9.3.3 The Nitrogen Cycle · 9. Ecosystems and Material Cycles | Edexcel GCSE Combined Science 1SC0: Biology

Section 1 of 8Why nitrogen matters

Living organisms need nitrogen to make amino acids, which are joined to form proteins, and to make the bases in DNA and RNA.

Air is about 78 per cent nitrogen gas (N₂), but plants and animals cannot use it directly because the two atoms are held together by a very strong triple bond.

Nitrates

Plants must take up nitrogen as dissolved nitrates (and some ammonium ions) through their roots.

Animals obtain nitrogen by eating plants or other animals.

Bacteria carry out the steps that make nitrogen available and recycle it.

Section 2 of 8The stages

1Nitrogen fixation

Nitrogen fixation nitrogen gas is converted into nitrogen compounds.

This is done by nitrogen-fixing bacteria in the soil (such as Azotobacter) and by mutualistic bacteria (Rhizobium) living in the root nodules of legumes such as peas, beans and clover.
Lightning also fixes small amounts, and in industry the Haber process makes ammonia for fertilisers.

2Decomposition (ammonification)

Decomposition (ammonification) decomposers, bacteria and fungi, break down proteins and urea in dead remains and waste, releasing ammonia, which forms ammonium ions.

3Nitrification

Nitrification nitrifying bacteria in the soil convert ammonium ions to nitrites and then to nitrates. They need oxygen, so well-aerated soil promotes it.

4Uptake and feeding

Uptake and feeding plants absorb nitrates, use them to make amino acids and proteins, and these pass along the food chain.

5Denitrification

Denitrification denitrifying bacteria convert nitrates to nitrogen gas, returning it to the air. They thrive in waterlogged soil with little oxygen.

Section 3 of 8Farming and fertilisers

When crops are harvested, nitrogen compounds in the plants are removed, and the soil becomes depleted.

Crop rotation

Farmers replace this by adding nitrate or ammonium fertilisers, by adding manure, which decomposes to supply minerals, or by using crop rotation, in which a legume such as clover or beans is grown in one year, and its root nodule bacteria fix nitrogen that benefits a cereal crop the following year.

Ploughing and draining the soil helps oxygen to enter and increases nitrification, whereas waterlogged fields lose nitrate through denitrification.

If too much fertiliser is applied it can be washed by rain into streams, which results in eutrophication, as explained in the lesson on human effects on the environment.

Section 4 of 8Linking the cycle to other biology

The nitrogen cycle shows the dependence of ecosystems on microorganisms.

Linking the cycle to other biology

It also links to the idea of mutualism, since the bacteria in nodules gain sugars and protection while the plant gains nitrogen compounds.

Note that a deficiency of nitrate in the soil leads to stunted growth and yellow older leaves in plants, since there is not enough nitrogen to make protein and chlorophyll.

When explaining the cycle in an exam, name the type of bacteria for each step and describe the change of the nitrogen compound; without this detail answers are too vague to earn marks.

Section 5 of 8Worked example: soil in two fields

A farmer finds that wheat in a waterlogged corner of a field is stunted with pale leaves, while the rest of the field is healthy.

Worked example: soil in two fields

In the wet area there is little oxygen, so nitrifying bacteria, which need oxygen, are less active and make fewer nitrates, and denitrifying bacteria convert more of the nitrate to nitrogen gas which escapes into the air.

The wheat therefore has too little nitrate to make amino acids and chlorophyll.

Draining the field, adding air by ploughing, or applying fertiliser would help.

On another farm, rotating peas, wheat and potatoes means that the pea crop supplies nitrogen compounds through its root nodules, so less artificial fertiliser has to be bought. This lowers costs and reduces the risk of nitrate run-off into streams.

Evidence and exam links

Gardeners notice the cycle when they add compost or manure: the protein in the dead plant material is broken down by decomposers to ammonium and then by nitrifying bacteria to nitrate, which the next crop absorbs. Tracing one nitrogen atom through all the stages, from nitrogen gas in the air through a nodule bacterium, a pea plant, a rabbit, its waste, and a soil bacterium, then back to the air, is a useful exam skill that shows you understand that the cycle is continuous.

Section 6 of 8Exam-style questions

Explain why farmers grow clover in a field before growing wheat in the same field. [3]3 marks
Describe the role of bacteria in returning nitrogen from a dead animal to a plant. [4]4 marks

Section 7 of 8Key terms

Term	Meaning
Nitrogen fixation	Conversion of nitrogen gas into nitrogen compounds by bacteria (or lightning).
Nitrification	Conversion of ammonium to nitrite and then nitrate by bacteria.
Denitrification	Conversion of nitrates to nitrogen gas by bacteria.
Nitrate	A nitrogen ion (NO₃^-) absorbed by plant roots.
Legume	A plant such as pea or clover whose roots have nitrogen-fixing nodules.
Crop rotation	Growing different crops in turn on a field to maintain soil fertility.
Fertiliser	A substance added to soil to supply minerals for plant growth.

Section 8 of 8Exam technique

1

Saying plants take in nitrogen gas. They need nitrates (or ammonium) from the soil.

2

Mixing up nitrification and nitrogen fixation. Fixation uses N₂; nitrification starts from ammonium.

3

Forgetting that denitrification returns nitrogen to the air and happens when oxygen is low.

4

Writing 'bacteria' with no named type or function.