Nitrogen fixation is the process in which atmospheric N2 is converted to compounds that can be utilized by plants. It is both a natural process that is mediated by certain microorganisms and an industrial process that requires large amounts of energy. Although all organisms require N compounds, very few are able to utilize N2, the most abundant readily-available form of the element. Most organisms require fixed forms of N in the form of NH3, NO3-, NO2-, or organic-N. The amount of fixed N may be a key limiting factor in the earth's ability to sustain life.
The N2 molecule is relatively inert and unreactive. It combines with other elements only at high temperature and pressure or in the presence of a catalyst. N can be fixed in the form of several oxides. Two sources of N oxides are internal combustion engines and lightning. Internal combustion engines produce NO and NO2 because the high internal temperatures and pressures cause atmospheric N2 and O2 to react. High-voltage electrical discharges, such as lightning, can oxidize N2. Cyanobacteria (blue-green algae) and bacteria associated with legumes can fix N2 by reducing it to ammoniacal (ammonia-like) N, mostly in the form of amino acids. N2 is reduced to NH3 industrially using high temperature, pressure, and a catalyst. NH3 production is the second-largest chemical industry in the United States, with an annual production of 17.2 million metric tons (14.2 million metric tons of N).
In pre-industrial times, the natural fixation of atmospheric N2 gas is estimated to be 40 to 200 million metric tons N/yr by marine ecosytems and 90 to 130 million metric tons N/yr by terrestrial ecosystems (Galloway, 1998). Lightning produces approximately 9.4 million metric tons of N oxides each year.
Estimates of global anthropogenic fixation of N2 include 80 million metric tons/yr of chemical N fertilizer, 20 million metric tons of N/yr released by combustion, 25 million metric tons of N/yr fixed by agricultural plantings of N-fixing legumes, and 25 million tons of N recycled in non-agricultural anthropogenic wastes (Kinzig and Socolow, 1994; Galloway et al., 1995; Galloway, 1998; Smil, 1999). U. S. production in 1999 included approximately 17 million metric tons of anhydrous NH3, 7 million metric tons of ammonium nitrate, 3 milion metric tons of ammonium sulfate, and 8 million metric tons of nitric acid (Anon. 2000). The combustion estimate includes oxidation of the organic-N in coal and oil to NO. Although this is not fixation of N2, some researchers feel it is equivalent to N2 fixation because the organic N would otherwise remain unavailable for cycling in the biosphere (Vitousek et al., 1997).
It is interesting to note that the amount of N2 released by combustion and fertilizer manufacturing is of the same order of magnitude as biological of N fixation.