12.6.1 Nitrogen Cycle

Apart from carbon, hydrogen and oxygen, nitrogen is the most prevalent element in living organisms.
Nitrogen is a constituent of amino acids, proteins, hormones, chlorophylls and many of the
vitamins. Plants compete with microbes for the limited nitrogen that is available in soil.

Thus, nitrogen is a limiting nutrient for both natural and agricultural eco-systems.
Nitrogen exists as two nitrogen atoms joined by a very strong triple covalent bond (N ≡ N).
The process of conversion of nitrogen (N2) to ammonia is termed as In nature, lightning and ultraviolet radiation
provide enough energy to convert nitrogen to nitrogen oxides (NO, NO2, N2O). Industrial combustions, forest fires,
automobile exhausts and power-generating stations are also sources of atmospheric nitrogen oxides.

Decomposition of organic nitrogen of dead plants and animals into ammonia is called ammonification.
Some of this ammonia volatilises and re-enters the atmosphere but most of it is converted into nitrate by soil
bacteria in the following steps:

2NH3  + 3O2  ---> 2NO2 + 2H + 2H2O

1NO2+O2 ---> 2NO3


Ammonia is first oxidised to nitrite by the bacteria Nitrosomonas and/or
Nitrococcus. The nitrite is further oxidised to nitrate with the help of the
bacterium Nitrobacter.

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