It is the process that occurs in nature to maintain the nitrogen concentration in the atmosphere, soil and water.
Nitrogen cycle involves the following key steps:
(i) Nitrogen fixation
Free atmospheric nitrogen is chemically inert and cannot be used in pure form by organisms. Hence, it is first converted into nitrates, which can be assimilated by plants. This conversion is carried out by industrial nitrogen fixation (by the manufacturing of ammonium salts or chemical fertilisers) or by nitrogen-fixing bacteria such as Azotobacter and Rhizobium. This conversion is called bio-fixation of nitrogen or nitrogen fixation. The nitrates are absorbed by plants and utilised by them for making organic matter.
Animals consume plant matter and break down the nitrogenous compounds of plants during their cell activities. Urea or uric acid excreted by animals and the nitrogenous compounds present in the dead plant matter are converted to ammonium ions by the process of ammonification. Ammonification is carried out by certain putrefying bacteria and fungi. Plants assimilate this ammonium ions or certain bacteria and convert them into nitrate through nitrification.
(iii) Nitrification: Microorganisms like Nitrosomonas and Nitrobacter convert ammonia into nitrates by the process of nitrification.
(iv) Denitrification: Decomposers such as Pseudomonas reduce nitrates back into nitrogen or ammonia or some other oxides by the process of denitrification and release free nitrogen back to the atmospheric pool.