The Nitrogen Cycle
Nitrogen is constantly taken, or fixed, from the atmosphere,
oxidised to a form able to be utilised by plants and some bacteria, to be
subsumed into metabolic pathways, and through the various routes described
above is then excreted into the environment as reduced nitrogen where it may be
reoxidised by bacteria or released back into the atmosphere as nitrogen gas.
These combined processes are known collectively as the nitrogen cycle. The
previous discussions have referred to the release of nitrogen during
degradation of proteins and nucleic acid bases, either in the form of ammonia,
the ammonium ion, urea or uric acid. The fate of all these nitrogen species is
to be oxidised to nitrite ion by Nitrosomas,
a family of nitrifying bacteria. The nitrite ion may be reduced and released as
atmospheric nitrogen, or further oxidised to nitrate by a different group of
nitrifying bacteria, Nitrobacter. The
process of conversion from ammonia to nitrate is sometimesfound as a tertiary
treatment in sewage works to enable the nitrate consent to be reached. The
process typically occurs in trickling bed filters which have, over time, become
populated with a Nitrosomas and Nitrobacter along with the usual flora
and fauna which balance this ecosystem. Denitrification may then occur to
release atmospheric nitrogen or the nitrate ion, released by Nitrobacter, may be taken up by plants
or some species of anaerobic bacteria where it is reduced to ammonium ion and
incorporated into amino acids and other nitrogen – carbon containing compounds.
To complete the cycle, atmospheric nitrogen is then fixed by nitrifying
bacteria, either free living in the soil or in close harmony with plants as
described earlier.
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