Nitrogen fixation

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Nitrogen fixation is a chemical process by which the abundant but relatively inert molecular dinitrogen (N2) is converted into bioavailable nitrogen compounds such as ammonia (NH3) and nitrates (NO3). It occurs both biologically and abiologically, the latter either naturally through weather phenomena (e.g. lightning) or artificially by chemical industries. Biological nitrogen fixation or diazotrophy is catalyzed by enzymes called nitrogenases, produced by microorganisms such as cyanobacteria and rhizobia. 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). Some nitrogen-fixing bacteria have symbiotic relationships with plants, especially legumes, mosses, and aquatic ferns such as Azolla. 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...

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