Nitrogen fixation is the process by which nitrogen is taken from its relatively inert molecular form (N₂) in the atmosphere and converted into nitrogen compounds useful for other chemical processes (such as, notably, ammonia, nitrate and nitrogen dioxide).

Nitrogen fixation is performed naturally by a number of different prokaryotes, including bacteria, and actinobacteria certain types of anaerobic bacteria. Many higher plants, and some animals (termites), have formed associations with these microorganisms.

Biological nitrogen fixation was discovered by the Dutch microbiologist Martinus Beijerinck.

Biological Nitrogen Fixation

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Biological Nitrogen Fixation (BNF) occurs when atmospheric nitrogen is converted to ammonia by a bacterial enzyme called nitrogenase. Microorganisms that fix nitrogen are called diazotrophs. The formula for BNF is:

N₂ + 8H⁺ + 8e⁻ + 16 ATP → 2NH₃ + H₂ + 16ADP + 16 P_i

Although ammonia (NH₃) is the direct product of this reaction, it is quickly ionized to ammonium (NH₄⁺). In free-living diazotrophs, the nitrogenase-generated ammonium is assimilated into glutamate through the glutamine synthetase/glutamate synthase pathway.

Leguminous nitrogen-fixing plants

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The best-known are legumes (such as clover, beans, alfalfa and peanuts) which contain symbiotic bacteria called rhizobia within nodules in their root systems, producing nitrogen compounds that help the plant to grow and compete with other plants. When the plant dies, the nitrogen helps to fertilize the soil. The great majority of legumes have this association, but a few genera (e.g., Styphnolobium) do not.

Non-leguminous nitrogen fixing plants

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Plants from many other families have similar associations, including:

• Lobaria lichen and some other lichens
• Mosquito fern (Azolla species)
• Cycads
• Gunnera
• Alder ('Alnus' species)
• Ceanothus ('Ceanothus' species)
• Wax myrtle ('Myrica' species)
• Mountain-mahogany ('Cercocarpus' species)
• Bitterbrush ('Purshia tridentata')
• Buffalo berry ('Shepherdia argentea')
• Ironwood ('Casuarina' species), Sheoak ('Allocasuarina' species), and other genera in the Casuarinaceae

Chemical nitrogen fixation

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Nitrogen can also be artificially fixed for use in fertilizer, explosives, or in other products. The most popular method is by the Haber process. Artificial fertilizer production has achieved such scale that it is now the largest source of fixed nitrogen in the Earth's ecosystem.

See also

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• Denitrification
• George Washington Carver
• Nitrification
• Nitrogen cycle
• Nitrogen deficiency
• Johanna Döbereiner

External links

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• Nitrogen Fixation

Botany | Nitrogen metabolism | Metabolism | Soil biology

Kvælstoffiksering | Stickstofffixierung | Nitrogena fiksado | Fixation biologique de l'azote | Azotofissazione | Stikstoffixatie | 窒素固定 | Fixação de nitrogênio | Азотфиксация