Role of Organisms in Nitrogen Cycle

Nitrogen fixation:

Nitrogen is required by plants to produce proteins, DNA, RNA, ATP, NAD+, NADP+, Chlorophyll etc.

Nitrogen fixation describes the conversion of nitrogen into nitrate, a form that can be used by plants. This is carried out by volcanic action, lightning, industrial activity and by some bacteria. Nitrogen-fixing bacteria can be found free in the soil or live in the root nodules of legumes (peas. beans, clover). The bacteria use the plant as a carbohydrate source of energy, protection & shelter (anaerobic conditions too) and the plant uses the nitrate produced by the bacterium (e.g. of mutualism).


Nitrates are converted into plant and animal protein, DNA and RNA.


Bacteria and fungi decay convert the dead remains of plants or animals or their waste products to ammonia (NH3).


The ammonia is converted to nitrites and then to nitrates by nitrifying bacteria. Some of the nitrate formed in the soil is absorbed and assimilated by the plants.


The conversion of nitrates to nitrogen gas.  It is carried out by denitrifying bacteria in the soil. These bacteria are anaerobic and live in swampy soil or deep down in the soil (where water accumulates).

Role of organisms in the nitrogen cycle:

  • Bacteria play a central role:
    • Nitrogen-fixing bacteria, which convert atmospheric nitrogen to nitrates.
    • Bacteria of decay, which convert decaying nitrogen waste to ammonia.
    • Nitrifying bacteria, which convert ammonia to nitrates/nitrites.
    • Denitrifying bacteria, which convert nitrates to nitrogen gas.
  • Fungi, like bacteria, help to convert dead plants and animals and their wastes into ammonia in the soil.
  • Plants absorb nitrates from the soil to make proteins.
  • Animals consume plants and use it to form animal protein.
  • Humans contribute to the cycle by adding nitrogen rich fertilisers to the soil and by using manure (The Physics Teacher, 2018).
Irish Farmers' Journal

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