The Nitrogen Cycle - Ecosystems

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Nitrogen fixation and Nitrification

Nitrogen Fixation:

  • Atmospheric nitrogen cannot be used directly by plants and animals. 
  • Nitrogen fixation occurs when lightning strikes or through the Haber process. 
  • Nitrogen-fixing bacteria such as Rhizobium live inside the root nodules of leguminous plants such as beans peas and clover. 
  • Rhizobium converts atmospheric nitrogen into ammonia which the plant can use as its fixed nitrogen source. They have a mutualistic relationship. 


  • Nitrifying/chemoautotrophic bacteria oxidise ammonium ions to nitrites (Nitrosomonas) and oxidise nitrites into nitrates (Nitrobacter)
  • Plants take the nitrates in through their roots from the soil and use them to make amino acids and proteins. 
  • Animals can then eat the plants to get proteins.
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Decomposition and Denitrification


  • Process where ammonium ions are produced from ammonium containing compouds such as urea.
  • Dead plant and animal waste contains nitrogenous waste. 
  • Over time, these wastes decay and ammonia is released. 
  • Nitrifying bacteria convert the ammonia to nitrates.
  • Some of the nitrates are taken up by the plants and used again (nitrification).
  • Some of the nitrates go through the process denitrification.


  • Denitrifying bacteria convert the nitrates back into atmospheric nitrogen which goes back to the atmosphere. 

Nitrogen fixation = Nitrogen gas --> Ammonia  (Nitrogen-fixing bacteria e.g. Rhizobium)

Nitrification = Ammonia --> Nitrites --> Nitrates  (Nitrifying bacteria - Nitrosomonas/Nitrobacter)

Ammonification/Decomposition = Decay and produce ammonia (-->Nitrification/Denitrification)

Denitrification = Nitrates -> Atmospheric nitrogen (Denitrifying bacteria)

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