The Nitrogen cycle

  • 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.

3. Ammonification:

  • 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.

4. Nitrification:

  • 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.

5. Denitrification:

  • 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.

The Nitrogen cycle