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Much of the terrestrial fixation is carried out by symbiotic bacteria in plant roots and to lesser extent by free living soil bacteria. Cyanobacteria also fix nitrogen and are much significance in aquatic systems. Some termites have mutualistic gut micro flora which fixes atmospheric nitrogen.
Specialised, widespread soil bacteria play vital roles in nitrification. Some bacteria oxidise ammonium ions to nitrite, and others complete the process by converting NO2 to nitrate. These chemoautotrophs obtain energy from such transformations. Denitrifying bacteria reduce nitrate to gaseous nitrogen in anaerobic conditions, returning a small proportion of that circulating in the biosphere to atmosphere.
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