- Nitrogen is an essential constituent of plant and animal proteins, vitamins and nucleic acids.
- The atmosphere is the main source of nitrogen gas which is about 78.1% of the atmosphere by volume.
- Plants and animals cannot use nitrogen gas directly except for some nitrogen fixing bacteria and some blue green algae.
- Nitrogen is used in the form of salts like nitrates and nitrites, thus nitrogen gas has to be converted into nitrates.
- There is a continuous exchange of nitrogen gas among plants, animals and the physical environment.
- Thus, nitrogen cycle can be defined as the cyclic process by which nitrogen element is circulated continuously through the non-living and living components of the biosphere.
- Various steps involved in the nitrogen cycle are as follows:
1. Nitrogen fixation:
- It is the process of converting free nitrogen gas of the atmosphere into the compounds of nitrogen which takes place in nature by the following methods:
a. Atmospheric nitrogen fixation:
- In this process, the nitrogen gas present in the atmosphere reacts with oxygen during lightning in the sky and produces different oxides of nitrogen.
- Those oxides of nitrogen get dissolved in rain water forming dilute nitric acid.
- Nitric acid then reacts with salts present in soil (like limestone) and forms nitrates in the soil which is then utilized by plants.
b. Industrial nitrogen fixation:
- In this process, atmospheric nitrogen is combined with hydrogen under high temperature and pressure to produce ammonia in industrial process (Haber’s process).
- Then ammonia is used to produce ammonium salts like ammonium sulphate, ammonium nitrate etc. which are used as chemical fertilizers rich in nitrogen.
c. Biological nitrogen fixation:
- in this process, living organisms (some bacteria and blue green algae) convert atmospheric nitrogen into compounds of nitrogen.
- Some free living bacteria like Azotobacter and Clostridium and symbiotic bacteria like Rhizobium convert atmospheric nitrogen into nitrates.
- Rhizobium live in the root nodules of leguminous plants like soyabean, pea, gram, bean etc. which convert atmospheric nitrogen gas directly into nitrates which can be absorbed by plants.
- Similarly, certain blue green algae like Anabaena and Nostoc also fix atmospheric nitrogen into nitrates.
2. Nitrogen assimilation:
- It is the process of conversion of inorganic nitrogen compounds into organic compounds like proteins that become a part of living organisms.
- In this process, plants absorb nitrogen compounds like nitrates from soil and water, and convert them into plant protein and other organic compounds that form the components of cells.
- It is the process in which complex organic compounds like proteins are converted into ammonia gas.
- The dead bodies of plants and animals are converted into simpler components with the release of gases like ammonia, carbon dioxide and water.
- Decomposers like fungi and bacteria present in soil bring about this process, after the putrefaction of proteins.
- It is the process of conversion of ammonia into nitrates and nitrites.
- The nitrifying bacteria present in soil are responsible for nitrification.
- Nitrosomonas bacteria oxidize ammonia into nitrites.
- Similarly, Nitrobacter bacteria convert those nitrites into nitrates.
- These nitrate compounds present in soil are absorbed by green plants.
- It is the process of conversion of nitrate salts present in soil into free nitrogen gas.
- It is carried out in the soil by the action of denitrifying bacteria like Pseudomonas.