Nitrogen cycle

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The nitrogen cycle is an important process where the atmospheric nitrogen is converted, through various biological and physical processes, to substances that can be used by organisms.The main processes in this cycling of nitrogen are : nitrogen fixation, nitrification, ammonification and denitrification.[1]


Nitrogen Fixation

Nitrogen fixation is a process where the atmospheric nitrogen is converted or 'fixed' into ammonia .The ‘fixing’ occurs in lightning, through industrial process or mostly is done by nitrogen fixing soil bacteria like the Cynobacteria and also bacteria living in the root nodules of legumes called Rhizobium.[2]

Fixation by lightning

The atmospheric nitrogen molecule is broken due to the energy from lightning and this enables the nitrogen atoms to bind with oxygen in air to form nitric oxide. The nitric oxide then dissolve in water in the form of rain and form (gets  'fixed' into)  nitrates and ammonia which pours into the soil. These nitrates and ammonia are then assimilated by the plants.[3]

Industrial Fixation

With high pressure, temperature of 450oC and an iron catalyst, the atmospheric nitrogen can combine with hydrogen to form ammonia through an industrial process called the harber process. Ammonia is further processed to form urea and ammonium nitrate [4]

Biological Fixation

Most of the nitrogen 'fixing' is done by bacteria. Cynobacteria lives freely in the soil and ‘fixes’ the nitrogen by converting it into ammonium. The Rhizobium bacteria produce ammonia which is then taken up by the legume plants through their root nodules. There is a mutualistic relationship present between the bacteria and the plant since, the bacteria reduces nitrogen to ammonia to be taken up by the plant in exchange for shelter and carbohydrates .Thus, legumes plant often increase the nitrogen content of nitrogen-deprived soils.[5]


Nitrification is a process where ammonia is converted to nitrites and then to nitrates, before it can be taken up directly by plants. The conversion of ammonia to nitrites is done by nitrifying bacteria called the Nitrosomonas. The nitrite is further converted to nitrate by another type of nitrifying bacteria called the Nitrobacter. Through the activities of these bacteria, nitrogen is made available to be taken up by plants and so, animals obtain nitrogen by eating these plants.[6]


When plants and animals die, they get decomposed by decomposers, which include aerobic and anaerobic bacteria and fungi. During decomposition, bacteria convert the nitrogen nutrients back into ammonium and ammonia, which then can go through nitrification and denitrification process again.[7]


It is the process of reducing nitrates and nitrites back into the atmospheric nitrogen. This process completes the nitrogen cycle. Denitrification is done by denitrifying bacteria, like Pseudomonas, in anaerobic conditions as these bacteria live deep in soil and in aquatic sediments. These bacteria use nitrates as the final electron acceptor, rather than oxygen, during respiration and releases nitrogen gas into the atmosphere. [8]


  1. Kennedy,P. & Sochacki,F. (2008) Biology A. Heinemann of Pearson Education Limited.
  2. Wagner, S. C. (2011) Biological Nitrogen Fixation. Nature Education Knowledge 3(10):15
  3. Meteorol, J. and Viemeister, P. E. (1960) Lightning and the origin of nitrates found in precipitation, 7, 681
  6. Appleman,M.D., Fisher, T. and Fisher,E. (1952) Nitrification by Certain Heterotrophic Bacteria present in Soil. Journal of Bacteriology. 64 (4). P.596.
  7. <a href="">Nitrogen Cycle - Ammonification And Nitrification</a>
  8. Kennedy,P. & Sochacki,F. (2008) Biology A. Heinemann of Pearson Education Limited.
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