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Nitrogen Cycle (Edexcel IGCSE Biology)
Revision Note
The Nitrogen Cycle
- Nitrogen is present as N2 gas in the atmosphere and within biological molecules, e.g. proteins, in the tissues of living organisms
- Nitrogen is cycled through ecosystems by the processes of the nitrogen cycle
Uptake of nitrogen by living organisms
- N2 in the atmosphere is made available to living organisms by the process of nitrogen fixation
- Nitrogen fixation is carried out by nitrogen-fixing bacteria which convert N2 gas into ammonium compounds; these compounds are converted into nitrates in the soil
- Nitrogen gas can also be fixed by lightning when it strikes the earth, or during the production of chemical fertilisers
- After nitrogen fixation has occurred plants absorb the nitrates in the soil and use the nitrogen to build plant proteins
Transfer of nitrogen between living organisms
- Animals feed on plants and digest the proteins in the plant tissues, providing nitrogen to build animal proteins
- Nitrogen may then be passed from one consumer to another up the food chain in the same way
Release of nitrogen from tissues
- Nitrogen from living organisms is returned to the soil in the form of ammonia by the action of decomposers, such as bacteria and fungi
- The plants can’t absorb ammonia so nitrifying bacteria convert it to nitrates, which can then be taken up again by plants
- The conversion of ammonium compounds to nitrates is known as nitrification
Returning nitrogen to the atmosphere
- Nitrates in the soil can be converted back into nitrogen gas by the action of denitrifying bacteria
- This process is known as denitrification
- Denitrifying bacteria are active in anaerobic conditions, e.g. in waterlogged or compacted soil
- Farmers can decrease the activity of denitrifying bacteria by ploughing the soil to increase aeration
The nitrogen cycle involves nitrogen fixation, decomposition, nitrification and denitrification
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