Nitrogen is both the most abundant element in the atmosphere and a building block of proteins and nucleic acid. The nitrogen is a complex biochemical cycle in which nitrogen is converted from its inert atmospheric molecular form. N₂ into a form that is useful in biological processes

The nitrogen cycle contains several stages:


  • Atmospheric nitrogen occurs primarily in inert form N₂ or non reactive form that few organisms can use, therefore it must be converted into a compound or fixed form in a process called nitrogen fixation.
  • Most atmospheric nitrogen is 'fixed' through biological processes. A number of bacteria and blue green algae are known to be able to fix atmospheric nitrogen into compounds in their own body.
  • These may be symbiotic e.g: Rhizobium or free living, e.g: Nitrosomonas respectively.
  • This organisms convert atmospheric nitrogen into organic nitrogen for their own cells. In leguminous plants like peas, beans, etc.. There is a symbiotic relationship of the nitrogen fixing bacteria with the plant, thus nitrogenous compounds are added to the soil after a leguminous crop is grown.
  • As they die rapidly (they grow rapidly as well), this nitrogen present in the soil as compounds become available to plants.
  • Nitrogen can also be fixed as nitrates by lighting.


  • Nitrates can also be converted to ammonia by the denitrifying bacteria in the soil.
  • The nitrifying bacteria may then use this ammonia to synthesize compounds for their own cell and eventually convert to proteins, nucleic acids, nitrites and nitrates.
  • Nitrites are produced mainly by Nitrosomonas, while nitrates by nitrifying bacteria that are also capable of utilizing nitrites and converting them to nitrates.
  • The death of these organisms adds nitrogenous compounds to the soil. Plants take up nitrates as well as ammonium ions from the soil to convert them to proteins and nucleic acid.


  • Nitrogen compounds mainly as nitrates or ammonium ions (NH₄⁺) are taken up from soils by plants which are then used in the formation of plant proteins and as animals these plants and animal proteins are synthesized.


  • Production of ammonia (NH₃) from nitrates, other nitrogenous compounds is known as ammonification.
  • Ammonification are also occurs when plants and animals die or when animals emit wastes, the nitrogen in the organic matter reenters the soil and water bodies where it is broken down by other organisms, known as decomposers.


  • Denitrification is the conversion principally by bacteria, of compounds of nitrogen in solid and aquatic systems to the gases, nitrogen (N₂),nitrous oxide (N₂O) eventual release of these into the atmosphere.
  • Denitrification occurs primarily in wet soils where water makes it difficult for microorganisms to get oxygen.
  • Under these conditions, certain organisms known as denitrifying bacteria will process nitrate to gain oxygen, leaving free nitrogen gas as by product. Thus, the nitrogen content of the earth and its atmosphere remains in a perfect balance.

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