# Explain the role of nitrogen-fixing bacteria and denitrifying bacteria in the nitrogen cycle.

> Edexcel GCSE Biology (1BI0) — 9.3 Material cycles · Explain · 4 marks

> Farmers growing legume crops such as peas notice that their soils remain fertile without the addition of nitrogen-containing fertilisers. However, in waterlogged fields, the concentration of nitrate ions in the soil can decrease significantly over time.

## Mark scheme (4 marks)

1. Nitrogen-fixing bacteria convert atmospheric nitrogen gas into ammonia (nitrogen-containing compounds)
2. Nitrogen-fixing bacteria are found in the soil or in root nodules of legumes, providing plants with usable nitrogen compounds
3. Denitrifying bacteria convert nitrate ions into nitrogen gas
4. Denitrifying bacteria are anaerobic and are found in waterlogged soils, which explains why nitrate levels fall in waterlogged conditions

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Biology (1BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-the-role-of-nitrogen-fixing-bacteria-d38ac75d) · Published by Druglandscape Ltd.