Nitrogen is an essential constituent of the organic molecules and also a major component of atmosphere. Though nitrogen is very abundant in its gaseous state, it cannot be used in that stage. Nitrogen cycle follows a regular path.
Atmospheric nitrogen is fixed in two ways. One is by very high energy fixation by lightening, cosmic radiation etc. In this, nitrogen combines with oxygen and hydrogen of water in the presence of energy to form nitrates and ammonia. Second is biological fixation; in this bacteria (Azatobacter and Clostridium) and blue green alga (Nostoc) can fix atmospheric nitrogen and convert it into organic substances. The bacteria may be symbiotic (Rhizobiurn) or free living.
Plants absorb ammonia and nitrates from the soil or may get them from symbiotic bacteria (of leguminous plants) and incorporate them in their tissue. These are incorporated as amino acids (proteins). This is called nitrate assimilation.
Animals feed on plants and they assimilate the plant amino acids. When animals excrete ammonia, urea and uric acid: the decay bacteria convert these into ammonia. They also decompose the nitrogenous waste compounds of dead plants and animals into NH2, CO2 & H2O. Ammonia formed by the decay bacteria (putrefaction) is oxidised by nitrite bacteria (Nitrosomonas) into nitrites.
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This is further oxidised by nitrate bacteria (Nitorbacter) into nitrates. The process is called nitrification and the bacteria involved are called nitrifying bacteria. Nitrates are added to the soil and thus cycle is completed. Some of the NH3 is converted into nitrogen by certain bacteria. This process is called dentrification.