They contain symbiotic bacteria called rhizobia within nodules in their root systems, producingnitrogencompounds that help the plant to grow and compete with other plants. When the plant dies, thefixed nitrogenis released, making it available to other plants; this helps to fertilize the soil

Nitrogen fixationis a process by whichnitrogenin theairis converted intoammonia(NH3) or related nitrogenous compounds.[1]Atmospheric nitrogen is moleculardinitrogen(N2), a relatively nonreactive molecule that is metabolically useless to all but a few microorganisms. Biological nitrogen fixation converts N2into ammonia, which is metabolized by most organisms.

Nitrogen fixation is essential to life because fixed inorganic nitrogen compounds are required for thebiosynthesisof all nitrogen-containingorganic compounds, such asamino acidsandproteins,nucleoside triphosphatesandnucleic acids. As part of thenitrogen cycle, it is essential foragricultureand the manufacture offertilizer. It is also, indirectly, relevant to the manufacture of all chemical compounds that contain nitrogen, which includes explosives, most pharmaceuticals, and dyes.

Nitrogen fixation is carried out naturally in thesoilby a wide range ofmicroorganismstermeddiazotrophsthat includebacteriasuch asAzotobacter, andarchaea. Some nitrogen-fixing bacteria havesymbioticrelationships with someplantgroups, especiallylegumes.[2]Looser non-symbiotic relationships between diazotrophs and plants are often referred to as associative, as seen in nitrogen fixation onriceroots. Nitrogen fixation also occurs between sometermitesandfungi.[3]It also occurs naturally in the air by means ofNOxproduction bylightning.[4][5]

All biological nitrogen fixation is effected by enzymes callednitrogenases.[6]These enzymes containiron, often with a second metal, usuallymolybdenumbut sometimesvanadium.

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