Nitrogen Cycle- Suitable for A-Level and IB Biology

Nitrogen Cycle- Suitable for A-Level and IB Biology

▪79% of the atmosphere is made up of nitrogen.

▪Plants are unable to absorb nitrogen gas directly.

▪Absorption of nitrogen into plants is only as ammonium ions and nitrates.

▪ animals fulfill their nitrogen requirements from other plants and animals.

▪Nitrogen is obtain in soil in the following ways

  • artificial fertilizers.
  • nitric acid formed due to lightning.
  • Due to biological assimilation.

Nitrogen Fixation

This is when nitrogen gas in the atmosphere changes nitrogen containing compounds.

Biological fixation is carried out in two ways.

Non symbiotic fixation: following organisms convert N2(g) into

ammonium ions: azotobacter and clostridium,

Symbiotic fixation 

  • Plants absorb ammonium ions and synthesise nitrogenous compounds including amino acids and proteins.
  • These are passed into other animals.
  • Ultimately these compounds are added again to the soil by their death.

Ammonification: the above products are converted to ammonium ions by saprophytic bacteria and fungi.


  • ammonium compounds in the soil are changed to nitrogen compounds.
  • Ammonium ions are converted into nitrites by Nitrosomonas.
  • Then nitrites are converted into nitrates by Nitrobacter


  • nitrates in the soil are converted into nitrogen gas this occurs by denitrifying bacteria.
  • This occurs in under anaerobic conditions.


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