The cyclic process of nitrogen being fixed, used by plants and animals and later returned to the atmosphere, is referred to as the nitrogen cycle. This transformation can be carried out through both biological and physical processes.
Important processes in the nitrogen cycle include–
- Nitrogen Fixation: The conversion of free atmospheric nitrogen into nitrates is known as nitrogen fixation.
Nitrogen fixation occurs by following two ways:
a. Lighting: When lightning strikes, nitrogen and oxygen of atmosphere react and form oxides of nitrogen.
b. Biological nitrogen fixation: Certain plants like peas and pulses contain Rhizobium bacteria in their root nodules. These bacteria convert atmospheric nitrogen to nitrates.
- Nitrogen Assimilation: The process of conversion of inorganic nitrogen compounds into organic compounds that become a part of living organism is called nitrogen assimilation. Plants absorb nitrates from the soil through their root system and convert them into plant proteins. When animals eat these plants, the plant proteins are converted into animal proteins.
- Ammonification: The process of conversion of plant and animal proteins into ammonium compounds by putrefying bacteria in soil is called ammonification.
- Nitrification: The ammonium salts are converted into nitrites by Nitrosomonas bacteria. The nitrites are then converted into nitrates by Nitrobacter bacteria. This process is called nitrification.
- Dentrification: The conversion of nitrates into free nitrogen gas by denitrifying bacteria (Pseudomonas) is called denitrification.
Thus, the nitrogen cycle maintains the percentage of nitrogen in the atmosphere more or less constant.