the diazotrophic actinomycete Frankia. The plant supplies energy materials to the diazotrophs, which in turn reduce atmospheric nitrogen to ammonia. This ammonia is transferred from the bacteria to the plant to meet the plant's nutritional nitrogen needs for the synthesis of proteins, enzymes, nucleic acids, chlorophyll, and so forth.
Biological nitrogen fixation is an essential natural process that supports life on this planet. Higher plants and animals obtain nitrogen ultimately from nitrogen-fixing organisms or from nitrogen fertilizers (including nitrogen compounds formed during lightning strikes). Available soil nitrogen, which originates from decomposing plant residues and microorganisms, is normally deficient for intensive crop production. This is the compelling reason to improve our understanding of BNF for application to agriculture and forestry production worldwide. In addition, the projected doubling in population over the next 50 years will put increasing pressure on food production, the environment, and the need for fixed nitrogen. Growing concerns about the environment, energy, nutrition, and agricultural sustainability make the need for BNF research even more compelling.
Concerns about the cost and supply of fossil-based energy were major reasons for the expansion of BNF research in the 1970s. Environmental quality and sustainability are equally compelling concerns in the 1990s.
Nitrogen fixation occurs both biologically and non-biologically. Asymbiotic and symbiotic biological systems fix an estimated 100-175 million metric tons of nitrogen annually (Burns and Hardy, 1975), and this probably has not changed substantially during the last century. Non-biological nitrogen fixation occurs through the effects of lightning, and in industry primarily by the Haber-Bosch process, which requires high levels of fossil fuel. Worldwide, lightning may fix 10 million metric tons of nitrogen a year, a value that probably has not changed over time. Industrial fixation for fertilizer nitrogen has increased from 3.5 million tons in 1950 to 80 million tons in 1989 (Figure 1) (Hardy, 1993) in response to the needs of high-yielding crops.
By the year 2050, world population is expected to double from its current level of more than 5 billion. It is reasonable to expect that the need for fixed nitrogen for crop production will also at least double. If this is supplied by industrial sources, synthetic fertilizer nitrogen use will increase to about 160 million tons of nitrogen per year, about equal to that produced by the biological process. This amount of nitrogen fertilizer will require burning some 270 million tons of coal or its equivalent. However, expanded exploitation of BNF could reduce, and in the longer