Natural process in which nitrogen from the atmosphere is removed in the form of compounds and then returned to the atmosphere via, plants and animal kingdom in order to maintain a balance between free nitrogen in the atmosphere and the bound nitrogen in the soil is called Nitrogen cycle.
It takes place through the following steps:
Transfer of atmosheric nitrogen to soil
- Leguminous plants like pea, bean etc., directly assimilate nitrogen from the atmosphere through the agency of symbiotic bacteria living at their roots.
- Also free nitrogen is converted into nitric oxide and ultimately to nitric and nitrous acid by lightning and those compounds finally comes down to that soil along with the rain.
The nitric acid reacts with base of the salt to form soluble nitrate.
This is assimilated by plants to form proteins. This vegetable protein is consumed by herbivorous animals which in turn are the food of carnivorous animal and man. Nitrogen consumed by animals is restored to the soil in the form of urea in the excrete form ammonia in the soil.
- Return of nitrogen from animals and plants in the soil:
Again, when animal and plant bodies undergo decay or animal excrete purify, most of the organic nitrogen in the protein is liberated as ammonia. This is oxidized in the soil by the combined action of nitrosifying and nitrofying bacteria into nitrate. Protein gives back nitrogen by the action of denitrifying bacteria present in the soil.
The nitrogen is shown in the following flow-sheet:
Nitrogen Cycle of the atmosphere