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The nitrogen cycle is the biogeochemical cycle by which nitrogen
is converted into multiple chemical forms as it circulates among atmosphere,
terrestrial, and marine ecosystems. The conversion of nitrogen can be
carried out through both biological and physical processes.
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Processes in Nitrogen Cycle:
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Nitrogen fixation
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Assimilation
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Ammonification
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Nitrification
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Denitrification
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Dissimilatory nitrate reduction to ammonium
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Anaerobic ammonia oxidation
Nitrogen fixation
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The conversion of nitrogen gas (N2) into nitrates and nitrites through
atmospheric, industrial and biological processes is called nitrogen fixation.
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Atmospheric nitrogen must be processed, or "fixed", into a
usable form to be taken up by plants.
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Non-symbiotic bacteria or free-living bacteria such as Azobacter and Beijemickia (aerobic); Clostridium and Rhodosprillium (anaerobic), some symbiotic bacteria such as Rhizobium and some cyanobacteria, blue-green
algae (Nostoc, Anabaena, Spirulina) are major sources of nitrogen fixation.
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Symbiotic nitrogen-fixing bacteria such as Rhizobium usually live in the
root nodules of legumes (such as peas, beans, alfalfa, locust trees, red
clover, peanut, lentil etc.).
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Here they form a mutualistic relationship with the plant, producing
ammonia in exchange for carbohydrates. Because of this relationship, legumes
will often increase the nitrogen content of nitrogen-poor soils.
Assimilation
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Plants can absorb nitrate or ammonium from the soil by their root hairs.
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If nitrate is absorbed, it is first reduced to nitrite ions and then
ammonium ions for incorporation into amino acids, nucleic acids, and
chlorophyll.
Ammonification
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Living organisms produce nitrogenous waste products like urea and uric
acid.
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These waste products and dead remains of the organisms are converted back into inorganic ammonia
and ammonium ions by bacteria by ammonification.
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When a plant or animal dies or an animal expels waste, the initial form
of nitrogen is organic. Bacteria or fungi convert the organic nitrogen within
the remains back into ammonium (NH4+)
Nitrification
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The conversion of ammonium to nitrate is performed primarily by
soil-living bacteria and other nitrifying bacteria. In the primary stage of
nitrification, the oxidation of ammonium (NH4+) is performed by bacteria such
as the Nitrosomonas species, which converts ammonia to nitrites (NO2).
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Ammonium ions are first oxidized to nitrite by Nitrosomonas/
Nitrococcus bacteria.
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Other bacterial species such as Nitrobacter bacteria (chemoautotrophs), are responsible for the oxidation of
the nitrites (NO2-) into nitrates (NO3).
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It is important for the ammonia (NH3) to be converted to nitrates or
nitrites because ammonia gas is toxic to plants.
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Plants absorb these nitrates and convert them
into amino acids.
Denitrification
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The process of reducing nitrate
in the soil to nitrogen is called denitrification.
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Soil and oceans have denitrifying
bacteria like Pseudomonas and Thiobacillus which convert nitrate/nitrites
to elemental nitrogen, under anaerobic conditions.
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This nitrogen is released into the atmosphere completing the cycle.
Dissimilatory nitrate reduction to ammonium
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Dissimilatory nitrate reduction to ammonium (DNRA), or nitrate/nitrite
ammonification, is an anaerobic respiration process.
Anaerobic ammonia oxidation
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In this biological process, nitrite and ammonia are converted directly
into molecular nitrogen (N2) gas.
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This process makes up a major proportion of nitrogen conversion in the
oceans.