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Question : Describe nitrogen cycle.

Answer»

SOLUTION :Apart from CARBON ,hydrogen and oxygen. Nitrogen is the most prevalent element in living organism.
Nitrogen: 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 be a very strong triple covalent bond (N=N)
The PROCESS of conversion of nitrogen `(N_2)` to ammonia is termed as nitrogen fixation.
IN nature, lightning and ultraviolet radiation provide enough energy to convert nitrogen to nitrogen oxides `(NO,NO_2,N_2O)` 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:
`2NH_3+3O_2 to 2NO_2^(-) +2H^(+) +2H_2O`..........(i)
`2NO_2^(-)+O_2 to 2NO_3^(-)`........(ii)

Ammonia is first oxidised to nitrite by the bacteria Nitrosomans and /or Nitrococcus.
The nitrite is further oxidised to nitrate with the help of the bacterium Nitrobacter.
These steps are called nitrification.
`Ammonia to^(Oxidation) Nitrite`
`Nitrite to_(Oxidation)^(Nitrobacter) Nitrate`
These nitrifying bacteria are chemoautotrophs.
The nitrate thus formed is absorbed by plants is transported to the leaves.
In leaves it is reduced to form ammonia
`Nitrate to leaves to^(REduction ) Ammonia to Amino acid`
It is finally forms the amine group of amino acids. Nitrate present in the soil is also reduced to nitrogen by the process of denitrification.
`Nitrate to^(Denitrification) Nitrogen`
Pseudomonas+Thiobacillus bacteria


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