Nitrogen Fixation

Nitrogen fixation refers to the chemical conversion of nitrogen gas (dinitrogen, N₂) to some oxidized form, usually nitric oxide (NO) or ammonia (NH₃). Nitrogen fixation can occur through inorganic reactions or as a result of biological processes. Because the nitrogen atoms in dinitrogen are bound by a very strong triple bond, this gas is very stable and cannot be utilized as a source of nutrition by any but a few highly specialized microorganisms. These nitrogen-fixing microbes are critically important ecologically because nitrogen fixation is the ultimate source of the capital of available organic nitrogen in ecosystems.

It has been estimated that terrestrial ecosystems fix about 135 million metric tons of nitrogen/year (10⁶ tons N/yr), and marine ecosystems 40 × 10⁶ tons N/yr. The terrestrial fixation most commonly occurs in agroecosystems in which legumes are cultivated which fix an estimated 44 × 10⁶ tons N/yr, while grasslands fix 45 × 10⁶ tons N/yr, forests 40 × 10⁶ tons N/yr, and all other terrestrial ecosystems 10 × 10⁶ tons N/yr. Industrial fixation of nitrogen for the manufacturing of synthetic fertilizers contributed another 30 × 10⁶ tons N/yr in the early 1980s, while fixation during combustion accounts for 19 × 10⁶ tons N/yr. Because of increasing demands for nitrogen fertilizers for agriculture, it has been estimated that industrial fixation could increase to 100 × 10⁶ tons N/yr by the year 2000.