# Nitrogen cycle

Nitrogen gas in the air cannot be used by most organisms. Nitrogen-fixing bacteria convert it to ammonia, nitrifying bacteria convert ammonia to nitrate that plants absorb, and denitrifying bacteria return nitrogen gas to the air.

Biology (university) · Part 10: Ecology and the environment · Card 7 of 9 · Last updated 2026-10-19

## Analogy

A relay of microbes passing nitrogen along.

## In practice

Farmers plant legumes such as peas because bacteria in their roots fix nitrogen into the soil.

## Practice

### Flashcards

- **What do nitrogen-fixing bacteria do?** They convert nitrogen gas into ammonia that plants can use after further conversion.

## Questions people ask

### Why is nitrogen so important to living things?

Nitrogen is a key part of amino acids, so it is needed to make proteins, and of nucleotides, so it is needed to make DNA and RNA. Although the air is about 78 percent nitrogen gas, most organisms cannot use it directly, so supplies in the soil often limit plant growth.

### What is eutrophication?

Eutrophication occurs when fertiliser runoff adds too many nutrients to water. Algae multiply quickly, then die. Bacteria that decompose them use up the oxygen in the water, so fish and other animals suffocate. It is a major problem in lakes and coastal waters.

## Related concepts

- [Carbon cycle](https://studyvisually.com/learn/science/biology-university/biology-university-part-10/biology-university-part-10-concept-6)

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