland and Coleman, 1987). Heterotrophs in soils with greater plant species diversity or greater abundance of C relative to N appear to convert a greater proportion of metabolized C to biomass (Aoyama et al., 2000; Fliessbach et al., 2000). The new molecular tools that make it possible to characterize abundance and activity of microbial functional groups open up new possibilities for intentional management of the microbial community to enhance N retention. Cavigelli and Robertson (2000, 2001) discovered that denitrifiers from an agricultural soil were more sensitive to O2 levels and produced a greater proportion of N2O compared to denitrifiers from an early successional plant community. Denitrifier community composition influenced both the rate of denitrification and the proportion of N2O to N2 produced. A second anaerobic NO 3 pathway, dissimilatory nitrate reduction to ammonium (DNRA), occurs in a variety of unmanaged terrestrial ecosystems (Silver et al., 2001) and could also be manipulated to enhance N conservation. Previously this process was thought to be limited to extremely anaerobic, C‐rich environments such as sewage sludge and submerged sediments (Maier et al., 2000). Silver et al. (2001) reported average rates of DNRA were threefold greater than denitrification in humid tropical forest soils and concluded that the resulting reduction in NO3 availability to denitrifiers and leaching may contribute to N conservation in these ecosystems. The presence of microbes capable of DNRA in agricultural systems has yet to be determined, but there is no reason to expect this process to be excluded from managed ecosystems. Agricultural soils with management‐induced increases in labile C pools have a greater NHþ 4:NO 3 ratio compared to soils where C is
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less abundant (Drinkwater et al., 1995) suggesting DNRA may be an important N‐conserving process in agroecosystems that could be enhanced through management of appropriate soil organic C pools. Microorganisms access P through several mechanisms that ultimately lead to increased P availability for primary producers (Illmer et al., 1995; Oberson et al., 2001). Direct excretion of phosphatase enzymes is an important mechanism for releasing P from recalcitrant organic forms such as phytic acid. Sparingly soluble P is made available through organic acid excretion, such as occurs in the soil fungus Penicillium radicum, isolated from a low‐P rhizosphere of unfertilized wheat (Whitelaw et al., 1999). In this system, PO 4 solubilization from insoluble or sparingly soluble complexes with metals was related to titratable acidity and gluconic acid concentration. Organic acid excretion not only alters pH, but also may chelate Al3þ or other cations directly, further enhancing the solubilization of PO 4 (Erich et al., 2002; Laboski and Lamb, 2003). An incubation study investigating the microbial community structure in legume‐maize cropping system found that fungal and gram‐negative bacteria abundance tracked was correlated with the organic P pool, and that the major driver determining the size of the labile organic P pool was crop rotation and the presence of plant residues with high soluble C content (Bunemann et al., 2004a).
V. PLANT ADAPTATION TO ECOSYSTEM‐BASED NUTRIENT MANAGEMENT In the last half‐century, plant breeding has occurred almost entirely under management regimes that include fumigated soils with luxurious additions of nutrients and suYcient water (Boyer, 1982) and has produced modern hybrids well adapted to a microbially deficient soil environment with abundant resources. This strategy of reducing environmental variation by providing ample resources reduces gene by environment interaction and enhances the power of selection for specific traits (Banziger and Lafitte, 1997). However, it has potentially selected against traits that allow plants to maintain high NPP and yields under nonsaturating nutrient conditions (Jackson and Koch, 1997) and has contributed to the need for surplus nutrient additions. The impact of conventional plant selection practices on belowground characteristics has rarely been investigated, although interest in this area is growing. In lettuce, comparison of wild genotypes and modern cultivars demonstrated a marked decrease in root system nutrient‐scavenging ability due to altered root architecture and reduced plasticity (Jackson, 1995). Cultivar eVects on root‐associated microorganisms have been found more often than not, and there is now considerable evidence that rhizosphere community
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composition varies at the cultivar level in agroecosystems (Briones et al., 2002; Dalmastri et al., 1999; Germida and Siciliano, 2001; Siciliano et al., 1998). Modern wheat varieties have rhizosphere communities distinct from preindustrial varieties (Germida and Siciliano, 2001). In most cases, the functional significance of these diVerences in plant‐associated microbial communities is not known. A notable exception is the case of traditional versus modern rice cultivars where plant‐induced species diVerences in rhizoplane NHþ 4 ‐oxidizing bacteria appear to account for the greater nitrification rates in the rhizosphere of the modern variety (Briones et al., 2002, 2003). Clearly, plant selection for high yields has had unintended cascading eVects on the associated microorganisms and biogeochemical functions. These examples demonstrate that plants do not exist as single organisms, but are more accurately viewed as a consortium consisting of a primary producer and many species of associated microbes. We see many opportunities for plant breeding to enhance plant–microbial–nutrient interactions in ways that contribute to restored ecosystem functions through some of the mechanisms we have highlighted in this chapter. Crop‐breeding programs should select for well‐adapted consortia that can achieve necessary yields by accessing nutrient reservoirs less susceptible to loss. Criteria for selection of cash crops should be expanded to include contributions to ecosystem function. For example, crop species with unique ecosystem functions such as N‐fixers should be selected under conditions that will enhance these abilities while optimizing yields. On the other hand, accessory crops can be selected to enhance their capability to provide ecosystem services while filling specific niches in space and time that are compatible with cash crop production. There are a few examples of breeding programs intentionally aimed at improving yields under conditions where nutrients are not saturating. A marked increase in N derived from BNF and yield adaptation to low‐input, N‐limited cropping systems was recently achieved in the Brazilian soybean‐ breeding program (Alves et al., 2003). More modest success was achieved by the seminal collaboration of a breeder, microbiologist, and plant physiologist to improve BNF in alfalfa (Barnes et al., 1984; Jessen et al., 1988). Research to improve yield potential of cereal grains in low‐nutrient environments has been sporadic, with mixed results until a recent concerted eVort showed that it is possible to improve yields of wheat and corn in low‐input environments (Banziger and Cooper, 2001).
VI. CONCLUSIONS While the concept of sustainability as a goal has become widely accepted, the dominant agricultural paradigm still considers high yields and reduced environmental impacts to be in conflict with one another (Keller and Brummer, 2002).
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The requirement for nutrient saturation in agriculture will not be easy to overcome given the economic constraints imposed on production‐oriented farming systems. Nutrient surpluses in agroecosystems, and hence environmental losses, can be reduced through intentional management of intrinsic ecosystem processes. Our approach shifts the emphasis of nutrient management away from soluble, inorganic plant‐available pools to organic and mineral reservoirs that can be accessed through microbially and plant‐ mediated processes. The goal of nutrient management under this framework would be to balance nutrient budgets as much as possible while maintaining these reservoirs. Management practices that increase the sink capacity of the ecosystem in ways that contribute to reduced needs for surplus additions should be emphasized in conjunction with breeding for cultivars and their associated microorganisms that do not require surplus additions of soluble nutrients. Understanding how plants alter microbial community structure and function to facilitate access to organic and mineral reservoirs (i.e., decomposition and assimilation of N and P from recalcitrant organic polymers, P mobilization from mineral reservoirs) will be an important consideration for cash crops. On the other hand, for accessory crops, understanding assimilatory processes that compete with P geochemical sinks and N loss pathways will be particularly important. The critical question remains, what productivity levels can be supported by this approach? We do not believe that this question can be fully answered based on available research. Levels of NPP comparable to those achieved in high‐yielding agricultural systems occur in unmanaged ecosystems (Zak et al., 1994) and monocultures composed of weedy species (Abul Fatih et al., 1979). Perhaps a more appropriate question is: What proportion of annual NPP would need to be allocated to provide internal ecosystem services while still meeting the goals for harvested exports? This will depend on ecosystem state factors such as climate, topography, and parent soil material, as well as socioeconomic constraints. Many climates with suYcient water and a long growing season support very high annual NPP. These conditions increase the potential for a portion of the NPP to be devoted to the provision of ecosystem functions. In contrast, areas with severe water, light, or temperature limitations, such as the arid tropics or cold temperate zones, will likely require strategic inputs of inorganic nutrients. It follows that an additional requirement for the success of this approach will be to match production systems with the strengths and limitations of the environment.
ACKNOWLEDGMENTS We thank Mark David, Valerie Eviner, Jørgen Olesen, and David Wolfe for helpful comments on an earlier draft. We also thank the many agricultural professionals, including farmers, who have shared their real world perspective
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with us over the years. This work was supported by NSF‐BE/CBC #0216316 to L.E.D. and others.
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B. A. M. Bouman,1 E. Humphreys,2 T. P. Tuong1 and R. Barker3 1 International Rice Research Institute, Los Ban˜os, Philippines CSIRO Land and Water, PMB 3 Griffith, NSW 2680, Australia 3 International Water Management Institute, Colombo, Sri Lanka 2
I. Introduction II. Trends and Conditions A. The Rice Plant B. Rice Environments C. Water Use and Water Productivity D. Ecosystem Services E. Environmental Impacts F. The Main Challenges Ahead III. Response Options A. Varietal Improvement B. Improved Management Practices C. Options at the Landscape Level IV. Summary and Recommendations Acknowledgments References
The Comprehensive Assessment of Water Management in Agriculture (CA) seeks answers to the question of how freshwater resources can be developed and managed to feed the world’s population and reduce poverty, while at the same time promoting environmental security. The CA pays particular attention to rice as this crop is the most common staple food of the largest number of people on Earth (about 3 billion people) while receiving an estimated 24–30% of the world’s developed freshwater resources. Rice environments also provide unique—but as yet poorly understood—ecosystem services such as the regulation of water and the preservation of aquatic and terrestrial biodiversity. Rice production under flooded conditions is highly sustainable. In comparison with other field crops, flooded rice fields produce more of the greenhouse gas methane but less nitrous oxide, have no to very little nitrate pollution of the groundwater, and use relatively little to no herbicides. Flooded rice can locally raise groundwater tables with subsequent risk of salinization if the groundwater carries salts, but is also an effective restoration crop to leach accumulated salts from the soil in combination with drainage. The production of rice needs to increase in the coming decades to meet the food demand of growing populations. To meet the dual challenges of producing enough food and alleviating poverty, more rice needs to be produced at a low 187 Advances in Agronomy, Volume 92 Copyright 2007, Elsevier Inc. All rights reserved. 0065-2113/07 $35.00 DOI: 10.1016/S0065-2113(04)92004-4
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B. A. M. BOUMAN ET AL. cost per kilogram grain (ensuring reasonable profits for producers) so that prices can be kept low for poor consumers. This increase in rice production needs to be accomplished under increasing scarcity of water, which threatens the sustainability and capability to provide ecosystem services of current production systems. Water scarcity is expected to shift rice production to more water‐abundant delta areas, and to lead to crop diversification and more aerobic (nonflooded) soil conditions in rice fields in water‐short areas. In these latter areas, investments should target the adoption of water‐saving technologies, the reuse of drainage and percolation water, and the improvement of irrigation supply systems. A suite of water‐saving technologies can help farmers reduce percolation, drainage, and evaporation losses from their fields by 15–20% without a yield decline. However, greater understanding of the adverse effects of increasingly aerobic field conditions on the sustainability of rice production, environment, and ecosystem services is needed. In drought‐, salinity‐, and flood‐prone environments, the combination of improved varieties with specific management packages has the potential to increase on‐farm yields by 50–100% in the coming 10 years, provided that investment in research and # 2007, Elsevier Inc. extension is intensified.
I. INTRODUCTION Rice is eaten by about 3 billion people and is the most common staple food of the largest number of people on earth (Maclean et al., 2002). In the 1960s, the combination of new high‐yielding varieties with increased input use, such as water, fertilizer, and biocides (agrochemicals to protect the crop from pests, diseases, and weeds), initiated a rapid increase in productivity that is called the Green Revolution (Khush, 1995). Because of this increased productivity, and an increase in cropped area, total rice production in the first two decades of the Green Revolution more than kept pace with the tremendous growth in population in Asia (Fig. 1). The increased productivity and profitability also contributed to food security and poverty reduction among farmers with irrigated land (Dawe, 2000). The growth in rice production outstripped growth in population, thus lowering prices, which reduced the daily expenses for food of poor consumers such as the rural landless, urban laborers, fishers, and farmers of nonrice crops. World rice prices (adjusted for inflation) fluctuated around US$1000 t1 between 1961 and 1981, saw a sharp decline between 1981 and 1984, and then a gradual decline until a record low around US$250 t1 in 2002 (Fig. 1). The world rice price is now just 25% of its level in the early 1980s and national prices show a similar trend. However, whereas the low price of rice has benefited rice consumers, it now threatens the livelihoods of rice farmers, the very segment of the population that helped to alleviate poverty in the first place.
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189 2250 2000
140 Production
1750
120
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100
1250 80 1000 60 40
750 Price
500
20
World rice price (US$ per t)
Per capita production (kg per person)
160
250 0
0 1961 1965 1969 1973 1977 1981 1985 1989 1993 1997 Year
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Figure 1 World market rice price and Asian per capita rice production. Rice export prices were adjusted for inflation (2000 ¼ 100). Source: FAOSTAT on‐line statistical service, available online at http://faostat.fao.org/faostat/collections; World Rice Statistics, available online at http://www.irri. org/science/ricestat/index.asp; International Financial Statistics, International Monetary Fund, 2005.
Because rice is mostly grown under flooded, or submerged, conditions, it is also one of the biggest users of the world’s developed freshwater resources (Tuong and Bouman, 2003). However, water is becoming increasingly scarce and grave concerns exist about the sustainability of irrigated agriculture (Rijsberman, 2006). The Comprehensive Assessment of Water Management in Agriculture (www.iwmi.cgiar.org/assessment) seeks answers to the question of how water can be developed and managed to feed the world’s population and reduce poverty, while at the same time promoting environmental security. To answer this question, it synthesizes existing knowledge and stimulates thought on ways to manage water resources to continue meeting the needs of both humans and ecosystems in the future. The results will enable better investment and management decisions in water and agriculture in the near future. Because of the importance of rice and the large amount of water used to grow it, the Comprehensive Assessment pays particular attention to the relationship between rice and water, between the rice–water interface and food security and poverty alleviation, and between rice ecosystems and the environment. This chapter presents the scientific analysis underpinning that assessment. In the first part, past and current trends and conditions are synthesized and the major challenges in the 25 years ahead are identified. The second part reviews response options to these challenges, at the levels of plant, field, and irrigation system. The analysis focuses on Asia, where some 90% of the world’s rice is produced and consumed (Maclean et al., 2002), complemented by information from other regions where this adds to our understanding of the relationships between rice and water.
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II.
TRENDS AND CONDITIONS A. THE RICE PLANT
Cultivated rice evolved from a semiaquatic, perennial ancestor, and rice separated evolutionarily from other Gramineae before grasses moved from the forest floor to more open habitats (Lafitte and Bennett, 2002). Like other cereals such as wheat and barley, rice belongs to the so‐called group of C3 grasses, whereas cereals like maize and sorghum belong to the group of C4 grasses. The C4 species have a more eYcient photosynthetic pathway than the C3 species, and produce more biomass per unit intercepted radiation and per unit transpiration. The wetland ancestry of rice is reflected in a number of morphological and physiological characteristics that are unique among crop species. Key diVerences between rice and other cereals include shoot and root anatomy, water loss patterns, and growth responses to soil water status drier than saturation (Lafitte and Bennett, 2002). Rice leaves are generally thin, with no diVerentiation of mesophyll cells into palisade and spongy parenchyma. The number of stomata is tenfold greater than in the leaves of dryland grasses, stomata are small in size, and cultivars diVer in the eVect of environment on stomatal frequency. Rice cuticles are thin, with less than 5% of the wax load of other crops, and cuticular resistance is comparatively low. Rice panicles are also characterized by thin cuticles. About 30% of the water transpired by lowland rice after flowering is lost through panicles. Rice is extremely sensitive to water shortage. When the soil water content drops below saturation, growth and yield formation are aVected, mainly through reduced leaf surface area, photosynthesis rate, and sink size (Bouman and Tuong, 2001; Yoshida, 1981). Leaf area expansion is reduced as soon as the soil dries below saturation in most cultivars, and when only about 30% of the available soil water has been extracted in cultivars with aerobic adaptation (Lilley and Fukai, 1994; Wopereis et al., 1996). Stomatal closure begins at higher leaf water potential than in other crops, and transpiration declines gradually starting at about 0.75 MPa (Dingkuhn et al., 1989). Water stress also induces leaf rolling and accelerated leaf senescence (Turner et al., 1986). The reduction in leaf area (by reduced leaf expansion, rolling, and senescence) results in reduced light interception, which reduces total crop photosynthesis and hence total biomass production. The closure of stomata reduces not only transpiration but also leaf photosynthesis, which reduces crop radiation‐use eYciency and hence again total biomass production. Rice is very sensitive to reduced water availability in the period around flowering as this greatly aVects spikelet sterility (Cruz and O’Toole, 1984; Ekanayake et al., 1989). At anthesis, there is a short time span when spikelet fertility is especially sensitive to drought.
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Although rice is adapted to waterlogging, complete submergence can be lethal. Most rice varieties can survive complete submergence of only 3–4 days although some rainfed lowland rice varieties can survive up to 10 days (Maclean et al., 2002). Tall plants tend to lodge when the water level recedes, resulting in additional yield losses and poor grain quality. Rice is a salt‐sensitive crop and yield reductions start at electric conductivity values in the soil of 3 dS m1, going up to 50% at 6 dS m1, and 90% at 10 dS m1 (Shannon, 1997). For comparison, yields of maize decline at around 2 dS m1, of wheat at 6 dS m1, and of barley at 8 dS m1. Rice is relatively tolerant of salinity during germination, active tillering, and toward maturity, but is sensitive during early seedling and reproduction. Tolerance at one stage does not correlate with tolerance at another stage. Salt stress aVects rice through osmotic stress, salt toxicity, and nutrient imbalances.
B. RICE ENVIRONMENTS Worldwide, there are about 150 million ha of rice land (Table I). Rice is unique among the major food crops in its ability to grow in a wide range of hydrological situations, soil types, and climates (Fig. 2) (Huke and Huke, 1997; Maclean et al., 2002). Depending on the hydrology of where rice is grown, the rice environment can be classified into irrigated lowland rice, rainfed lowland rice, flood‐prone rice, and upland rice. Irrigated lowland rice is grown in bunded fields with assured irrigation for one or more crops per year. Usually, farmers try to maintain 5–10 cm of water (floodwater) on the field. Rainfed lowland rice is grown in bunded fields that are flooded with rainwater for at least part of the cropping season to water depths that exceed 100 cm for no more than 10 days. In both irrigated and rainfed lowlands, fields are predominantly puddled (i.e., wet land preparation), with transplanting as the conventional method of crop establishment. However, puddling is not a prerequisite for rice production; for example, puddling is not practiced in some of the world’s highest‐yielding rice production systems such as in Australia and California. In flood‐prone environments, fields suVer periodically from excess water and uncontrolled deep flooding. Deepwater rice and floating rice are found in these environments. Upland rice is grown under dryland conditions (no ponded water) without irrigation and without puddling, usually in nonbunded fields. 1.
The Irrigated Environments
Worldwide, there are about 79 million ha of irrigated lowland rice, which provide 75% of the world’s rice production (Maclean et al., 2002). Approximately 56% of the world’s irrigated area of all crops is in Asia, where rice
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Table I Rice Production and Consumption Statistics Worldwide, 2002
Country China India Indonesia Bangladesh Vietnam Thailand Myanmar Philippines Japan Brazil United States Pakistan Korea, Republic of Egypt Nepal Cambodia Nigeria Iran, Islamic Republic of Sri Lanka Madagascar Laos Colombia Malaysia Korea, DPR Peru Italy Ecuador Australia Coˆte d’Ivoire World
Production (103 t)
Area (103 ha)
Yield (t ha1)
Consumption (kg capita1 year1)
Calories from rice in diet (%)
176,342 116,500 51,490 37,593 34,447 26,057 21,805 13,271 11,111 10,457 9569 6718 6687
28,509 40,280 11,521 10,771 7504 9988 6381 4046 1688 3146 1298 2225 1053
6.19 2.89 4.47 3.49 4.59 2.61 3.42 3.28 6.58 3.32 7.37 3.02 6.35
83 83 149 164 169 103 205 105 58 35 9 18 83
28 34 50 74 65 41 68 43 22 12 3 8 29
6105 4133 3823 3192 2888
613 1545 1995 3160 611
9.97 2.67 1.92 1.01 4.73
38 102 149 24 37
12 38 69 9 12
2859 2604 2417 2348 2197 2186 2119 1379 1285 1192 1080 577,971
820 1216 783 469 677 583 317 219 327 150 470 147,633
3.49 2.14 3.09 5.01 3.25 3.75 6.69 6.31 3.93 7.95 2.30 3.91
91 95 168 30 73 70 49 6 47 10 63 57
37 49 64 12 25 32 19 2 16 3 22 20
Data only for countries producing more than 1 million t of rice annually. Area refers to harvested area and includes multiple cropping. Yield is unhulled and unmilled rice, consumption is milled rice. Yield is calculated as total production divided by total area, and is thus an average across all rice environments and across all seasons in which rice is grown in the country. Source: FAOSTAT on‐line statistical service, last updated July 14, 2005 (available on‐line at http:// faostat.fao.org/faostat/collections).
accounts for 40–46% of the net irrigated area of all crops (Dawe, 2005). In Southeast Asia, rice occupies 64–83% of the irrigated area, in East Asia 46–52%, and in South Asia 30–35%. At the field level, rice receives up to two to three times more water than other irrigated crops (Tuong et al., 2005;
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see Section II.C), but an unknown proportion of the water losses from individual fields is reused by other fields downstream. Assuming a reuse fraction of 25%, it can be calculated that irrigated rice receives an estimated 34–43% of the total world’s irrigation water. With irrigation accounting for about 70% of the world’s developed freshwater resources, irrigated rice receives a share of 24–30%. Irrigated rice is mostly grown with supplementary irrigation in the wet season, and is entirely reliant on irrigation in the dry season. The proportion of the Asian rice area that is irrigated (excluding China, where essentially all rice is irrigated) increased substantially from the late 1970s (35%) to the mid‐ 1990s (44%) (Dawe, 2005). This occurred because of an increase in the irrigated area coupled with a large decline in upland and deepwater rice cultivation. In many irrigated areas, rice is grown as a monoculture with two crops per year. However, significant areas of rice are also grown in rotation with a range of other crops, including about 15–20 million ha of rice–wheat systems (Dawe et al., 2004; Timsina and Connor, 2001). At the turn of the millennium, country‐average irrigated rice yields in Asia ranged from 3 to 9 t ha1, with an overall average of about 5 t ha1 (Maclean et al., 2002). 2.
The Rainfed Environments
Worldwide, there are about 54 million ha of rainfed lowlands, which contribute 19% of the world’s total rice production, and 14 million ha of rainfed uplands, which contribute 4% of the world’s total rice production (Maclean et al., 2002). The rainfed rice environments experience multiple abiotic stresses and are characterized by high levels of uncertainty, particularly with regard to the timing, duration, and intensity of rainfall (Tuong et al., 2000). Approximately 27 million ha of rainfed rice are frequently aVected by drought, the largest and most frequently and severely aVected areas being eastern India (about 20 million ha) and northeastern Thailand and Laos (7 million ha) (Huke and Huke, 1997). Further constraints arise from the widespread incidence of problem soils with poor physical and chemical properties (Garrity et al., 1986). National‐average rice yields are currently only some 2.3 t ha1 in lowlands and 1 t ha1 in uplands (Maclean et al., 2002). a. Rainfed Lowlands. These are characterized by small to medium topographic diVerences, which have important consequences for water availability, soil fertility, and flooding risk (Tuong et al., 2000). The unpredictability of rainfall often results in field conditions that are too dry or too wet. Figure 2 Distribution of rice environments in Asia: irrigated lowland rice (A), rainfed lowland rice (B), upland rice (C), and flood‐prone (deepwater) rice (D). Source: GIS Laboratory, International Rice Research Institute, Philippines.
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Besides imposing water‐related stresses on crop growth, these conditions also prevent timely and eVective management operations such as land preparation, transplanting, weed control, and fertilizer application. If such operations are delayed or skipped, large yield losses often ensue, even though the plants may not suVer physiological water stress. b. Rainfed Uplands. Rainfed uplands are highly heterogeneous areas with climates ranging from humid to subhumid, soils from relatively fertile to highly infertile, and topography from flat to steeply sloping (Piggin et al., 1998). Historically, with low population density and limited market access, shifting cultivation with long fallow periods (more than 15 years) was the dominant land‐use system. Increasing population and improved market access have put pressure on these systems. Some 70% of Asia’s upland rice areas have made the transition to permanent systems where rice is grown every year. About 14% of the Asian upland rice area, mainly in Laos, northeastern India, and Vietnam, still practices shifting cultivation with shorter fallow periods (3–5 years). As market access remains limited, most of the world’s upland rice farmers tend to be self‐suYcient by producing a range of agricultural outputs.
3.
Flood‐Prone Environments
Flood‐prone environments include deepwater areas that are submerged at more than 100 cm for durations of more than 10 days to a few months, areas that are aVected by flash floods of longer than 10 days, extensive low‐lying coastal areas where plants are subject to daily tidal submergence, and areas with problem soils (acid sulfate and sodicity), where the problem is often excess water but not necessarily prolonged submergence (Maclean et al., 2002). Altogether, there are some 11 million ha of flood‐prone rice areas, with average yields of around 1.5 t ha1.
4.
Salinity‐Affected Environments
Salinity and/or alkalinity are most widespread in coastal areas, where salinity is predominant, and in inland areas, where both salinity and alkalinity are major problems and often coexist (Garrity et al., 1986). Both problems occur in irrigated as well as rainfed lowlands. In coastal areas, rice can suVer from salinity because of seawater intrusion during high tides. In inland areas, the sources of salinity are salt deposits inherently present in the soil or bedrock, or the use of saline irrigation water. In the mid‐1980s, the extent of rice‐growing areas aVected by salinity and/or alkalinity was estimated to be
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around 1.3 million ha (Garrity et al., 1986), but current estimates are on the order of 9–12 million ha, with 5–8 million ha in India, 1 million ha each in Vietnam, Bangladesh, and Thailand, and about 1 million ha in Myanmar and Indonesia combined (estimates from A. Ismail, R. K. Singh, G. Gregorio, and T. P. Tuong, IRRI). 5. Shifting Comparative Advantages Within Asia, the comparative regional advantage in rice production is shifting (Barker and Dawe, 2002; Dawe, 2005). Before World War II, the delta regions (in eastern India, Bangladesh, Thailand, Vietnam, Cambodia, and Myanmar) held a comparative advantage in rice production and were the main sources of rice exports. The early beneficiaries of Green Revolution technology were those areas where it was possible to irrigate two crops of rice with the construction of reservoir storage of water. Another area that benefited was the northwest Indo‐Gangetic Plain, where public investment in irrigation systems and private investment in groundwater pumping, together with favorable policies (subsidies for inputs and minimum price support schemes for grain), favored rice production. For political reasons and/or because of the inability to manage floods, the deltas initially were unable to take advantage of the new rice technologies. However, over the past 15–20 years, the delta areas have regained a comparative advantage with the aid of low‐cost pump technology and new cropping systems based on short‐ duration rice varieties that can escape floods (Dawe, 2005). During this period, the delta regions have shown the most rapid growth in rice production and exports. With improved water control provided by pumps, the delta regions have been able to shift out of deepwater and floating rice to more productive systems by planting one crop before and one after floods. In short, rice production is gaining in those regions with a plentiful water supply and cheap labor relative to areas of water scarcity. In the northwest Indo‐Gangetic Plain, concerns are now grave about the sustainability of irrigated rice production at current levels because of rapidly falling groundwater tables (Singh, 2000) and the need to reduce the large fiscal costs associated with government policies that promote rice production.
C. WATER USE
AND
WATER PRODUCTIVITY
About 90% of the world’s rice production is harvested from irrigated or rainfed lowland rice fields (also called ‘‘paddies’’). Usually, lowland rice is raised in a seedbed and then transplanted into a main field. Rice can also be established by direct wet seeding (broadcasting pregerminated seeds onto wet soil) or dry direct seeding (broadcasting dry seeds onto dry or moist soil)
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directly on the main field. After crop establishment, the main field is kept under continuous (irrigated) or intermittent (rainfed) ponded water conditions. Keeping the fields continuously flooded helps control weeds and pests. Wet land preparation for transplanted and wet‐seeded rice consists of soaking, plowing, and puddling (i.e., harrowing or rotavating under shallow submerged conditions). Puddling is done for weed control and to reduce soil permeability and percolation losses, and it eases field leveling and transplanting. Because of the flooded nature of lowland rice, its water balance and water productivity are diVerent from those of other cereals such as wheat.
1.
Water Flows from a Rice Field
Water for lowland rice is needed for land preparation and to match the outflows by seepage, percolation, evaporation, and transpiration during crop growth (Fig. 3). The amount of water used for wet land preparation can be as low as 100–150 mm when the time lag between soaking and transplanting is a few days only or when the crop is direct wet seeded. However, in large‐scale irrigation systems that have poor water control, the time lag between soaking and transplanting can go up to 2 months and water inputs for land preparation up to 940 mm (Tabbal et al., 2002). After crop establishment, the soil is usually kept ponded until shortly before harvest. Seepage is the lateral subsurface flow of water and percolation is the down flow of water below the root zone. Typical combined values for seepage and percolation vary from 1 to 5 mm day1 in heavy clay soils to 25–30 mm day1 in sandy and sandy loam soils (Bouman and Tuong, 2001). Evaporation is water released into the air as vapor from the ponded water layer or from the surface of the soil, and transpiration is water released as vapor by the plants. Typical combined evapotranspiration rates of rice fields are 4–5 mm day1 in the wet season and 6–7 mm day1 in the dry season, but can be as high as 10–11 mm day1 in subtropical regions before the onset of the monsoon (Tabbal et al., 2002). About 30% of combined evapotranspiration is evaporation and 70% transpiration (Bouman et al., 2005a). In direct‐seeded rice, the proportion of evaporation in the combined evapotranspiration is higher than in transplanted rice (Cabangon et al., 2002). In water‐seeded rice culture in temperate Australia, where the fields are ponded for about 5 months from presowing to the end of grain filling, about 40% of combined evapotranspiration is evaporation from the floodwater and 60% is transpiration (Simpson et al., 1992). Overbund flow or surface runoV is the spillover when water depths rise above the paddy bunds. Seepage, percolation, overbund flow, and evaporation are all nonproductive water flows and can be considered loss flows at the field level. Total seasonal water input to rice fields (rainfall plus irrigation) is up to two to three times more than for other cereals (Tuong et al., 2005). It varies
RICE AND WATER Figure 3 Water balance of a lowland (paddy) rice field. C, capillary rise; E, evaporation; I, irrigation; O, overbund flow; P, percolation; R, rainfall; S, seepage; T, transpiration.
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from as little as 400 mm in heavy clay soils with shallow groundwater tables that supply water for crop transpiration by capillary rise to more than 2000 mm in coarse‐textured (sandy or loamy) soils with deep groundwater tables (Bouman and Tuong, 2001; Cabangon et al., 2004). Around 1300–1500 mm is a typical value for irrigated rice in Asia. Outflows of water by seepage and percolation account for about 25–50% of all water inputs in heavy soils with shallow water tables of 20–50 cm depth (Cabangon et al., 2004; Dong et al., 2004), and 50–85% in coarse‐textured soils with deep water tables of 1.5‐m depth or more (Sharma et al., 2002; Singh et al., 2002). Although seepage and percolation are losses at the field level, they are often captured and reused downstream and do not necessarily lead to true water depletion at the irrigation area or basin scales. 2.
Water Productivity
Water productivity denotes the amount of marketable grain produced for each volume of water used, which can be taken as transpiration, evapotranspiration, irrigation, or irrigation plus rainfall. Modern rice varieties, when grown under flooded conditions, have water productivity with respect to transpiration (transpiration eYciency) similar to that of other C3 cereals such as wheat, at about 2 kg grain m3 water transpired (Bouman and Tuong, 2001; Tuong et al., 2005). The few available data indicate that water productivity with respect to evapotranspiration is also similar to that of wheat, ranging from 0.6 to 1.6 kg grain m3 of evapotranspired water, with a mean of 1.1 kg grain m3 (Tuong et al., 2005; Zwart and Bastiaanssen, 2004). Compared with wheat, the higher evaporation rates from the water layer in rice than from the underlying soil in wheat are apparently compensated for by the higher yields of rice. For maize, being a C4 crop, the water productivity with respect to evapotranspiration is higher, ranging from 1.1 to 2.7 kg grain m3 water, with a mean of 1.8 kg grain m3. Water productivity of rice with respect to total water input (irrigation plus rainfall) ranges from 0.2 to 1.2 kg grain m3 water, with 0.4 as the average value, which is about two times smaller than that of wheat (Tuong et al., 2005).
D. ECOSYSTEM SERVICES The benefits that people derive from ecosystems (defined as a dynamic complex of plants, animals, and microorganisms and the nonliving environment interacting as a functional unit) can be divided into (Millennium Ecosystem Assessment, 2005): Provisioning (e.g., fresh water and commodities such as food, wood, timber, and fuel)
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Regulating (e.g., water purification and climate, flood, and disease regulating) Supporting (e.g., nutrient cycling, soil formation, primary production) Cultural (e.g., aesthetic, spiritual, educational, recreational) Agriculture’s role in providing noncommodity outputs in ecosystems is also referred to as multifunctionality. Although only a few studies have been conducted so far, awareness is increasing that lowland rice environments provide an unusually rich variety of ecosystem services (PAWEES, 2005). The most important provisioning function of rice environments is the production of rice. Recent findings of 30 long‐term continuous cropping experiments at 24 sites in Asia indicate that, given assured water supply, lowland rice fields are extremely sustainable and able to produce continuously high yields (Dawe et al., 2000). Examples of provisioning services, beside the production of rice, are the raising of fish and ducks in rice fields, ponds, or canals. Frogs and snails are collected for consumption in some countries. As part of regulating services, bunded rice fields may increase the water storage capacity of catchments and river basins, lower the peak flow of rivers, and increase groundwater flow. For example, in 1999 and 2000, 20% of the floodwater in the lower Mekong River Basin was estimated to be temporarily stored in upstream rice fields (Masumoto et al., 2004). Other possible regulatory services of bunded rice fields and terraces include the prevention or mitigation of land subsidence, soil erosion, and landslides (PAWEES, 2005). Percolating water from rice fields, canals, and storage reservoirs recharges groundwater systems (Mitsuno et al., 1982). The moderation of air temperature by rice fields has been recognized as an important function in periurban areas where paddy and urban land are intermingled (Oue et al., 1994). This function is attributed to relatively high evapotranspiration rates resulting in reduced ambient temperature of the surrounding area in summer, and in lateral heat emission from the water body in winter. Despite its salt sensitivity, rice can be used as a desalinization crop because the continuously percolating water leaches salts from the topsoil (Bhumbla and Abrol, 1978). Rice soils that are flooded for long periods of the year contribute to the mitigation of the greenhouse eVect by taking CO2 from the atmosphere and sequestering the carbon (C) in the soil, even with complete removal of above‐ ground plant biomass (Bronson et al., 1997a). A significant input of C is derived from biological activity in the soil–floodwater system. Average soil organic C content in irrigated double and triple rice systems of Asia is about 14–15 g C kg1 in the upper 20–25 cm of soil (Dobermann et al., 2003). Assuming an average bulk density of about 1.25 t m3 soil and a physical land area of about 24 million ha, these monoculture systems alone store about 45 t C ha1 or a total of 1.1 Pg C (109 t) in the topsoil. Overall, however, reliable information on soil C stocks is not available for rice systems in most countries.
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As a supporting service, flooded rice fields and irrigation channels form a comprehensive water network, which, together with the contiguous dry land, provides a complex mosaic of landscapes. Irrigated rice land has been classified as human‐made wetlands by the Ramsar Convention on wetlands (Ramsar, 2004). Surveys show that such landscapes sustain a rich biodiversity, including unique as well as threatened species, and also enhance biodiversity in urban and periurban areas (Fernando et al., 2005). The cultural services of rice environments are especially valued in Asian countries where, for centuries, rice has been the main staple food and the single most important source of employment and income for rural people. Many old kingdoms as well as small communities have been founded on the construction of irrigation facilities to stabilize rice production. Rice aVects daily life in many ways and the social concept of rice culture gives meaning to rice beyond its role as an item of production and consumption (Hamilton, 2003). Many traditional festivals and religious practices are associated with rice cultivation and rice fields are valued for their scenic beauty.
E. ENVIRONMENTAL IMPACTS 1. Reactive Nitrogen Irrigated rice systems contribute to the accumulation of reactive nitrogen (N) compounds in the environment. Reactive N is defined as all biologically, photochemically, and/or radiatively active forms of N. This diverse pool þ includes mineral N forms such as nitrate (NO 3 ) and ammonium (NH4 ), gases that are chemically active in the troposphere (NOx and ammonia, NH3), and gases such as nitrous oxide (N2O) that contribute to the greenhouse eVect. Asia accounts for nearly 50% of the net global creation of reactive N from anthropogenic sources (Boyer et al., 2004), with irrigated rice systems accounting for about 15% of Asia’s total share. On average across irrigated environments in Asia, fertilizer N input is 118 40 kg ha1, with the highest levels found in southern China, with up to 300 kg ha1 (Witt et al., 1999). Annually, irrigated rice consumes about 8–9 million t of fertilizer N or roughly 10% of global fertilizer N production. On average, only 30–40% of the applied N is recovered by the crop (Dobermann et al., 2002), leading to large losses of reactive and nonreactive N forms. 2.
Ammonia Volatilization
NH3 volatilization from the application of urea fertilizer is the major pathway of N loss in flooded rice systems, often causing losses of 50% or more of the applied urea‐N in tropical transplanted rice (Buresh and
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De Datta, 1990). NH3 emissions are generally negligible from direct‐seeded rice in temperate regions, where the majority of the fertilizer is incorporated into the soil prior to flooding (Humphreys et al., 1988). NH3‐N emissions from lowland rice fields are estimated to be roughly 3.6 Tg year1 (compared with a total of 9 Tg year1 emitted from all agricultural fields), which is some 5–8% of the estimated 45–75 Tg of globally emitted NH3‐N per year (Kirk, 2004). The magnitude of NH3 volatilization largely depends on climatic conditions and the method of N fertilizer application. Volatilized NHþ4 can be deposited on the earth by rain, which can lead to soil acidification (Kirk, 2004) and unintended N inputs into natural ecosystems.
3. Greenhouse Gases Irrigated double‐ and triple‐cropping rice systems are a significant sink for atmospheric CO2 (Section II.D), a significant source of methane (CH4), and a small source of N2O. a. Methane. In the early 1980s, it was estimated that lowland rice fields emitted some 50–100 Tg of CH4 per year, or about 10–20% of the then estimated global CH4 emissions (Kirk, 2004). Recent measurements, however, show that many rice fields emit substantially less than those investigated in the early 1980s, especially in northern India and China. Also, CH4 emissions have actually decreased since the early 1980s because of changes in crop management such as a decreased use of organic inputs. At the same time, techniques for upscaling of greenhouse gas emissions have improved with the use of simulation models coupled with GIS databases on soil and land use (Matthews et al., 2000). However, the uncertainty about CH4 emissions from rice fields is higher than about most other sources in the global CH4 budget (Van der Gon et al., 2000). Current estimates of annual CH4 emissions from rice fields are in the range of 20–60 Tg, being 5–10% of total global emissions of about 600 Tg (Kirk, 2004). The magnitude and pattern of CH4 emissions from rice fields are mainly determined by water regime and organic inputs, and to a lesser extent by soil type, weather, tillage, residue management, fertilizer use, and rice cultivar (Bronson et al., 1997a,b; Wassmann et al., 2000). Organic manure generally enhances CH4‐ emissions. Flooding of the soil is a prerequisite for sustained emissions of CH4. Mid‐season drainage, a common irrigation practice adopted in major rice‐growing regions in China and Japan, greatly reduces CH4 emissions. Similarly, rice environments with an uneven supply of water, such as rainfed environments, have a lower emission potential than fully irrigated rice.
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b. Nitrous Oxide. Few accurate assessments have been made of emissions of N2O from rice fields (Abao et al., 2000; Bronson et al., 1997a,b; Dittert et al., 2002), and the contribution to global emissions has not yet been assessed. In irrigated rice systems with good water control, N2O emissions are small except when excessively high fertilizer N rates are applied. In irrigated rice fields, the bulk of N2O emissions occur during fallow periods and immediately after flooding of the soil at the end of the fallow period. However, in rainfed systems, nitrification during aerobic phases and denitrification during subsequent waterlogged phases might contribute to considerable emission of N2O (Abao et al., 2000).
4.
Water Pollution
Changes in water quality associated with rice production may be positive or negative depending on the quality of the incoming water and management practices such as fertilizer and biocide use. The quality of the water leaving rice fields may be improved as a result of the capacity of the wetland ecosystem to remove N and phosphorus (P) (Feng et al., 2004; Ikeda and Watanabe, 2002). On the other hand, N transfer from flooded rice fields by direct flow of dissolved N in floodwater through runoV/drainage warrants more attention. High N pollution of fresh waters can be found in lowland rice‐growing regions where fertilizer rates are excessively high, for example, in Jiangsu Province in China (Cui et al., 2000). Many of the rural poor in Asia obtain water for drinking and household use from shallow aquifers under agricultural land (Bouman et al., 2002). Among the agrochemicals that pose the greatest threats to domestic use of groundwater are NO 3 , biocides and their residues, and, more recently, arsenic (As). Salinization and acidification are other forms of water pollution that can be associated with rice cultivation. a. Nitrate. NO 3 leaching from flooded rice fields is normally negligible because of rapid denitrification under anaerobic conditions (Buresh and De Datta, 1990). For example in the Philippines, NO 3 pollution of groundwater under rice‐based cropping systems was found to surpass the 10 mg liter1 limit for safe drinking water only when highly fertilized vegetables were included in the cropping system (Bouman et al., 2002). In the Indian Punjab, 1 however, an increase in NO was recorded between 3 of almost 2 mg liter 1982 and 1988, with a simultaneous increase in fertilizer N consumption of 56–188 kg ha1, most of which would have been used on rice–wheat (Bijay‐Singh et al., 1991). The relative contribution to this increase from rice, however, is not clear.
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b. Biocides. In traditional rice systems, relatively few herbicides are used as puddling, transplanting, and ponding water are eVective weed control measures. Mean biocide use in irrigated rice systems varies from some 0.4 kg active ingredients (a.i.) ha1 in Tamil Nadu, India, to 3.8 kg a.i. ha1 in Zhejiang Province, China (Bouman et al., 2002). In the warm and humid conditions of the tropics, volatilization is a major process of biocide loss, especially when biocides are applied on the surface of water or on wet soil (Sethunathan and Siddaramappa, 1978). The relatively high temperatures further favor rapid transformation of the remaining biocides by (photo)chemical and microbial degradation, but little is known about the toxicity of the residual components. In case studies in the Philippines, mean biocide concentrations in groundwater underneath irrigated rice‐based cropping systems were one to two orders of magnitude below the single (0.1 mg liter1) and multiple (0.5 mg liter1) biocide limits for safe drinking water, although temporary peak concentrations of 1.14–4.17 mg liter1 were measured (Bouman et al., 2002). As for N, however, biocides and their residues may be directly transferred to open water bodies through drainage water flowing overland out of rice fields. The potential for water pollution by biocides is greatly aVected by field water management. DiVerent water regimes result in diVerent pest and weed populations and densities, which farmers may combat with diVerent amounts and types of biocides. Residual biocides interact diVerently with soil under diVerent water regimes (Sethunathan and Siddaramappa, 1978). c. Arsenic. In Asia, the number of shallow tubewells for irrigation purposes has increased dramatically in the last two decades. In parts of the Indo‐Gangetic Plain in Bangladesh and India, the lowering of the groundwater table associated with the pumping has led to a severe problem of As pollution (Ahmed et al., 2004; Alam et al., 2002). As accumulates in the topsoil as a result of irrigation. Duxbury et al. (2003) estimated that 10 years of irrigation with As‐contaminated water would add 5–10 mg As kg1 soil to 41% of the 456 study sites included in their study. Rice fields receive relatively high amounts of irrigation water, and therefore accumulate more As than nonrice fields. Under the flooded conditions in which rice is grown, redox potentials are low, making As potentially bioavailable. To date, however, it has not been possible to predict As uptake by plants from the soil (Meharg, 2004). Also, it is not known whether high As concentrations in the soil or water aVect crop production. Jahiruddin et al. (2004) reported that As concentrations of 10 mg kg1 soil caused a yield reduction of more than 45%, but, in general, our understanding of the (long‐ term) behavior of As in agriculture is too limited to assess the risks. d. Salinization. In many areas, percolating water from lowland rice fields can raise the groundwater table and cause waterlogging. Where the
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groundwater is saline, this can salinize the root zone of nonrice crops, such as in parts of the northwest Indo‐Gangetic Plain (Hira et al., 1998). e. Acidic Pollution. Many rice‐growing areas of the deltas of Southeast Asia include acid sulfate soils. Amelioration of these soils for rice cultivation entails the leaching of toxicities from the soil, which means transferring acidic pollutants from the soil to surrounding water (Tuong et al., 1998, 2003). Acidic pollution with low pH and high aluminum in the water may result in reduced aquatic biodiversity, including fish and shrimp, which constitute important protein sources for the rural poor as well as a supplemental source of income for landless laborers and small farmers. However, leachates from rice fields are less harmful (having higher pH and lower aluminum levels) than those from upland crops on raised beds.
F. 1.
THE MAIN CHALLENGES AHEAD
Food Production and Poverty Alleviation
A main challenge facing many Asian countries is to keep providing suYcient and aVordable food for their growing and urbanizing populations. Future demand for rice is a function of population growth, the age structure of the population, income, and urbanization (Pingali et al., 1997). Annual population growth in rice‐producing Asia was about 1.2% from 2000 to 2005, but is forecast to decline to 0.1% by 2050 (FAOSTAT). As incomes rise, particularly in urban areas, per capita rice consumption declines. At the national level, per capita consumption is declining not only in East Asia (Japan, Republic of Korea, and China) but also in Malaysia and Thailand. However, for the next two decades, the increasing demand for rice by increasing populations is still expected to outstrip the decline in per capita consumption caused by changed consumer preferences. Rice demand in Asia is expected to grow by approximately 1% per year until 2025, assuming a mild decline in world rice prices (Sombilla et al., 2002). Increased rice production has to be accomplished under increasing pressure on land, water, and labor resources that threatens the sustainability of the rice production base. Although major advances have been made in poverty alleviation in the past few decades, absolute numbers of the poor have declined very little, especially in South Asia. Despite the successes of the Green Revolution, poverty and hunger still occur among considerable numbers of rice farmers within irrigated areas (Magor, 1996). Moreover, the rainfed areas have largely been bypassed by the Green Revolution and now constitute major poverty hot spots of the world (Sanchez et al., 2005). Asia is rapidly urbanizing and more people will shift from being net rice producers to net rice
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consumers (Pingali et al., 1997). Also, the total number of urban poor is expected to increase. Therefore, a major challenge is not only to produce more rice, but to keep its price low to contribute to livelihood improvement of the urban and rural poor who are net rice purchasers. Since a low rice price depresses the profitability of rice farming, the simultaneous challenge is to decrease the cost of rice production (per kilogram) so that the profitability of rice farming can increase.
2.
Increasing Water Scarcity
Worldwide, water for agriculture is becoming increasingly scarce (Rijsberman, 2006). The causes are diverse and location specific, but include decreasing resources (e.g., falling groundwater tables, silting of reservoirs), decreasing quality (e.g., chemical pollution, salinization), malfunctioning of irrigation systems, and increased competition from other sectors such as urban and industrial users. However, there is no systematic inventory, definition, or quantification of water scarcity in rice‐growing areas. Tuong and Bouman (2003) estimated that, by 2025, 15–20 million ha of irrigated rice will suVer some degree of water scarcity. There are no reports of water scarcity in some of Asia’s largest irrigated rice ecosystems in the river deltas of the Yangtze, the Mekong, or the Irrawady. However, in South Asia, the Ganges and Indus rivers have little outflow to the sea in the dry season, aVecting downstream rice‐growing areas (Postel, 1997). Overexploitation of groundwater during the last decades has caused serious problems in northern China and South Asia (Postel, 1997; Shu Geng et al., 2001; Singh, 2000), aVecting rice–wheat‐growing areas. Groundwater tables have dropped on average by 1–3 m year1 in the North China Plain; by 0.5–0.7 m year1 in the Indian states of Punjab, Haryana, Rajasthan, Maharashtra, Karnataka, and northern Gujarat; and by about 1 m year1 in Tamil Nadu and southern India, where flooded rice is the dominant cropping system. In Bangladesh, the heavy use of groundwater has led to shallow wells falling dry by the end of the dry season and to severe problems of As pollution in rice‐growing areas (Ahmed et al., 2004). Heavy competition for river water between states and diVerent sectors (city, industry) is causing water scarcity in southern India’s Cauvery delta and in Thailand’s Chao Phraya delta (Postel, 1997), which are major regional rice bowls. Several case studies suggest local hot spots of water scarcity because of increased competition between diVerent users of water, even in areas generally not considered water scarce, for example, the Zanghe Irrigation System in the middle reaches of the Yangtze (Dong et al., 2004) and Angat reservoir near Manila, Philippines (Bhuiyan and Tabbal, as referenced in Pingali et al., 1997). In principle, water is always scarce in the dry season
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when the lack of rainfall makes cropping impossible without irrigation. Overall, increasing water scarcity can be expected to (further) shift rice production to more water‐abundant delta areas (Section II.B.5), and to lead to crop diversification and less flooded conditions in rice fields where water is becoming scarce.
III.
RESPONSE OPTIONS
A key response option to achieve food security and alleviate poverty is to increase yields and productivity on existing crop land to avoid the environmental degradation, destruction of natural ecosystems, and loss of biodiversity that are associated with an expansion of cropped area (Tilman et al., 2002). For many poor farming households, increasing rice productivity is often the first step out of poverty as it provides food security and frees up land and labor resources (Hossain and Fischer, 1995). With increased rice yields, part of the farm land can be taken out of rice production and converted into more profitable cash crops. Freed‐up labor can be invested in oV‐farm employment. Increased income can be used to invest in the education of children, which is a potential pathway out of farming and poverty. Increased yield and increased total production mean that, with current management practices, more water will be needed to meet the increased transpiration requirements. With increasing water shortage, this means that the water productivity of rice needs to increase. To increase the productivity of rice, options exist for varietal improvement and for better management of natural and man‐made resources at the field to landscape levels.
A. VARIETAL IMPROVEMENT 1.
Yield Potential
The genetic yield potential of a crop variety is expressed when it is grown with an adequate supply of water (i.e., flooded conditions for rice) and nutrients, and when no pests, diseases, or weeds are present. The key attributes of the high‐yielding varieties of the Green Revolution were semidwarf stature (that increased harvest index and lodging resistance) and photoperiod insensitivity (Khush, 1995). There are no indications that these factors can be further exploited to significantly increase the yield potential of inbred varieties under fully irrigated conditions (Peng et al., 1999). For example, since the introduction of IR8 in the 1960s, the yield potential of semidwarf tropical indica inbred varieties has stagnated at about 10 t ha1. Yield potential under temperate conditions with longer daylength, longer ripening period, and lower night
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temperature is around 14 t ha1. Significant yield improvement has recently come only from the development of hybrid rice, which has increased yield potential by 5–15% over inbred varieties in the same environment (Peng et al., 1999). China’s ‘‘super’’ rice breeding program has developed several hybrid varieties with yields of 12 t ha1 in on‐farm demonstration fields, 8–15% higher than the hybrid check varieties (Yuan, 2001). Transforming the C3 rice plant into a C4 plant by genetic engineering could be a long‐term approach for increasing rice yield potential (Sheehy et al., 2000), but its feasibility and potential benefits are still debated. Traditional breeding programs for irrigated environments have selected under conditions of continuously ponded water. With increasing water scarcity in irrigated systems, breeding programs should include selection under conditions of water‐saving technologies such as alternate wetting and drying (AWD) (Section III.B.1.b.iii) or aerobic cultivation (Section III.B.1.b.iv). 2. Water Productivity The modern, improved japonica varieties have 25–30% higher transpiration eYciency than the older indica varieties, suggesting that significant variation exists for this trait in rice germplasm (Peng et al., 1999). The potential for exploiting this trait, however, has not been investigated. It has been proposed to increase the waxiness of rice leaves to reduce nonstomatal transpiration (Lafitte and Bennet, 2002), but no notable progress has been made so far. Transforming the C3 rice plant into a C4 plant could potentially also increase transpiration eYciency. Overall, the scope to increase the water productivity of the rice plant with respect to transpiration seems to be small compared with the scope to increase the productivity of total water inputs (irrigation, rainfall). The reduced growth duration of modern high‐yielding rice varieties reduced total outflows of evaporation, seepage, and percolation at the field level. The combined eVect of increased yield and reduced growth duration is that these varieties have a water productivity with respect to total inputs that is three times higher than that of traditional varieties grown with similar water management (Tuong and Bouman, 2003). A range of breeding strategies can be explored to further increase water productivity with respect to evapotranspiration, such as early vigor to reduce soil evaporation, and weed suppression to reduce weed transpiration (Bennett, 2003). 3.
Tolerance of Water‐Related Stresses
a. Drought. For drought tolerance, most progress so far has come from the development of short‐duration varieties that escape drought at the end of the rainy season, using conventional breeding approaches (Bennett, 2003). During the last decade, substantial genetic variability for grain yield
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under drought stress has been documented in both cultivated Asian rice, Oryza sativa (Atlin et al., 2006) and its hardy African relative O. glaberrima (Jones et al., 1997a). Drought tolerance has been demonstrated to be moderately heritable, with repeatability similar to that of yield in nonstress environments (Atlin et al., 2004). A promising strategy for developing drought‐tolerant yet high‐yielding cultivars is to combine selection for yield potential under favorable conditions with managed‐stress screening for yield under treatments that impose severe stress bracketing the drought‐sensitive flowering period. This approach is resulting in the development of both lowland and upland rice varieties that have improved tolerance of periods of severe water stress during the sensitive flowering and grain‐filling stages while retaining the ability to produce high yields when water supplies are not limiting. Breeding eVorts need to be specifically directed to well‐defined target environments. Two specific examples for upland environments are ‘‘aerobic rice’’ and NERICA (New Rice for Africa). Aerobic rice is higher yielding than traditional upland varieties and combines input responsiveness with improved lodging resistance and harvest index (Atlin et al., 2006). These new varieties are specifically designed for nonflooded, aerobic soil conditions in either rainfed or water‐short irrigated environments. Bouman et al. (2005b) demonstrated that the capacity to retain spikelet fertility and hence the harvest index is one of the successes of aerobic adaptation of these newly developed aerobic rice varieties. Examples of adoption in China are given in Section III.B.1.b.iv. At the African Rice Center (WARDA), breeders started crossing O. glaberrima with O. sativa species in the mid‐1990s to combine the toughness of the former with the productivity of the latter (Dingkuhn et al., 1998; Jones et al., 1997b). These crosses have subsequently been named NERICA and aim to combine resistance to local stresses with higher yield, shorter growth duration, and higher protein content than traditional rice varieties. Molecular tools were used in the development of NERICA to overcome hybrid sterility, which accelerated the breeding program from 5 to 7 years to 2 years or less. NERICA is specifically targeted at the upland and dryland areas of sub‐Sahara Africa, and no notable success has been reported in Asia so far. Molecular tools are starting to identify genes controlling the responses of plants to water stress, but no quantitative trait loci (QTLs) have been identified so far for tolerance of either reproductive‐ or vegetative‐stage drought stress with eVects large enough to be useful in breeding (Bennett, 2003). b. Submergence. Although breeding for submergence tolerance and enhanced yield in flash‐flood areas has been going on for over three decades, only a few tolerant lines with improved agronomic characteristics have been developed so far. For flash‐flood areas, some tolerant landraces were discovered that can withstand complete submergence for 10–14 days, such as FR13A, FR13B, Goda Heenati, Kurkaruppan, and Thavalu. A few
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submergence‐tolerant breeding lines with improved agronomic characteristics have now been developed by transferring this tolerance into semidwarf breeding lines (Mackill et al., 1993). Recently, fast progress has been made with the development of submergence‐tolerant lines using marker‐assisted selection (Mackill and Xu, 1996). Using a population developed from a cross between an indica submergence‐tolerant line (IR40931) and a susceptible japonica line (PI543851), a major QTL was mapped to chromosome 9, designated as Sub1 (Mackill and Xu, 1996). This Sub1 QTL was fine mapped (Xu et al., 2000) and markers were developed and successfully used to transfer it into Swarna, a popular rainfed lowland variety sensitive to submergence, which became substantially tolerant of flooding under field conditions without changing its agronomic or quality traits. EVorts are currently ongoing to transfer the Sub1 QTL into other ‘‘mega’’ varieties in rainfed lowlands such as BR11, IR64, Mahsuri, Samba Mahsuri, and CR1009. For deepwater areas, elongation ability of leaves and internodes is essential to keep pace with rising water and to escape complete submergence. Some breeding progress has been made and a few new lines with reasonable yield and grain quality have been released. Recently, three main QTLs for elongation ability were identified (Sripongpangkul et al., 2000). Fine mapping and tagging of these QTLs should facilitate their eYcient incorporation into modern popular varieties through marker‐assisted selection. c. Salinity. Despite its high sensitivity to salinity, considerable variation in tolerance exists in rice (Flowers and Yeo, 1981). Combining new eYcient screening techniques with conventional, mutation, and anther culture techniques, salinity tolerance was successfully introduced into high‐ yielding plant types (Gregorio et al., 2002). Some newly released varieties have demonstrated more than a 50% yield advantage over current salt‐ sensitive varieties. Breeding cultivars with much higher tolerance is possible if component traits are combined in a suitable genetic background. The opportunity to improve salinity tolerance by incorporating useful genes and/or pyramiding superior alleles appears very promising (Prasad et al., 2000). A recently mapped major QTL, designated ‘‘Saltol,’’ accounted for more than 70% of the variation in salt uptake in the studied population (Bonilla et al., 2002). Marker‐assisted backcrossing is currently being used to incorporate this QTL into popular varieties that are sensitive to salt stress.
B. IMPROVED MANAGEMENT PRACTICES 1.
Irrigated Environments
a. Closing the Yield Gap. Increasingly, technologies that aim to close the gap between actual (farmers’) yields and yield potential apply holistic approaches that integrate various components of crop, soil, and water
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management. Examples for irrigated rice are the system of rice intensification (SRI) (Stoop et al., 2002) and site‐specific nutrient management (Dobermann et al., 2002). However, care must be taken in promoting a ‘‘one‐fit‐all solution’’ across environments, and site‐specific adaptations must be allowed for. For example, SRI in its original form as developed in Madagascar has strict rules about using young seedlings, wide row spacing, careful transplanting of single seedlings, transplanting in squares, AWD, manual or mechanical weed control, and large amounts of organic fertilizer use (Stoop et al., 2002). Although the fabulously high yields that are reportedly obtained with SRI are contested (McDonald et al., 2006; Sheehy et al., 2004), many farmers modify SRI to suit their needs and environments, often to the extent that the original SRI recipe can no longer be recognized (UphoV et al., 2002). However, SRI has relatively high labor requirements and, partly because of this, disadoption of the system has been reported in its country of origin, Madagascar (Moser and Barett, 2003). Although water flows have hardly been studied in these integrated technologies, yield increases are likely to be accompanied by relative increases in transpiration and by relative decreases in evaporation, seepage, and percolation (Bouman, 2006). In terms of water savings, any agronomic practice that increases harvest index will result in more grains per unit water transpired by the crop and hence in increased crop water productivity. The productivity of total water input can also be increased by agronomic practices that reduce evaporation or nonbeneficial transpiration, such as weed and nutrient management, and therefore increase the proportion of water that goes to crop transpiration. b. Water‐Saving Technologies. Various ‘‘water‐saving technologies’’ exist or are being developed to assist farmers to cope with water scarcity in irrigated environments (Humphreys et al., 2005; Tuong and Bouman, 2003; Tuong et al., 2005). These water‐saving technologies increase the productivity of total water inputs (rainfall, irrigation), mainly by reducing unproductive seepage and percolation losses, and to a lesser extent by reducing evaporation (Belder et al., 2004; Bouman et al., 2005a). i. Soil Amelioration. The resistance to water flow (percolation) can be increased by changing the soil physical properties. Cabangon and Tuong (2000) showed the beneficial eVects of an additional shallow soil tillage before land preparation to close cracks that cause rapid bypass flow at land soaking. Thorough puddling results in a good compacted plow soil that impedes vertical water flow. Soil compaction using heavy machinery has been shown to decrease soil permeability in northeast Thailand in sandy and loamy soils with at least 5% clay (Sharma et al., 1995). ii. Reducing Turn‐Around Time and Direct Seeding. Minimizing the turn‐around time between wet land preparation and transplanting reduces the period when no crop is present and the outflows of water from the field
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that do not contribute to production. Especially in large‐scale irrigation systems with plot‐to‐plot irrigation, water losses during the turn‐around time can be very high when farmers maintain seedbeds in their fields and have to keep the whole area flooded for the full duration of the seedbeds (Tabbal et al., 2002). In such systems, the turn‐around time can be minimized by installing field channels, adopting common seedbeds, or adopting direct wet or dry seeding. With field channels, water can be delivered to the individual seedbeds separately and the main field does not need to be flooded. Common seedbeds, either communal or privately managed, can be located strategically close to irrigation canals and irrigated as one block. With direct seeding, the crop starts growing and using water from the moment of establishment onward (Tabbal et al., 2002). Direct dry seeding can also increase the eVective use of rainfall and reduce irrigation needs as shown for the MUDA irrigation scheme in Malaysia (Cabangon et al., 2002). However, direct dry seeding does not necessarily reduce the total amount of water or increase the productivity of total water input or of evapotranspired water. A major driving force for the adoption of direct seeding in Asia is the scarcity of labor since direct seeding does not use labor for transplanting and can be a mechanized operation. As of the late 1990s, it is estimated that one‐fifth of the area in Asia is direct seeded (Pandey and Velasco, 2002). iii. Saturated Soil Culture. In saturated soil culture (SSC), the soil is kept as close to saturation as possible, thereby reducing the hydraulic head of the ponded water, which decreases seepage and percolation flows. SSC in practice means that a shallow irrigation is given to obtain about 1‐cm ponded water depth a day or so after the disappearance of ponded water. Analyzing a data set of 31 published field experiments with an SSC treatment, Bouman and Tuong (2001) found that water input decreased on average by 23% (range: 5–50%) from the continuously flooded check, with a nonsignificant yield reduction of 6%. Thompson (1999) found that SSC in southern New South Wales, Australia, reduced both irrigation water input and yield by a bit more than 10%, thus maintaining the irrigation water productivity. Borell et al. (1997) experimented with raised beds (120‐cm wide, with furrows of 30‐cm width and 15‐cm depth) in Australia to facilitate SSC practices. The water in the furrows kept the beds at saturation. Compared to flooded rice, water savings were 34% and yield losses 16–34% (though not always significant). iv. Alternate Wetting and Drying. In AWD, irrigation water is applied to obtain flooded conditions after a certain number of days have passed after the disappearance of ponded water. Although some researchers have reported a yield increase under AWD (Zhang and Song, 1989), recent work indicates that this is the exception rather than the rule (Belder et al., 2004; Cabangon et al., 2004; Tabbal et al., 2002). In 31 field experiments analyzed by Bouman and Tuong (2001), 92% of the AWD treatments resulted in yield reductions varying from just more than 0% to 70%
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compared with the flooded checks. In all these cases, however, AWD increased the productivity of total water input because the reductions in water inputs were larger than the reductions in yield. Bouman and Tuong concluded that the large variability in results of AWD was caused by diVerences in the number of days between irrigations and in soil and hydrological conditions. Experimenting with AWD in lowland rice areas with heavy soils and shallow groundwater tables in China and the Philippines, Cabangon et al. (2004), Belder et al. (2004), and Lampayan et al. (2005) reported that total (irrigation and rainfall) water inputs were reduced by around 15–30% without a signifi-cant impact on yield (Table II). In all these cases, groundwater depths were extremely shallow (between 10 and 40 cm), and ponded water depths hardly dropped below the root zone during the drying periods, thus turning AWD eVectively into a kind of near‐SSC. More water can be saved and water productivity further increased by prolonging the periods of dry soil and imposing a slight drought stress on the plants, but this usually comes at the expense of yield loss (Bouman and Tuong, 2001). Research in more loamy and sandy soils with deeper groundwater tables in India and the Philippines showed reductions in water inputs of more than 50% together with a yield loss of more than 20% compared with the flooded check (Sharma et al., 2002; Singh et al., 2002; Tabbal et al., 2002) (Table II). To keep the yield under AWD similar to that under flooded conditions, the number of days without ponded water in AWD may vary with soil type, climatic conditions, and groundwater depth. Instead of ‘‘number of days without ponded water’’ as a guide to ‘‘safe AWD,’’ Tuong (2005) suggested that irrigation should be based on a threshold soil water potential at 10‐cm depth of 20 kPa. AWD is a mature technology that has been widely adopted in China and can now be considered the common practice of lowland rice production in that country (Li and Barker, 2004). It is also a recommended practice in northwest India. Very little research has been done to quantify the impact of AWD on the diVerent water outflows: evaporation, seepage, and percolation. The little work done so far suggests that AWD mostly reduces seepage and percolation flows and has only a small eVect on evaporation flows. Belder et al. (2006) and Cabangon et al. (2004) calculated that evaporation losses were reduced by 2–33% compared with fully flooded conditions. v. Aerobic Rice. In the system of aerobic rice, specially adapted input‐ responsive aerobic rice varieties are grown under dryland conditions just like other cereals such as wheat, with or without supplemental irrigation. In experiments in the Philippines and northern China, water inputs in aerobic rice systems were 30–50% less than in flooded systems, with yields that were 20–30% lower, with a maximum of about 5.5 t ha1 (Table III). Reductions in evaporation losses were on the order of 50–75% (Bouman et al., 2005a). In both experiments, water productivity with aerobic rice was
Table II Yield, Water Use, and Water Productivity of Rice Under Alternate Wetting and Drying (AWD) and Continuously Flooded Conditions
Year
Treatment
Yield (t ha1)
Total water input (mm)
Water productivity (g grain kg1 water)
Guimba, Philippines (Tabbal et al., 2002)
1988
Flooded AWD Flooded AWD Flooded AWD Flooded AWD Flooded AWD Flooded AWD Flooded AWD
5.0 4.0 5.8 4.3 5.3 4.2 4.9 3.3 8.4 8.0 8.1 8.4 7.2 7.7
2197 880 1679 700 2028 912 3504 1126 965 878 878 802 602 518
0.23 0.46 0.35 0.61 0.26 0.46 0.14 0.29 0.90 0.95 0.92 1.07 1.20 1.34
1989 1990 1991 Tuanlin, Huibei, China (Belder et al., 2004)
1999 2000
Mun˜oz, Philippines (Belder et al., 2004)
2001
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Table III Yield, Water Use, and Water Productivity of Rice Under Aerobic and Continuously Flooded Conditions
Location
Year
Treatment
Yield (t ha1)
Total water input (mm)
Water productivity (g grain kg1 water)
Philippines (Bouman et al., 2005a)
2001
Flooded Aerobic Flooded Aerobic Flooded Aerobic Flooded Aerobic 1 Aerobic 4 Flooded Aerobic 1 Aerobic 4
5.1 4.4 7.3 5.7 6.8 4.0 5.4 4.7 3.4 5.3 5.3 4.6
1718 787 1268 843 1484 980 1351 644 524 1255 917 695
0.29 0.55 0.58 0.67 0.46 0.41 0.40 0.73 0.66 0.42 0.58 0.66
2002 2003 China (Yang et al., 2005)
2001
2002
higher than with flooded rice, suggesting that aerobic rice is an attractive option to ‘‘produce more rice with less water’’ in situations where water is scarcer than land. It is estimated that aerobic rice systems are currently being pioneered by farmers on some 80,000 ha in northern China (Wang et al., 2002). However, the development of aerobic rice systems for irrigated environments is in its infancy and more research is needed to develop high‐yielding aerobic rice varieties and sustainable management systems (Section III.B.1.c). In aerobic rice systems, resource‐conserving technologies, such as mulching and zero or minimum tillage as practiced in upland crops, become available to rice farmers as well (Hobbs and Gupta, 2003). Various methods of mulching (e.g., using dry soil, straw, and plastic sheets) are being experimented with in nonflooded rice systems in China and have been shown to reduce evaporation while maintaining high yields (Dittert et al., 2002). Growing rice under aerobic conditions on raised beds shows promise but is also still in its infancy of development (Humphreys et al., 2005; Kukal et al., 2005). c. Sustainability and Environmental Impacts with Water Scarcity. The transformation of rice fields to upland crops will have consequences for sustainability and the environment. In a long‐term experiment at International Rice Research Institute (IRRI), where a continuous rice system is compared with a maize–rice system, 12 years of maize–rice cropping caused a 15% decline in soil organic C and indigenous N supply relative to rice–rice cropping (Roland Buresh, personal communication). Also, more leaching of NO 3 is expected under upland crops than under flooded rice.
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Flooding has beneficial eVects on soil acidity (pH), P, iron (Fe), and zinc (Zn) availability, and biological N fixation (Kirk, 2004). A change to more aerobic soil conditions within rice fields will negatively aVect the soil pH in some situations and decrease the availability of P, Fe, and Zn for rice. There are indications that soil‐borne pests and diseases (e.g., nematodes, root aphids, fungi) and micronutrient disorders occur more in nonflooded than in flooded rice systems (Sharma et al., 2002; Singh et al., 2002; Ventura and Watanabe, 1978; Ventura et al., 1981). Current experience is that under fully aerobic soil conditions, rice cannot be grown continuously on the same piece of land each year (as can be successfully done with flooded rice) without a yield decline (George et al., 2002). Figure 4 presents recent data from a continuous aerobic rice cropping experiment at IRRI (Bouman et al., 2005a; Peng et al., 2006). Since 2001, the aerobic rice variety Apo has been continuously grown under flooded and aerobic conditions in the same field. Flooded yields in the dry season are usually 6.5–7 t ha1, except in 2001, when diseases depressed yields. In 2001, the yield under aerobic conditions was 86% of that under flooded conditions, but this gradually declined until it was only 45% in 2005. In 2003, half of the flooded fields were converted to aerobic conditions, and aerobic yields returned to 85% of the flooded yields, as in the first year 2001. In 2004, half of the continuous aerobic fields were left fallow or were flooded for the whole year, and returned to aerobic conditions in 2005. This ‘‘restoration’’ attempt brought aerobic yields up to 65% of flooded yields. The mechanisms behind the gradual yield decline and the restoration eVect are not yet understood, although high levels of the nematode Melodoigyne graminicola are found in the aerobic rice fields (up to 3000 counts g1 fresh root) compared with the flooded fields (6–400 counts g1 fresh root; unpublished data). Varieties that are tolerant of soil‐borne pests and diseases need to be developed alongside suitable crop rotations. Rice that is not permanently flooded tends to have more weed growth and a broader weed spectrum than rice that is permanently flooded (Mortimer and Hill, 1999). It is expected that water shortages will lead to more frequent use of herbicides. With less water, the numbers and types of pests and predators (e.g., spiders) may change as well as predator–pest relationships. The possible shift in the use of pesticides by farmers in response to these changes, and what this means for the environment, is as yet unknown. In general, fewer CH4 emissions are expected under aerobic than under flooded conditions, but higher N2O emissions are expected (Bronson et al., 1997a,b). However, the relative emissions of these greenhouse gases vary with environment and management practices. Data from Dittert et al. (2002) illustrate the variability in greenhouse gas emissions from conventional flooded rice fields (control) and from two water‐saving systems, unsaturated soil covered by plastic film (film) and unsaturated soil covered by straw mulch (mulch), at three sites in China (Fig. 5). CH4 emissions were highest from flooded rice at all three sites. N2O emissions
B. A. M. BOUMAN ET AL.
218 A 8 Flooded yield (t ha−1)
7 6 5 4 3 2 1 0 2001
2002
2003 Year
2004
2005
2001
2002
2003 Year
2004
2005
B 100 90 Aerobic yield (%)
80 70 60 50 40 30 20 10 0
Continuous
New
Restore
Figure 4 Yield of aerobic rice variety Apo under flooded conditions (A) and relative yield of Apo under aerobic conditions (as percentage of the Apo yield under flooded conditions) (B). In (B), the white columns indicate continuous aerobic conditions, the striped columns indicate yield under new aerobic conditions after conversion of flooded fields, and dotted columns indicate yield under restored aerobic conditions after 1 year of fallow or flooded conditions (average is given). Source: Bouman et al. (2005a), Peng et al. (2006), and unpublished data, IRRI.
were lowest from flooded rice at Nanjing and Guangzhou, but similar among all three systems at Beijing. When both CH4 and N2O emissions were converted into equivalent CO2 emissions and summed, flooded rice had the lowest global warming potential at Nanjing and highest global warming potential at Guangzhou, whereas all three systems had similar global warming potentials at Beijing. Thus, the overall impact of an adoption
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CH4 flux (mg CH4 m −2 per season)
A
219 20,379
4000 3500 3000 2500 2000 1500 1000 500 0 Control
Film Beijing
Straw Control
Film
Nanjing
Straw Control
Film
Straw
Guangzhou
B N2O flux (mg N2O m−2 per season)
1500 1250 1000 750 500 250 0 Control
Film
Straw Control
Film
Straw Control
Film
Straw
Film
Straw Control
Film
Straw Control
Film
Straw
C CO2 equivalent flux (mg CO2 m −2 per season)
600 500 400 300 200 100 0 Control
Figure 5 Emissions of CH4 (A) and N2O (B), and combined global warming potential (C) of rice fields under continuous flooding (control), under plastic film with unsaturated soil underneath, and under straw mulch with aerobic soil conditions underneath, at three sites in China. Source: Dittert et al. (2002).
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of water‐saving management practices in rice production on global warming is unknown and needs more study.
2.
Stress‐Prone Environments
The high variability of rainfed environments exposes farmers to great risk of yield loss. However, the development of stress‐tolerant and input‐responsive varieties will reduce this risk and increase the incentive to use external inputs and to intensify the cropping system. Adjusted cropping systems and management technologies are needed to make the best use of the possibilities oVered by the new varieties. These technologies should aim at reducing stress intensity, enhancing survival and robustness of the crop to withstand stress and stabilize yields, and avoiding stress at sensitive crop stages. a. Rainfed Lowlands. Two promising technologies are dry direct seeding and improved nutrient management. Dry direct seeding potentially oVers better use of early‐season rainfall, better drought tolerance as a result of better root development, lower risk from late‐season droughts, better use of indigenous soil N supply, and an increased possibility for a second crop after rice (Sharma et al., 2005; Tuong et al., 2002). In most cases of rainfed lowland rice, fertilizer input increases grain yields (Boling et al., 2004; Wade et al., 1999), unless very severe drought occurs at the flowering stage (Castillo et al., 2006). Site‐ and season‐specific nutrient management, combined with an appropriate rice variety, can reduce nutrient losses and chemical pollution of the environment (Haefele et al., 2006). Both technologies have already enabled substantial productivity increases in some more favorable rainfed areas. In Lombok, Indonesia, the introduction of short‐duration and input‐ responsive varieties with direct seeding and the use of inorganic fertilizer increased and stabilized yields (Fagi and Kartaatmadja, 2002). In Laos, rainfed lowlands contributed considerably to achieving self‐suYciency in rice within a decade after the introduction of improved varieties and crop management (Pandey, 2001). Similar successes should be feasible in drought‐prone lowlands in eastern India (Sharma et al., 2005). b. Rainfed Uplands. Strategies should be aimed at sustainable intensification to break the spiral of resource degradation caused by shorter fallow periods in traditional shifting cultivation systems. For rice, a promising option is the establishment of lowland fields in valley bottoms in mountainous areas, also referred to as ‘‘Montane paddy rice’’ (Castella and Erout, 2002). These lowland fields could benefit from irrigation water supplied by mountain streams that converge in the valley bottoms. The aerobic rice production system oVers scope where seasonal rainfall is some 600 mm or more, or where farmers have access to supplementary
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irrigation. In the hilly regions of Yunnan Province, southern China, farmers grow rainfed aerobic rice under intensified management, realizing yields of 3–4 t ha1 (Atlin et al., 2006). The combination of aerobic rice with the establishment of terraces on sloping land oVers even greater scope for intensification (Hill, 2006). Aerobic rice also holds promise for permanent arable production systems in rotation with other crops. In Brazil, a breeding program to improve upland rice has resulted in aerobic varieties with a yield potential of up to 6 t ha1 (Pin˜heiro et al., 2006). Farmers grow these varieties in rotation with crops, such as soybean and fodder, on large commercial farms with supplemental sprinkler irrigation on an estimated 250,000 ha of flat lands in the Cerrado region, realizing yields of 3–4 t ha1. c. Flood‐Prone Environments. The new submergence‐tolerant varieties need to be combined with adapted crop and nutrient management to improve seedling and plant survival as well as the ability to recover after submergence. Seedlings of submergence‐tolerant varieties that are enriched in nutrients, particularly Zn and P, and possibly silicon, have greater chances of survival because they have enhanced growth and accumulated higher carbohydrate reserves (Ella and Ismail, personal communication). The application of certain nutrients after the recession of floodwater also helps in faster recovery, better tillering, and higher grain yield. d. Salinity‐AVected Environments. EVorts to develop and deploy salt‐ tolerant varieties should be accompanied by improved crop management that enhances survival and robustness of the crop to withstand stress and stabilize yields. Tolerant varieties are much more responsive to amendments or mitigating strategies, basically because they are exposed to a much lower level of internal stress at a particular level of salt in the soil. Soil amendments, particularly the use of gypsum, have repeatedly proven to be beneficial for reclaiming salt‐aVected soils, but they require a large investment. New approaches involving the use of farmyard manure, industrial wastes such as pressmud, and tolerant varieties can help reduce the need for gypsum by more than 50% while achieving similar results.
C. OPTIONS AT THE LANDSCAPE LEVEL 1.
Irrigation Systems
Irrigated rice fields are characterized by large volumes of outflows by surface drainage, seepage, and percolation (Fig. 3). Although these outflows are losses from an individual field, there is great scope for reuse of these flows within a landscape that consists of many interconnected fields (Fig. 6). Surface drainage and seepage water usually flow into downstream fields and
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Figure 6 Surface and subsurface water flows across lowland rice fields. D, drainage (overbund flow); I, irrigation; P, percolation; S, seepage.
are only ‘‘lost’’ at the bottom of a toposequence when they flow into drains or ditches. However, farmers can use small pumps to lift water from drains to irrigate fields that are inadequately or not serviced by irrigation canals. In many irrigation systems in low‐lying deltas or floodplains with impeded drainage, the continuous percolation of water has created shallow groundwater tables close to the surface (Belder et al., 2004; Cabangon et al., 2004; Lampayan et al., 2005). Again, farmers can either directly pump water up from the shallow groundwater or pump groundwater when it becomes surface water as it flows into creeks or drains. Recent studies of rice‐based irrigation systems in China and the Philippines indicate that irrigation eYciency increases with increasing spatial scale level because of the reuse of water (Hafeez, 2003; Loeve et al., 2004a,b). Much of this reuse is done informally by farmers who take their own initiative to pump water, block drainage waterways, or construct small on‐farm reservoirs for secondary storage. Most of these farmers are found in tail‐end portions of irrigation systems where water does not reach because too much water is lost upstream (e.g., by upstream farmers taking too much water, by canal seepage losses, and by operational losses). Although water can be eYciently reused this way, it does, however, come at a cost, especially to downstream farmers, and may not alleviate inequities among farmers in irrigation systems. The current debate on the improvement of irrigation systems focuses on the relative benefits and costs of system modernization vis‐a`‐vis those of internal (mostly informal) reuse of water. System modernization aims to improve
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the irrigation system delivery infrastructure and operation scheme to supply each farmer with the right amount of water at the right time. Gains in water productivity are possible by providing more reliable irrigation supplies, for example, through precision technology and the introduction of on‐demand delivery of irrigation supplies (Gleick, 2000; Rosegrant, 1997). The argument is that when farmers have control over timing and amount of water supplies to their farm, they need not take their turn in a fixed rotational schedule of deliveries if the soil is still wet from rainfall. Matching system delivery and field‐level demand needs further research, as optimal scheduling of irrigation is diYcult when a large part of the crop water requirement is met from rainfall. This is especially true in large irrigation systems with considerable lag time between diversion of water at the source (river or reservoir) and its arrival at the farmer’s gate. In some parts of China, although the main system is supply driven, farmers have control over the timing and amount of water at the farm gate because water is stored in small farm ponds, which can provide water also for other uses, but with the trade‐ oV of evaporative losses from the pond (Mushtaq et al., 2006). Further studies on the improvement of irrigation water delivery should also establish to what extent seasonal weather forecasts could be used to reduce risks of over‐ or underwatering and to increase water productivity.
2.
Field Versus Irrigation System Level
The relationships between water use at the field level and at the irrigation system level are complex and involve hydrological, infrastructural, and economic aspects. At the field level, farmers can increase water productivity and reduce water use by adopting water‐saving technologies. If they pay for the cost of the water they use, they can thereby increase the profitability of rice farming. At the irrigation system level, the adoption of field‐level water‐saving technologies by farmers will reduce the total amount of water lost as evaporation from rice fields, but by relatively small amounts only (Section III.B.1.b). However, most of the water saved at the field level is by reduced seepage, percolation, and drainage flows. On the one hand, this results in more water retained at the surface (in the irrigation canals), which is available for downstream farmers. On the other hand, it reduces the amount of water reentering the hydrological cycle and thus reduces the options for informal reuse downstream. Reducing percolation from rice fields can lower groundwater tables. This can adversely aVect yields since rice plants may be less able to extract water directly from the groundwater (Belder et al., 2004). Deeper groundwater tables will also increase the cost of pumping for reuse downstream. Any adoption of water‐saving technologies requires considerable water control by farmers. This is not much of a problem for
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farmers using their own pump, but it is so for farmers in large‐scale surface irrigation systems that lack flexibility in, and reliability of, water delivery. It is also a problem for farmers using electricity to pump groundwater where supplies are unreliable, as in northwest India. To allow farmers to profit from water‐saving technologies, such irrigation systems need to be modernized, which brings about an economic cost. 3.
Integrated Approaches in Irrigation Systems
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Approaches that integrate agronomic measures, improved policies, institutional reforms, and infrastructural upgrading may have the best chance of achieving impact. A recent success story on ‘‘producing more rice using less water’’ is the Zanghe Irrigation System (ZIS) in the middle reaches of the Yangtze Basin, China (Loeve et al., 2004a,b). ZIS has a command area of about 160,000 ha and services mainly rice in the summer season. Since the early 1970s, the amount of water released to agriculture has been steadily reduced in favor of increased releases to cities, industry, and hydropower (Fig. 7). Since the mid‐1990s, the amount of water received by agriculture has been less than 30% of the amount received in the early 1970s. In the same period, however, total rice production has increased, with a peak of around 650,000 t in the late 1980s that was nearly twice the amount produced in the
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late 1960s (Fig. 7). Although rice production has leveled oV to a stable 500,000 t in the last decade, more rice has been produced with less water over the past 30 years. This feat has been accomplished by a variety of integrated measures (Dong et al., 2004; Hong et al., 2001; Loeve et al., 2001, 2004a,b; Moya et al., 2004): Double rice cropping has been replaced by more water‐eYcient single rice cropping. This was possible because of the availability of modern short‐ duration high‐yielding varieties. The AWD water‐saving technology has been promoted and widely adopted. Policies, such as volumetric water pricing, and institutional reforms, such as water‐user associations, have been introduced that drive and promote eYcient use of water by farmers. The irrigation system has been upgraded (e.g., canal lining). Secondary storage has been developed through the creation of thousands of small‐ to large‐size ponds and reservoirs. The ZIS case study suggests that win‐win situations can exist where rice production can be maintained, or even increased, while freeing up water for other purposes. 4.
Stress‐Prone Environments
Many interventions at the landscape level are eVective in alleviating water‐related stresses. On‐farm water harvesting is an eVective means of reducing drought risk and increasing productivity in drought‐prone rainfed environments by having small amounts of extra water available to bridge critical periods with dry spells (Pal and Bhuiyan, 1995). A large proportion of rainfed lowland rice is cultivated at diVerent toposequence levels in sloping areas, resulting in variations in groundwater depth and in nutrient and water availability along the toposequence (Tsubo et al., 2006; Wade et al., 1999). Crop management must be adapted according to position along the slope to take these variations into account. Water tables are rising in many areas because of excessive irrigation applications. Better management of irrigation supplies can reduce the rate of rise, but, when irrigation water contains salt and leaching is required to maintain the salt balance of the root zone, a downward flow of water is essential. Soil drainage can be quite eVective in controlling water tables and in preventing salt accumulation in the root zone and, therefore, has a positive impact on water productivity at the field and agroecological system level. The development of reservoirs and canal networks to store fresh water from rain or rivers before they become saline can help extend the growing
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season in saline coastal areas and can substantially improve productivity (Mondal et al., 2006). Water management through large‐scale construction of coastal embankments and sluices has been reasonably successful in preventing seawater intrusion in many deltaic coastal areas, substantially reducing soil salinity in the wet season. Coastal embankments have increased rice yields in coastal Bangladesh by more than 200% in 20 years (Nishat, 1988). The technology also opens up the possibility of growing high‐yielding, modern rice varieties in these areas, as in the coastal areas of the Mekong Delta of Vietnam (Tuong et al., 2003) and Bangladesh (Mondal et al., 2006). However, water needs to be managed judiciously to avert undesirable long‐ term environmental consequences and local conflicts with other water users, especially the landless poor who depend on brackish‐water fisheries for their livelihood. The use of pumps for shallow groundwater (as in Bangladesh) or surface water (as in the Mekong Delta) allows cultivation of short‐duration varieties in nonflooded periods (Section II.B.5) in many deltas. Quantifying the level and timing of hydrological stresses is essential to designing cropping patterns that can avoid these production constraints (Tuong et al., 1991). Farmers in the flood‐prone areas of the Mekong Delta also build community dikes, protecting areas from ten to a few hundred hectares, which allows them to harvest the crop before the arrival of the flood. These dikes are lower than the maximum flood depths. They delay the onset of the flood rather than prevent the peak of the flood to enter the protected area, thus avoiding possible adverse consequences on biodiversity of complete prevention of flooding.
IV. SUMMARY AND RECOMMENDATIONS Around one‐fourth to one‐third of the world’s developed freshwater resources are used to irrigate rice, the staple food of 3 billion people. More than 90% of the world’s rice is produced and consumed in Asia, where rice is a political commodity and where millennium‐old practices of growing rice have resulted in specific rice cultures. The collective approach needed for the development, operation, and maintenance of rice systems requires collective and strong community approaches. Rice environments also provide unique— but as yet poorly understood—ecosystem services such as the regulation of water and the preservation of aquatic and terrestrial biodiversity. Because of unprecedented growth in production in the last five decades, the supply of rice has kept up with population growth and the price of rice has gone down. The production of rice still needs to increase in the coming decades to meet the food demand of growing populations, despite changing consumer preferences in diet as their economy improves. High yields and
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low prices of rice have contributed considerably to poverty alleviation among poor rice consumers, however poverty still exists in rural rice‐ growing areas and is increasing in urban areas. To meet the dual challenges of producing enough food and alleviating poverty, more rice needs to be produced at a low cost per kilogram grain (ensuring reasonable profits for producers) so that prices can be kept low for poor consumers. Approximately 79 million ha of highly productive irrigated lowlands provide 75% of the world’s rice supply. The food security of many poor consumers relies heavily on the productive capacity of these irrigated areas. Rice production under flooded conditions is highly sustainable and may have fewer adverse environmental eVects than many other crops. Although rice has the same water productivity with respect to transpiration as other C3 crops, it requires more water at the field level than other grain crops because of high outflows (percolation and seepage) from the field. As these outflows can often be captured and reused downstream, water‐use eYciency may be higher at the irrigation system level than at the field level. Increasing water scarcity, however, is expected to (further) shift rice production to more water‐abundant delta areas, and to lead to crop diversification and more aerobic soil conditions in rice fields in water‐short areas. In these latter areas, investments should target the adoption of water‐saving technologies, the reuse of drainage and percolation water, and the improvement of irrigation supply systems. A suite of water‐saving technologies can help farmers reduce percolation, drainage, and evaporation losses from their fields by 15–20% without a yield decline. However, greater understanding of the adverse eVects of increasingly aerobic (nonflooded) field conditions (as induced by water scarcity) on the sustainability of rice production, the environment, and ecosystem services is needed. Maintaining vital ecosystem services—beyond that of producing food—of rice environments will become increasingly important and should be explicitly recognized and protected. In drought‐, salinity‐, and flood‐prone environments, the combination of improved varieties with specific management packages has the potential to increase on‐farm yields by 50–100% in the coming 10 years, provided that investment in research and extension is intensified.
ACKNOWLEDGMENTS Contributors to the Comprehensive Assessment of Rice and Water were G. N. Atlin, V. Balasubramanian, J. Bennett, D. Dawe, K. Dittert, A. Dobermann, T. Facon, N. Fujimoto, R. K. Gupta, S. M. Haefele, K. L. Heong, Y. Hosen, A. M. Ismail, D. Johnson, S. Johnson, S. Khan, Lin Shan, I. Masih, Y. Matsuno, S. Pandey, S. Peng, T. M. Thiyagarajan, and R. Wassman.
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Earlier drafts of the assessment were reviewed by G. Castillo, M. A. Ghani, P. Kiepe, N. Magor, K. Palanisamy, D. Renault, D. Seckler, A. K. Singh, P. Van Mele, I. Willett, and Yuanhua Li. This chapter was codeveloped as part of the theme Crop Water Productivity of the CGIAR Challenge Program on Water and Food.
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Index A Acrylamide (AMD) monomer, 82, 115–17, 119, 128–30 Active bacterial in soils, treated with polyacrylamide (PAM), 125–7 Agricultural interventions, individual nutrient delivery, 8–13 Agriculture. See also Agroecosystems human health interaction, 4–5 nutrients management in, 165–6 Agroecosystems biogeochemical processes in, 167–8 nutrients management, 163–79 ecosystem-based approach, 168–77 efficiency, 168–77 microbially mediated processes, 175–7 plant adaptation, 177–8 plant adaptation for, 177–8 plant diversity use, 172–4 plant–microbial interactions, 174–5 to restore ecosystem functions, 172–7 Atrazine in soils, sorption and desorption, 121–2, 125
Center-pivot irrigation, and polyacrylamide, 104 Central America bean food systems, 31–6 bean production, 33–4 nutritional and health status, 31–3 role of bean in diet, 34–5 subsistence food systems, 31–6 Central American Nutrition Institute (INCAP), Guatemala, 34 Chemical oxygen demand (COD), 75, 119 Chlorothalonil loss, by polyacrylamide, 122 Coliform bacteria migration, by polyacrylamide, 123 Common lambsquarters runoff loss, and polyacrylamide, 124 Communities, nutrient delivery system, 54–60 Comparative advantages, in rice production, 196–7 Comprehensive Assessment of Water Management in Agriculture, 188 Construction sites, polyacrylamide (PAM) for, 133–7 Cut back irrigation, and polyacrylamide, 102
B Bangladesh Ca in crops of, 27–8 nutrient-deficient soils in, 28–9 remedial food system strategies, 29–30 Bean food systems, in Central America, 31–6 Biodegradation of polyacrylamide (PAM) based agriculture, 129 Biofortification, 65 Biogeochemical processes in, agroecosystems, 167–8 Biological oxygen demand (BOD), 83, 119 Biopolymers surrogates, of polyacrylamide, 139–41 Bupirimate loss, by polyacrylamide, 122
C Calcium and polyacrylamide, 131–3 Canal sealing, by polyacrylamide, 137–9
D Death causes, 2–4 Degradation of polyacrylamide (PAM) based agriculture, 127–31 2,4-Dichlorophenoxyacetic acid, sorption and desorption in soils, 121–2, 125 Diet diversification, through food systems, 13–14 Diets low in Zn, global distribution, 12 Disease, and nutrition, 3, 49 Dissimilatory nitrate reduction to ammonium (DNRA), 176–7 Dissolved organic carbon (DOC), 83 Dissolved reactive phosphorus (DRP), 75, 119–20 Disturbed lands, polyacrylamide (PAM) for, 133–7 Dysfunctional rice–pulse food system Souteast Bangladesh, 27–31 remedial strategies for, 29–30
239
240
INDEX E
F
Eastern and Southern Africa, subsistence food systems, 44–54 Ecosystem based nutrients management in agroecosystems, 163–79 approach to improve efficiency, 168–77 bioavailability of, 7 microbially mediated processes, 175–7 plant adaptation to, 177–8 plant diversity use, 172–4 plant–microbial interactions, 174–5 to restore ecosystem functions, 172–7 Ecosystem functions restoration microbially mediated processes, 175–7 plant diversity use, 172–4 plant–microbial interactions, 174–5 Ecosystems services, rice production, 200–1 Endosulfan loss, by polyacrylamide, 122 Environmental impacts polyacrylamide (PAM) based agriculture, 85, 88, 115–22 on rice production, 201–7, 211–26 ammonia volatilization, 202 greenhouse gases, 202–3, 218–19 reactive nitrogen (N), 201–2 water pollution, 203–5 Environmental land management and PAM-based agricultural. See Polyacrylamide (PAM) based agriculture Environmental Quality Incentives Program (EQIP), 97 Environments rice production, 190–1, 194–7 flood-prone rice, 190, 193, 195–6, 221 irrigated environments, 194 irrigated lowland rice, 190, 192, 194 rainfed environments, 190, 192–5, 220–1, 222 rainfed lowland rice, 190, 192, 195, 220, 222 rainfed upland rice, 190, 193, 195, 220–1 salinity-affected environments, 196, 221 Erosion control and infiltration management, by polyacrylamide, 75–6, 96–102, 127 Exchangeable sodium percentage (ESP), 77
Fecal coliform bacteria migration, by polyacrylamide, 123 Fecal streptococci migration, by polyacrylamide, 123 Fe deficiency, 4, 21 Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA), 115 Fertilizer strategies, for individual nutrient delivery, 10–11 Field retention, by polyacrylamide, 115–22 Flocculants, 78 Food security and poverty alleviation challenges, 205–7, 216–20 water scarcity, 206–7, 216–20 management practices improvement for, 211–21 irrigated environment, 211–19 landscape level, 221–6 stress-prone environment, 216–20 rice varietal improvement for, 208–11 Food systems. See also Subsistence food systems bean food systems, 31–6 bean production, 33–4 in Central America, 31–6 information gaps, 36 nutritional status, 31–3 other components, 35–6 role bean in diet, 34–5 bioavailability imperative, 7 diet, 6–7 diet diversification through, 13–14 disease and, 6–7 farming for health, 7 individual nutrient delivery through, 13–14 maize-based food systems, 16 nutritional analysis, 20–2, 28 potato-based food systems diet, nutrition and, 41–3 dietary coverage, 42–3 food access and availability, 40–1 in Huancavelica Department, Peru, 36–44 malnutrition and, 39–40 potentially beneficial interventions, 43–4 rice-based food systems, 17–60 potato and wheat roles in, 24
INDEX rice/potato-based food systems, 23–7 crop diversification, 24–5 cropping systems, 23–4 limitations to crop diversification, 25 nutritional benefits of potato, 26–7 profitability, 25–6 sustainability, 25–6 rice–pulse food system analysis of, 28–9 preferred nutrient-balanced, 30–1 remedial strategies, 29–30 of Southeast Bangladesh, 27–31 rice–wheat food system, 17–23 additional crop, 22 agronomic practices, 19–20 breeding potential, 18–19 diversification, 22–3 nutritional analysis, 20–2 socioeconomic and policy environments, 59–66 agricultural development, 60–3 food prices, 60–3 households income, 60–3 trace elements addition to soils, 63–5 Foundation for Promotion of Eating Brown Rice, 16 Fungal biomass in soils, treated with polyacrylamide, 125–7 Furrow irrigation, and polyacrylamide, 95–102, 105, 109, 112, 114, 121, 123, 134, 140
G Genetically modified organism (GMO) rice, 18–19 Great Lakes Region, Central Africa breeding for nutrition, 50 cash crops, 46 cropping/nutrition balance implications, 49–50 cropping seasons, 45 cropping system, 45–6 future scenarios, 50–1 human nutrition, 46–9 nutrition and disease, 49 potato and sweet potato/bean food systems, 44–51 vitamin A deficiency and Fe deficiency anemia, 48
241
Greenhouse gases environmental impacts, 202–3, 218–19 methane, 202–3, 218–19 nitrous oxide, 203, 218–19 Green Revolution, 6, 8, 13, 15, 17–18, 61, 66, 187
H Hairy nightshade runoff loss, and polyacrylamide, 124 HarvestPlus Challenge Program, 2, 9, 34–5 Hidden hunger, 12 Households, nutrient delivery system, 54–60 Huancavelica Department, Peru agroecological zones, 37–8 biophysical conditions, 37–8 component crops in, 37–8 diet and nutrition, 41–3 dietary coverage from potato and total diet, 42–3 food access and availability, 40–1 malnutrition, 39–40 potato crop production systems, 37, 38 potentially beneficial interventions, 43–4 Human life, essential nutrients for sustaining, 6 Hydrolyzed polyacrylonitrile (HPAN) soil conditioner, 79 Hydrolyzed starch-polyacrylonitrile graft polymers (H-SPANs) soil conditioner, 80
I Individual nutrient delivery agricultural interventions, 8–13 fertilizer strategies, 10–11 food systems approaches, 13–14 plant breeding strategy, 9–10 rice–fish farming systems, 54–60 Zn deficiency, 11–13 Indo-Gangetic Plains rice-based food systems, 17–23 rice/potato-based food systems, 23–7 Infiltration management, by polyacrylamide, 75–6, 96–102, 127
242
INDEX
Infiltration process, and polyacrylamide, 107–15 International Center for Tropical Agriculture (CIAT), Colombia, 35 International Potato Center (CIP), Andes, 36 International Rice Research Institute (IRRI), 216, 218 Isobutylene maleic acid (IBM) soil conditioner, 79
K Kelthane, 121 Kochia runoff loss, and polyacrylamide, 124 Krilium soil conditioner, 79
L Landscape level improvement rice production, 221–6 field versus irrigation systems, 223–4 integrated approaches to irrigation systems, 224–5 irrigation systems, 221–3 stress-prone environments, 225–6
N Napropamide sorption and desorption, polyacrylamide, 122 National Institute of Occupational Safety and Health (NIOSH), 116 Natural Resource Conservation Service (NRCS), 94–6, 117, 119, 135–6 Nephelometric turbidity units (NTUs) runoff, polyacrylamide, 134 New agriculture paradigm, 4–5 Nutrient delivery system, communities and households, 54–60 Nutrient-dense crops, 13, 65 Nutrients management in agroecosystems, 163–79 bioavailability, 7 conceptual framework, 170, 171 disease, 3, 49 ecosystem-based approach, 168–77 efficiency, 168–77 microbially mediated processes, 175–7 plant diversity use, 172–4 plant–microbial interactions, 174–5 to restore ecosystem functions, 172–7
O Organisms in runoff and soil, and polyacrylamide, 122–7
M Maize–Cowpea Intercrop System, Zimbabwe, 51–4 Malnutrition global crisis, 5 Management practices improvement for rice production, 211–21 closing yield gap, 211–12 irrigated environments, 211–19 stress-prone environments, 220–1, 222 water-saving technologies, 212–21 Metolachlor sorption and desorption, polyacrylamide, 122 Microbial biomass in soils, treated with polyacrylamide, 125–7 Micronutrient malnutrition, 3–4 Microorganisms, 75 Milling, 16 Montane paddy rice, 220
P PAM-based agricultural. See Polyacrylamide (PAM) based agriculture Patch method, 96–7 Pesticides, 75 Photodegradation of polyacrylamide (PAM), 129 Picloram sorption and desorption, and polyacrylamide, 122 Plant adaptation, ecosystem based nutrients management, 177–8 Plant breeding strategy, individual nutrient delivery, 9–10 Polyacrylamide (PAM) based agriculture atrazine sorption and desorption, 121–2, 125
INDEX bacterial biomass in soils treated with, 125–7 biodegradation, 129 biopolymers surrogates, 139–41 bupirimate loss, 122 calcium and, 131–3 canal sealing, 137–9 center-pivot irrigation and, 104 chlorothalonil loss, 122 coliform bacteria migration, 123 common lambsquarters runoff loss, 124 for construction sites, 133–7 current use overview, 81–4 cut back irrigation and, 102 definition and description, 84–6 degradation, 127–31 2,4-dichlorophenoxyacetic acid sorption and desorption in soils, 121–2, 125 for disturbed lands, 133–7 early contributions, 93–5 effects on organisms in runoff and soil, 122–7 endosulfan loss, 122 environmental impacts, 85, 88, 115–22 erosion control and infiltration management, 75–6, 96–102, 127 fecal coliform bacteria migration, 123 fecal streptococci migration, 123 field retention, 115–22 formulations, 84–6 fungal biomass in soils treated with, 125–7 furrow irrigation and, 95–102, 105, 109, 112, 114, 121, 123, 134, 140 hairy nightshade runoff loss, 124 infiltration process, 107–15 kochia runoff loss, 124 metolachlor sorption and desorption, 122 microbial biomass in soils treated with, 125–7 napropamide sorption and desorption, 122 nephelometric turbidity units (NTUs) runoff, 134 photodegradation, 129 picloram sorption and desorption, 122 pond sealing, 137–9 properties affecting efficacy, 86–93 redroot pigweed runoff loss, 124 role in stabilizing surface structure, 105–6 safety, 115–22
243
sprinkler irrigation and, 102–7, 112 structure unit, 84–5 surface chemistry of soils and, 86 surface irrigation and, 95–102, 122 weed seeds runoff loss, 124 Poly (diallyldimethylammonium chloride) (poly-DADMAC) polymers, 81 Polyvinyl alcohol (PVA) soil conditioner, 79 Pond sealing, and polyacrylamide, 137–9 Potato, nutritional benefits, 26–7 Potato and sweet potato/bean food systems, 44–51 breeding for nutrition, 50 cash crops and, 46 cropping/nutrition balance implications, 49–50 cropping system, 45–6 in Great Lakes region, 44–51 human nutrition, 46–9 nutrition and disease, 49 Potato based food systems agroecological zones, 37–8 biophysical conditions, 37–8 component crops, 37–8 diet and nutrition, 41–3 dietary coverage from potato and total diet in, 42–3 food access and availability, 40–1 in Huancavelica Department, Peru, 36–44 malnutrition and, 39–40 potato crop production systems, 37–8 potentially beneficial interventions, 43–4 Preferred nutrient-balanced food system, 30–1
R Ramsar Convention on wetlands, 201 Redroot pigweed runoff loss, and polyacrylamide, 124 Rice based food systems, 17–60 in Indo-Gangetic Plains, 17–23 rice/potato system, 23–7 rice–pulse system, 27–31 rice–wheat system, 17–23 Rice fish farming system, 54–60 agronomic management practices, 55–7 constraints, 59–60 food composition values, 58–9 genetic variability, 58–9
244
INDEX
Rice fish farming system (cont.) improvements, 60 productivity levels, 55 reliability of food composition data, 58–9 socioeconomic and policy environments, 59–60 sustainability issues, 57 Rice/potato-based food systems, 23–7 crop diversification, 24–5 cropping systems, 23–4 in IGP, 23–7 limitations to crop diversification, 25 nutritional benefits of potato, 26–7 profitability, 25–6 sustainability, 25–6 Rice production challenges, 205–6 food production and poverty alleviation, 205–6 water scarcity, 206–7, 216–20 consumption, 191 ecosystems services, 200–1 environmental impacts, 201–7, 211–26 acidic pollution, 205 ammonia volatilization, 202 arsenic pollution, 205 biocides water pollution, 204–5 greenhouse gases, 202–3, 218–19 methane, 202–3, 218–19 nitrate water pollution, 204 nitrous oxide, 203, 218–19 reactive nitrogen (N), 201–2 salinization of water, 205, 211 water pollution, 203–5 environments, 190–7 flood-prone rice, 190, 193, 195–6, 221 irrigated environments, 194 irrigated lowland rice, 190, 192, 194 rainfed environments, 190, 192–5, 220–2 rainfed lowland rice, 190, 192, 195, 220, 222 rainfed upland rice, 190, 193, 195, 220–1 salinity-affected environments, 196, 221 growth in, 187–8 landscape level improvement, 221–6 field versus irrigation systems, 223–4 integrated approaches irrigation systems, 224–5
irrigation systems, 221–3 stress-prone environments, 225–6 management practices improvement, 211–21 closing yield gap, 211–12 irrigated environments, 211–19 stress-prone environments, 220–2 water-saving technologies, 212–21 plant, 189–90 recommendations on, 226–7 response options, 207–26 landscape level improvement, 221–6 management practices improvement, 211–21 varietal improvement, 208–11 shifting comparative advantages, 196–7 stress-prone environments, 220–2 flood-prone environment, 221 rainfed lowlands, 220, 222 rainfed uplands, 220–1 salinity affected environment, 221 sustainability and environmental impacts with water scarcity, 216–20 trends and conditions, 189–207 varietal improvement, 208–11 drought tolerance, 209–10 salinity tolerance, 211 submergence tolerance, 210–11 water productivity, 208–9 water-related stresses tolerance, 209–11 yield potential, 208 water-saving technologies, 212–21 aerobic rice, 214, 216 alternate wetting and drying, 213–15 direct seeding, 212–13 reducing turn-around time, 212–13 saturated soil culture, 213 soil amelioration, 212 water use and production, 197–201, 215–16 water balance, 198 water flows from rice field, 197–9 water pollution, 203–5 water productivity, 199–201, 208–9, 215–16 Rice pulse food system analysis of, 28–9 preferred nutrient-balanced in, 30–1 remedial strategies, 29–30 of Southeast Bangladesh, 27–31
INDEX Rice wheat food system, 17–23 additional crop, 22 agronomic practices, 19–20 breeding potential, 18–19 diversification of, 22–3 in IGP, 17–23 nutritional analysis, 20–2
245
Surface irrigation, and polyacrylamide, 95–102, 122 Surface structure, and polyacrylamide role in stabilization, 105–6 Surfactants, 78
T S Sodium adsorption ratio (SAR), 77 Sodium polyacrylate (SPA) soil conditioner, 79 Soil conditioner application strategies, 77–81 early uses, 76–7 mineral conditioners, 77–8 organic conditioners, 77–8 synthetic conditioners, 77–81 types, 77–8 water-soluble polymeric conditioners, 79 Southeast Bangladesh, dysfunctional rice–pulse food system, 27–31 Sprinkler irrigation, and polyacrylamide, 102–7, 112 Stress-prone environments and rice production flood-prone environment, 221 rainfed lowlands, 220, 222 rainfed uplands, 220–1 salinity affected environment, 221 Subsistence food systems. See also Food systems analysis of, 14–60 bean food systems, 31–6 in Central America, 31–6 of Eastern and Southern Africa, 44–54 in IGP, 23–7 potato-based food systems, 36–44 potato/bean based food systems, 44–51 rice–fish food system, 54–60 rice/potato-based food systems, 23–7 rice–pulse food system, 27–31 rice–wheat food system, 17–23 in Southeast Bangladesh, 27–31 sweet potato based food systems, 44–51 Super Slurper soil conditioner, 80 Surface chemistry of soils, and polyacrylamide, 86
Trace elements addition to soils food systems and, 63–5 fortification of fertilizers, 64–5 through public action, 64
U US Environmental Protection Agency (USEPA), 119
V Varietal improvement for rice production, 208–11 drought tolerance, 209–10 salinity tolerance, 211 submergence tolerance, 210–11 water productivity, 208–9 water-related stresses tolerance, 209–11 yield potential, 208 Vinylacetate maleic acid (VAMA) soil conditioner, 79 Vitamin A deficiency, 4, 20–1
W Water pollution impacts acidification, 205 arsenic, 205 biocides, 204–5 nitrate, 204 salinization, 205, 211 Water-saving technologies rice production, 212–21 aerobic rice, 214, 216–17 alternate wetting and drying, 213–15 direct seeding, 212–13 reducing turn-around time, 212–13 saturated soil culture, 213
246 Water-saving technologies (cont.) soil amelioration, 212 Water scarcity, sustainability and environmental impacts on rice production, 216–20 Water use and production in rice production, 197–201, 215–16 water balance, 198 water flows from rice field, 197–9 water pollution, 203–5 water productivity, 199–201, 208–9, 215–16
INDEX Weed seeds runoff loss, and polyacrylamide, 75, 124 Women in Burundi, seasonal energy and protein intakes among, 47 Z Zanghe Irrigation System (ZIS), 224 Zimbabwe, Maize–Cowpea intercrop system, 51–4 Zn deficiency, 11–13, 20–1