The chemical or biological conversion of atmospheric nitrogen (N₂) into compounds which can be used by plants, and thus become available to animals and humans. In the 1990s, chemical and biological processes together contributed about 260 million tons (230 million metric tons) of fixed nitrogen per year globally. Industrial production of nitrogen fertilizer accounted for about 85 million tons (80 million metric tons) of nitrogen per year, while spontaneous chemical processes, such as lightning, ultraviolet irradiation, and combustion, leading to the synthesis of nitrogen oxides from O₂ and N₂, may have accounted for 44 million tons (40 million metric tons) per year. The remainder, roughly half of the global input of newly fixed nitrogen, arose from biological processes. World agriculture, which is very dependent on nitrogen fixation, is increasingly reliant on chemical nitrogen sources.

Three chemical processes for fixing atmospheric nitrogen have been developed. All require considerable thermal or electrical energy and yield different products. In arc processes, which are now rarely used, air is passed through an electric arc and about 1% nitric oxide is formed, which can be chemically converted to nitrates. In the cyanamide process, which is now obsolete, heating calcium carbide in nitrogen generates calcium cyanamide, which when moistened hydrolyzes to urea and ammonia. In the widely used Haber process, hydrogen (generated by heating natural gas) is mixed with nitrogen (from air), and burned to yield a nitrogen-hydrogen mixture. The nitrogen-hydrogen mixture is compressed (10–80 megapascals) and heated (200–700°C or 390–1300°F) in the presence of a metal oxide catalyst to give ammonia. The Haber process is the major source of ammonia used for fertilizer. Fertilizer

Only prokaryotes—bacteria, archaea, and cyanobacteria (earlier called blue-green algae)—fix nitrogen. Nitrogen-fixing microbes, called diazotrophs, fall into two main groups, free-living and symbiotic. Archaebacteria Bacteria Cyanobacteria Prokaryotae

The free-living diazotrophs are subclassified. Aerobic diazotrophs, of which there are over 50 genera, including Azotobacter, methane-oxidizing bacteria, and cyanobacteria, require oxygen for growth and fix nitrogen when oxygen is present. Azotobacter, some related bacteria, and some cyanobacteria fix nitrogen in ordinary air, but most members of this group fix nitrogen only when the oxygen concentration is low. Free-living diazotrophs, which fix nitrogen only when oxygen is absent or vanishingly low, are widespread. The genera Bacillus and Klebsiella include many strains of this type, and representatives of symbiotic diazotrophs behave in this way as well. Algae Bacterial physiology and metabolism

The best-known symbiotic bacteria belong to the genus Rhizobium. Species of Rhizobium, or related genera, such as Bradyrhizobium and Sinorhizobium, colonize the roots of leguminous plants and stimulate the formation of nodules within which they fix nitrogen microaerobically. Both plants and bacteria show specificity; for example, certain types of plants require special strains of rhizobia. Some types of rhizobium, such as Bradyrhizobium, can fix nitrogen in the absence of plant tissue, but require low oxygen, though most rhizobia fix nitrogen only within the nodules. Soil microbiology

The enzymes responsible for nitrogen fixation are called nitrogenases. The most common nitrogenase consists of two proteins, one large containing molybdenum, iron, and inorganic sulfur (the MoFe-protein or dinitrogenase), the other smaller containing iron and inorganic sulfur (the Fe-protein or dinitrogenase reductase). Nitrogenase reduces one molecule of N₂ to two of ammonia (NH₃), a reaction which is accompanied by the conversion of 16 molecules of adenosine triphosphate (ATP) to adenosine diphosphate (ADP) and the release of one molecule of H₂ as a by-product. Nitrogenase is irreversibly destroyed by air, so all aerobic diazotrophs have developed means of restricting access of oxygen to the active enzyme.