The nitrogen cycleSpec 4.11B
In short
The nitrogen cycle is how nitrogen moves between the air, soil and living things. Plants need nitrates to make proteins but cannot use nitrogen gas. Decomposers break down proteins and urea into ammonium ions, nitrifying bacteria convert these into nitrates, nitrogen-fixing bacteria convert nitrogen gas into nitrogen compounds, and denitrifying bacteria turn nitrates back into nitrogen gas.
Plants need nitrates from the soil to make amino acids and proteins. Although about 78% of the air is nitrogen gas, plants cannot use it directly. Bacteria in the soil have key roles in making nitrates available. You need to know what each type of bacteria does, but not the names of the species.
| Process | Organisms | What happens |
|---|---|---|
| Decay | Decomposers (bacteria and fungi) | Break down proteins and urea in dead material and waste, releasing ammonia (ammonium ions) |
| Nitrification | Nitrifying bacteria | Convert ammonium ions in the soil into nitrates, which plants absorb |
| Nitrogen fixation | Nitrogen-fixing bacteria, in the soil or in root nodules of legumes | Convert nitrogen gas from the air into nitrogen compounds, which become nitrates |
| Denitrification | Denitrifying bacteria | Convert nitrates in the soil into nitrogen gas, which returns to the air and reduces soil fertility |
Plants take up nitrates through their roots and build them into proteins. Animals get nitrogen by eating plants or other animals. Nitrogen compounds return to the soil in waste and when organisms die.
Nitrogen-fixing bacteria are not the bacteria that make ammonia from dead material. That is decay. Fixing means turning nitrogen gas into nitrogen compounds.
(opens full size in a new tab)Quick check
Which process removes carbon dioxide from the air?
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Photosynthesis.
Name three processes that return carbon dioxide to the air.
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Any three of: respiration, decomposition, combustion.
Why are decomposers important in the carbon cycle?
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They break down dead material and release carbon dioxide by respiration.
Biology only What do nitrogen-fixing bacteria do?
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Convert nitrogen gas from the air into nitrogen compounds.
Biology only What do denitrifying bacteria do?
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Convert nitrates in the soil into nitrogen gas.
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026