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Biological Nitrogen Fixation: RESEARCH CHALLENGES Executive Summary In the developed world, agriculture is dependent on chemical fertilizers and pesticides for achieving and maintaining the high yields that are possible with modern crop cultivars. However, recent documentation of adverse effects from these chemicals emphasizes the importance of developing new production methods that are sustainable both agronomically and economically. They should be environmentally benign and should produce food that is safe for human consumption. The great challenge lies in devising more sustainable farming systems without compromising food production levels; indeed, increased productivity will be necessary to accommodate global population growth. Nitrogen is the soil nutrient element needed in greatest quantity by crops. A key component to the success of the “green revolution ” in improving the yields of rice and wheat was the increased input of fertilizer nitrogen. Likewise, high yields of hybrid maize require abundant applications of nitrogen. Synthetic nitrogen use has grown from 3 million to 80 million tons over the last 40 years. This increase occurred in both developed and developing countries. The current annual worldwide expenditure for fertilizer nitrogen exceeds $20 billion—an amount comparable to that for synthetic chemical pesticides. Modern industrial production of fertilizer nitrogen demands large inputs of energy in the form of natural gas, a finite natural resource; fertilizer constitutes a major energy cost in the production of a high-yield corn or rice crop. Moreover, carbon dioxide is released by the consumption of natural gas. Food production may thus
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Biological Nitrogen Fixation: RESEARCH CHALLENGES contribute indirectly to global warming. Of the fertilizer nitrogen applied to a crop, seldom is more than 50 percent assimilated, and often the efficiency of utilization is much less. Whatever type of fertilizer nitrogen is applied, microbial action converts it to nitrate, a mobile form that is assimilated by plants and is subject to loss from surface-water movement, thereby polluting streams and rivers and eventually affecting estuarine and marine ecosystems. Furthermore, nitrate may leach into groundwater aquifers, contaminating wells and placing human health at risk. In wet soils, denitrifying bacteria convert nitrate to nitrous oxide and gaseous nitrogen. The former is a greenhouse gas that has an energy reflectivity per mole 180-fold higher than that of carbon dioxide. Thus, the use of fertilizer nitrogen may contribute to global warming. Key components of the global nitrogen cycle are being increasingly affected by the industrial conversion of atmospheric nitrogen and the accumulation of nitrous oxide. The consequences of these disequilibria are unclear, but prudence dictates that further perturbations of this major natural cycle be minimized. BIOLOGICAL NITROGEN FIXATION The natural process of Biological Nitrogen Fixation (BNF) has a critical role in the achievement of environmentally benign, sustainable farming systems. Its increased use will mitigate the need for fertilizer nitrogen, with concomitant benefits accruing in terms of effects on the global nitrogen cycle, global warming, and ground- and surface-water contamination. This natural process is dependent on microorganisms, and a plant may serve as a partner. Some species of microorganisms have the ability to convert atmospheric nitrogen into forms that are usable by plants and animals. BNF occurs in bacteria that possess the enzyme nitrogenase. Plants and microbes form symbiotic associations in legumes, lichens, and some woody plants. The system most important for agriculture is the legume-rhizobia symbiosis: the fixation of atmospheric nitrogen occurs within root nodules after rhizobial penetration of the root. Thus, many legumes can grow vigorously and yield well under nitrogen-deficient conditions, and may contribute nitrogen to the farming system in the vegetative residues after grain harvest, or more significantly as green manure incorporated in the soil. Legumes are important sources of protein, mainly for feed in the developed world and for food in the developing world. They have been exploited as sources of nitrogen most notably in the agricultural systems of Australia and New Zealand. The successful introduction of exotic legume crops, such as alfalfa and soybean into the United States, necessitated the simultaneous introduction of compatible rhizobia bacte-
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Biological Nitrogen Fixation: RESEARCH CHALLENGES ria; such inoculants, in various forms, have been in use for about 100 years. Molecular genetic research has made available the tools for possibly conferring upon cereals and other nonlegumes the ability to fix atmospheric nitrogen. Although realization of this goal represents a long-term endeavor, the possibility of either substantially reducing or eliminating the economic and environmental cost of the use of fertilizer nitrogen justifies the effort. Fundamental knowledge is now in hand to provide the basis for focused efforts on BNF in legumes and cereals. Benefits are expected from research with legumes in the nearer term, whereas benefits from research with cereals could be very large but are in the longer term. Legumes and BNF are very important in the developing world, whence much of the increases in food production must come to accommodate increasing world population. It is essential that tropical legumes be exploited to replace fertilizer nitrogen, to avoid compounding recalcitrant environmental problems of local and global proportions. The need for food, feed, fuel, and building material has made deforestation an increasingly pressing problem in the developing world; legumes and other nitrogen-fixing trees offer a means of reversing this trend, especially the use of fast-growing nitrogen-fixing trees. RESEARCH GRANTS PROGRAMS Research grants involving BNF in developing countries were included in the Program in Science and Technology Cooperation (PSTC), U.S.-Israel Cooperative Development Research Grants Program (CDR), and Committee on Research Grants (CRG) of the BOSTID Research Grants Program. Research topics included various legumes, breeding, farming systems, trees and shrubs, azolla, environment, modeling, pathology, pests, drought tolerance, tissue culture, molecular biology, salt tolerance, strain improvement, competitiveness, genetic engineering, DNA identification, associative nitrogen fixation, interaction with micorrhizae, and microbial antagonisms. Clearly, these research grants had a major impact both scientifically and in terms of professional development. Perhaps the most important accomplishment was the production of scientists and technicians capable of conducting research in BNF and related areas in developing countries. There were contributions also to undergraduate education and farmer training. The funds afforded the grantees opportunities to attend national and international meetings for important exposure of their research work and discussions with colleagues with shared interests. The scientific yield was high, at more than
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Biological Nitrogen Fixation: RESEARCH CHALLENGES six publications per project, as proceedings and peer-reviewed papers. The results of the projects are impacting on the environment, agricultural sustainability, and national development. There are also possible scientific implications for the United States, in addition to the fruitful exchange of ideas, materials, and philosophies. The projects even resulted in commercialization of products based on this new-found knowledge. GENERAL RECOMMENDATION The committee produced several recommendations based on its considerable and diverse knowledge and experience in BNF and its review of the AID-funded research. Specific recommendations contained in the body of the report all support one general recommendation: The outstanding global potential of BNF in agriculture and forestry will be realized only with the long-term investment of significant funds. The energy crisis of the early 1970s prompted an influx of funding for BNF. Ambitious expectations of positive effects on food production were only partially realized in the short term. However, there now exists a sound technological base, much detailed knowledge of nitrogen-fixing processes, and molecular genetic tools to foster the formulation and accomplishment of realistic goals as outlined in this report. A high priority for increased investment in BNF is justified by the opportunities to reduce or replace the growing cost of fertilizer nitrogen, currently more than $20 billion per year, to minimize the multifaceted local and global environmental damage that it causes, and to sustain the food needs of an expanding world population. Expansion of our knowledge and development of economic applications and management systems for developed and developing countries should be pursued.
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