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Outline
of
Nitrogen
Cycle |
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Outline
of
Nitrogen
Cycle - The nitrogen cycle consists of the following events
(1) Nitrogen Fixation
The conversion of atmospheric nitrogen to ammonia is called nitrogen fixation. Fixation of nitrogen is carried out principally by microorganisms, usually in association with plant roots.
This type of fixation is called biological fixation. About 175 million tones of nitrogen is fixed annually by this method. Another type of nitrogen fixation is by spontaneous chemical fixation. Thus, atmospheric nitrogen can be converted into nitrogen oxides as a result of lightning flashes or combustion. These are then washed into the soil as nitrates.
The third form of nitrogen fixation is the industrial production of fertilizers, e.g., by the Haber Bosch, Berkland Eyde and Cynamide processes. More than 30 million tonnes of industrial fertilizer are produced annually.
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(2) Nitrification is the process by which soil ammonia is converted into soil nitrites (NO2-) and then nitrates (NO3-). Nitrification is carried out by two specialized groups of aerobic bacteria. Ammonia is oxidized to nitrite by Nitrosomonas and nitrite to nitrate by Nitrobacter.
(3) Ammonification is the restoration of nitrogen to the soil as ammonia in the decay of organic matter. The nitrogen in the excretory products of animals, and of dead plant and animal bodies is converted by microorganisms in to ammonia.
(4) Denitrification is the loss of nitrogen to the atmosphere by microbial reductions of nitrates. The denitrifying bacteria use nitrate as a final hydrogen acceptor.
Molecular nitrogen is formed during denitrification, and is therefore removed from the cycle.
(5) Assimilation Plants can assimilate inorganic nitrogen compounds which are transformed into the constituents of the cell.
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