Nitrogen Fixation

Abstract

Nitrogen fixation converts inert atmospheric molecular nitrogen gas into reduced forms whereby the nitrogen may be used by all life forms for protein and nucleic acid production. The biological process occurs only in microorganisms and is the principal contributor of fixed nitrogen to the biosphere. Fixed nitrogen is also derived from nonbiological processes, eg, fires, volcanoes, and lightning, and from commercial fertilizer production. The Haber‐Bosch ammonia process is highly energy efficient and is the most economical industrial process available. The most important source of biologically fixed nitrogen for agriculture is the symbiotic system that involves leguminous plants, which harbor rhizobia bacteria in nodules on their roots. Smaller contributions are made by other, less formal systems and by free‐living microbes. All of the biological systems use a similar metalloenzyme complex, called nitrogenase. Biological nitrogen fixation is discussed from a biochemical‐genetic viewpoint, as well as in terms of model chemistry and comparisons with the commercial process. Chemical approaches aimed at duplicating the biological process are also presented.