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Nitrogen cycle. Nitrification is the biological oxidation of ammonia to nitrate via the intermediary nitrite.Nitrification is an important step in the nitrogen cycle in soil.The process of complete nitrification may occur through separate organisms [1] or entirely within one organism, as in comammox bacteria.
Important processes in the nitrogen cycle include fixation, ammonification, nitrification, and denitrification. The majority of Earth's atmosphere (78%) is atmospheric nitrogen , [ 16 ] making it the largest source of nitrogen.
[12] [13] Complete nitrification, the conversion of ammonia to nitrate in a single step known as comammox, has an energy yield (∆G°′) of −349 kJ mol −1 NH 3, while the energy yields for the ammonia-oxidation and nitrite-oxidation steps of the observed two-step reaction are −275 kJ mol −1 NH 3, and −74 kJ mol −1 NO 2 − ...
Figure 1: The nitrogen cycle in a soil-plant system. One potential pathway: N is fixed by microbes into organic compounds, which are mineralized (i.e. ammonification) and then oxidized to inorganic forms (i.e. nitrification) that are assimilated by plants (NO 3 −).
The oxidation of ammonia is a rate-limiting step in nitrification and plays a fundamental role in the nitrogen cycle, because it transforms ammonia, which is usually extremely volatile, into less volatile forms of nitrogen. [29]
The most common denitrification process is outlined below, with the nitrogen oxides being converted back to gaseous nitrogen: 2 NO 3 − + 10 e − + 12 H + → N 2 + 6 H 2 O. The result is one molecule of nitrogen and six molecules of water. Denitrifying bacteria are a part of the N cycle, and consists of sending the N back into the atmosphere.
Nitrification has traditionally been thought to be a two-step process, where ammonia-oxidizing bacteria and archaea oxidize ammonia to nitrite and then nitrite-oxidizing bacteria convert to nitrate. [ 2 ] [ 3 ] Complete conversion of ammonia into nitrate by a single microorganism was first predicted in 2006. [ 1 ]
The oceanic nitrogen cycle with the role of DNRA. Blue line represents the ocean surface, with the atmosphere above. Notice how NH 4 produced by DNRA can be taken up by biota and converted into organic nitrogen, while N 2 produced by denitrification is removed from the system, and may only re-enter via nitrogen fixation.