Nitrogen Cycle


The Nitrogen Cycle


The atmosphere contains 78% of nitrogen gas. As it is very unreactive, it can't be directly used by plants or animals. It is needed to make protiens for growth so living organisms need a way to get it.

Nitrogen in the air has to be turned into mineral ions such as nitrates before plants can use it. They then absorb the ions from the soil and use the nitrogen to make proteins. Nitrogen is then passed along food chains as proteins.


Decomposers break down proteins in rotting animals and plants, and urea in animal waste. This returns the nitrogen to the soil, meaning it's recycled.

Nitrogen fixation is the process of turning nitrogen in the air into nitrogen-containing ions in the soil which the plants can use. There are two ways this happens:

  • Lighning - there is so much energy in a lightning bolt that it is enough to make nitrogen react with oxygen in the air to give nitrates.
  • Nitrogen-fixing bacteria in soil and plant roots. 

There are four different types of bacteria involved in the nitrogen cycle:

  • Decomposers - decompose urea and proteins to turn them


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