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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