Nitrogen cycle · Year 12
In Year 12 you learn: the nitrogen cycle; nitrogen-fixing, nitrifying and denitrifying bacteria; farming and the nitrogen cycle.
Plants need nitrogen to make proteins, but they cannot use the nitrogen gas in the air. Nitrogen-fixing bacteria, some in the root nodules of legumes, and lightning turn it into nitrogen compounds. Decomposers turn proteins and urea into ammonium compounds, nitrifying bacteria turn these into nitrates that plants take in through their roots, and denitrifying bacteria in waterlogged soil turn nitrates back into nitrogen gas.
Key facts
- Nitrogen gas makes up most of the air, but plants cannot use it directly. Nitrogen-fixing bacteria (some in the root nodules of legumes such as peas and clover) and lightning turn it into nitrogen compounds.
- Decomposers turn the proteins and urea in dead matter and waste into ammonium compounds. Nitrifying bacteria turn ammonium compounds into nitrites and then nitrates. Plants take in nitrates through their roots and use them to make proteins.
- Denitrifying bacteria turn nitrates back into nitrogen gas. They live in waterlogged soils with little oxygen.
Worked examples
Example 1
What happens in nitrogen fixation?
Answer: nitrogen gas from the air is turned into compounds such as ammonia or nitrates
Example 2
What happens in nitrification?
Answer: ammonium compounds are turned into nitrites and then nitrates
Example 3
What happens in denitrification?
Answer: nitrates in the soil are turned back into nitrogen gas
Example 4
Which type of bacteria carries out each step: nitrogen gas → ammonium ions; ammonium ions → nitrite → nitrate; nitrate → nitrogen gas?
Answer: nitrogen-fixing bacteria; nitrifying bacteria; denitrifying bacteria
Example 5
Describe how the nitrogen in a dead leaf can end up in a protein in a new plant.
Answer: decomposers make ammonium compounds; nitrifying bacteria make nitrates; plant roots take in the nitrates and make proteins
Example 6
Explain why ploughing and draining waterlogged soil increases the amount of nitrate available to crops.
Answer: it lets oxygen into the soil: nitrifying bacteria, which need oxygen, make more nitrate, and denitrifying bacteria, which work without oxygen, turn less nitrate into nitrogen gas
Example 7
Why do some farmers grow clover or beans in a field one year and then plough the plants into the soil?
Answer: legumes such as clover have nitrogen-fixing bacteria in their root nodules; when ploughed in, the plants decay and their nitrogen compounds become nitrate, so less fertiliser is needed