For all living organisms, nitrogen is extremely important, as it is a constituent of the proteins, nucleic acids and other essential molecules. All living cells consist of protoplasm which contains proteins. In absence of nitrogen neither proteins nor protoplasm can form. This clearly shows that nitrogen is the very basis of life.
The Nitrogen Cycle explains how nitrogen compounds circulate in nature through the various organisms to which nitrogen is essential.Nitrogen gets added to the soil mainly in two ways. First, during lightning, nitrogen and oxygen combine to form nitric oxide.
The nitric oxide so formed then reacts with more oxygen to form nitrogen dioxide.
Nitrogen dioxide so formed, gets washed down by rain and forms nitrates and nitrites in the soil. Plants absorb these of nitrates and nitrites from the soil and convert them into plant proteins. Animals consume plant proteins and change them into animal proteins. Animal waste and decay of animal bodies after death, cause nitrogen to return to the soil in the form of simple nitrogen compounds.
In addition to this main cycle, certain leguminous plants also absorb nitrogen directly from the atmosphere. A certain amount of nitrogen is fixed with the help of some bacteria found in the roots of these plants. Some denitrifying bacteria causes the release of nitrogen from plants and animal decay directly into the atmosphere. Thus, like carbon dioxide and oxygen, the percentage of nitrogen maintained in the atmosphere is also fairly constant.