Nitrogen fixation

Nitrogen fixation is a chemical process by which molecular dinitrogen (N
₂) is converted into ammonia (NH
₃).^[1] It occurs both biologically and abiologically in chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases.^[2] These enzyme complexes are encoded by the Nif genes (or Nif homologs) and contain iron, often with a second metal (usually molybdenum, but sometimes vanadium).^[3]

Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses and aquatic ferns such as Azolla.^[4] Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation on rice roots. Nitrogen fixation occurs between some termites and fungi.^[5] It occurs naturally in the air by means of NO_x production by lightning.^[6][7]

Nitrogen fixation is essential to life on Earth because fixed inorganic nitrogen compounds are required for the biosynthesis of all nitrogen-containing organic compounds such as amino acids, polypeptides and proteins, nucleoside triphosphates and nucleic acids. As part of the nitrogen cycle, it is essential for soil fertility and the growth of terrestrial and semiaquatic vegetations, upon which all consumers of those ecosystems rely for biomass. Nitrogen fixation is thus crucial to the food security of human societies in sustaining agricultural yields (especially staple crops), livestock feeds (forage or fodder) and fishery (both wild and farmed) harvests. It is also indirectly relevant to the manufacture of all nitrogenous industrial products, which include fertilizers, pharmaceuticals, textiles, dyes and explosives.