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The ammonia present in the soil is then converted into nitrates and nitrites
by the bacteria. This process is called Nitrification. For Example, Nitrobacter
is a nitrifying bacterium.
The reaction involved in the process of Nitrification is as follows:

2NH4+ + 3O2 → 2NO2– + 4H+ + 2H2O

2NO2– + O2 → 2NO3–

The various nitrogen compounds are utilized by plants and animals. Plants
form amino acids with these nitrites and nitrates. The amino acids are
further used to make proteins in plants. This is the process of assimilation.

When the plants and animals die or excrete wastes, the nitrogen present in
the organic matter enters the soil. The decomposers present in the soil break
down these nitrogen compounds into Ammonia. This process is called
ammonification. For Example, Bacillus and Clostridium are ammonifying
bacteria.

Nitrogen makes its way back into the atmosphere through a process called
denitrification, in which nitrate (NO3-) is converted back to gaseous nitrogen (N2).
Denitrification occurs primarily in wet soils where the water makes it difficult for
microorganisms to get oxygen. Under these conditions, certain organisms –
known as denitrifiying bacteria – process nitrate to gain oxygen, leaving free
nitrogen gas as a byproduct. For Example, Pseudomonas and Clostridium are
denitrifying bacteria.

1. The
burning of fossil fuels from various
industrial processes adds nitrogen
and nitrous oxide compounds to the
atmosphere, which upsets the
balance of natural nitrogen, polluting
ecosystems and altering the ecology
of entire regions. The increase in
nitrogen and nitrous oxide is caused
by automobiles, power plants and a
wide variety of industries.

2. The use of
fertilizers has increased the amount of usable nitrogen in the soil. Excess
nitrogen not taken up by plants and unable to be converted back into
atmospheric form by bacteria, leaches deeper into the soil and out of the cycle,
contaminating groundwater supplies and encouraging the growth of toxic algae