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?

  1. Nitrogen fixation: nitrogen gas from the air is turned into compounds such as ammonia or nitrates.

Answer: nitrogen gas from the air is turned into compounds such as ammonia or nitrates

Example 2

What happens in nitrification?

  1. Nitrification: ammonium compounds are turned into nitrites and then nitrates.

Answer: ammonium compounds are turned into nitrites and then nitrates

Example 3

What happens in denitrification?

  1. Denitrification: nitrates in the soil are turned back into nitrogen gas.

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?

  1. Nitrogen-fixing bacteria (some free in the soil, some in the root nodules of legumes) turn nitrogen gas into ammonium ions.
  2. Nitrifying bacteria turn ammonium ions into nitrite and then nitrate. They need oxygen.
  3. Denitrifying bacteria turn nitrate back into nitrogen gas. They live where there is little oxygen, such as waterlogged soil.

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.

  1. Decomposers break down the proteins in the leaf into ammonium compounds.
  2. Nitrifying bacteria turn ammonium compounds into nitrites, then nitrates.
  3. Root hair cells take in the nitrates by active transport.
  4. The plant uses the nitrogen to make amino acids and proteins.

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.

  1. Ploughing and draining let air, and so oxygen, into the soil.
  2. Nitrifying bacteria need oxygen. With more oxygen they turn more ammonium into nitrate.
  3. Denitrifying bacteria work in soil with little oxygen. With more oxygen they are less active, so less nitrate is lost as nitrogen gas.

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?

  1. Legumes such as clover and beans have nitrogen-fixing bacteria in nodules on their roots, so they make nitrogen compounds from the air.
  2. When the plants are ploughed in, decomposers break down their proteins into ammonium, and nitrifying bacteria turn this into nitrate.
  3. The next crop can take up this nitrate, so the farmer needs less fertiliser.

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