# Describe the role of microorganisms in the nitrogen cycle.

> Eduqas GCSE Biology — 1.6.2 Nutrient cycles · Describe · 4 marks

> Nitrogen makes up around 78% of the atmosphere, yet most organisms cannot use it directly. Microorganisms play key roles in making nitrogen available to living things and returning it to the atmosphere.

## Mark scheme (4 marks)

1. Nitrogen-fixing bacteria convert nitrogen gas into ammonia / nitrates (compounds usable by plants)
2. Nitrifying bacteria convert ammonia into nitrates in the soil
3. Decomposers / bacteria / fungi break down dead organisms / protein / urea, releasing ammonia
4. Denitrifying bacteria convert nitrates back into nitrogen gas, returning it to the atmosphere

## Key terms

- [nitrogen cycle](https://www.gradenine.co.uk/glossary/nitrogen-cycle)

## Related

- [Revision notes for Eduqas GCSE Biology](https://www.gradenine.co.uk/learn)
- [How to answer "Describe" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/describe-the-role-of-microorganisms-in-ae4334ee) · Published by Druglandscape Ltd.