Section 1 of 5Stages of the Cycle
Nitrogen is needed to build proteins, but most organisms cannot use the nitrogen gas that makes up much of the air.
Nitrogen fixation
- Bacteria convert nitrogen gas into ammonium compounds that are then changed into nitrates in the soil.
Absorption
- Plants take up nitrates through their roots and use them to build amino acids and plant proteins.
Feeding
- Animals eat plants and digest the proteins, and the nitrogen is passed along food chains.
Decomposition
- Decomposers break down dead organisms and waste, and release the nitrogen as ammonia.
Nitrification
- Nitrifying bacteria convert ammonia into nitrites and then nitrates, which plants can absorb.
Denitrification
- Denitrifying bacteria convert nitrates back into nitrogen gas, which returns to the atmosphere.
Section 2 of 5Nitrogen Fixation
Fixation is the way that atmospheric nitrogen first becomes available to living things.
Nitrogen Fixation
Root nodule bacteria
- These live in the roots of peas, beans and clover and have a mutualistic relationship with the plant.
Free-living bacteria
- Some nitrogen-fixing bacteria live independently in the soil and fix nitrogen without a host plant.
Lightning
- The energy of lightning can cause nitrogen and oxygen to react, forming compounds that reach the soil in rain.
Fertiliser production
- Nitrogen is also fixed industrially when chemical fertilisers are manufactured.
Section 3 of 5Following Nitrogen Around
1
Nitrogen gas in the air is fixed by bacteria and forms ammonium compounds and then nitrates in the soil.
2
Plants absorb the nitrates and use them to make proteins, and animals obtain nitrogen by eating plants or other animals.
3
When organisms die or produce waste such as urea, decomposers release ammonia into the soil.
4
Nitrifying bacteria oxidise ammonia to nitrites and then to nitrates, so the nitrogen can be used by plants again.
5
In waterlogged soil denitrifying bacteria convert some nitrates to nitrogen gas, which returns to the air.
Section 4 of 5Bacteria and Conditions
Bacteria and Conditions
Aerobic soil
- Nitrification needs oxygen, so well-aerated soil keeps nitrates available for crops.
Waterlogged soil
- Little oxygen allows denitrifying bacteria to thrive and remove nitrates from the soil.
Farming
- Ploughing aerates the soil and reduces the loss of nitrates by denitrification.
Crop rotation
- Growing legumes in rotation adds fixed nitrogen to the soil naturally.
| Process | Organisms | Change |
|---|---|---|
| Nitrogen fixation | Nitrogen-fixing bacteria | N₂ to ammonium compounds and nitrates |
| Decomposition | Bacteria and fungi | Dead material to ammonia |
| Nitrification | Nitrifying bacteria | Ammonia to nitrites to nitrates |
| Denitrification | Denitrifying bacteria | Nitrates to N₂ |
Section 5 of 5Exam Checklist & Tips
3.3.2 – Key Points for the Exam
- Nitrogen fixation Conversion of nitrogen gas into nitrogen compounds by bacteria, lightning or industry.
- Nitrification Conversion of ammonia into nitrites and then nitrates by nitrifying bacteria.
- Denitrification Conversion of nitrates into nitrogen gas by denitrifying bacteria.
- Decomposer A bacterium or fungus that breaks down dead material and releases nutrients.
- Mutualism A relationship in which both organisms benefit, as in legume root nodules.
Common Mistakes
- Saying that plants absorb nitrogen gas from the air, when they take up nitrates through their roots.
- Mixing up nitrification and denitrification; nitrification makes nitrates and denitrification removes them.
- Forgetting that decomposers release ammonia, which still has to be converted to nitrates before plants can use it.
- Stating that denitrifying bacteria work best in well-aerated soil, when they are active in waterlogged, low-oxygen soil.