Submitted to: Book Chapter
Publication Type: Book / Chapter
Publication Acceptance Date: June 23, 1997
Publication Date: N/A
Technical Abstract: Sustainable agriculture is broadly defined as agriculture that is managed toward greater resource efficiency and conservation while maintaining an environment favorable for evolution of all species. One of the driving forces behind agricultural sustainability is effective management of N in the environment. Moreover, judicious management of N inputs into cropping systems is a prerequisite for land stewardship. Successful manipulation o N inputs through the use of biologically fixed N results in farming practices that are economically viable and environmentally prudent. For example, use of N fixing species in cropping systems reduces the need for N fertilizers and increases soil tilth. Additionally, biologically fixed N is bound in soil organic matter and thus is much less susceptible to soil chemical transformations and physical factors that lead to volatilization and leaching. Although many diverse associations contribute to symbiotic N2 fixation, in most agricultural settings the primary source (80%) of biologically fixed N is through the soil bacteria Rhizobium, Bradyrhizobium, Sinorhizobium, and Azorhizobium-Legume symbiosis. Legumes provide 25 to 35% of the worldwide protein intake. Approximately 250 million hectares of legumes are grown world wide and they fix about 90 Tg N/yr. The amount of N fixed by legumes is quite amazing because the total amount of nitrogenase in the world amounts to only a few kilograms. To replace the N fixed by legumes with anhydrous ammonia produced by the Haber-Bosch process would require 288 Tg of fuel and cost approximately $30 billion annually. Obviously, important goals for agriculture are enhancing the use of and improving the management of legume biologically fixed N for both humanitarian and economic reasons.