Nutrient cycles

A nutrient cycle is the path a chemical element such as nitrogen, phosphorus or carbon takes as it moves from the air, soil or water into living things and back again. Plants take up the nutrient, animals get it by eating, and when organisms die or excrete, microorganisms break the remains down and release the nutrient to be used again. At A-level the two you need are the nitrogen cycle and the phosphorus cycle.

What is a nutrient cycle?

The Earth has a fixed amount of each element, so living things can only keep growing if the elements are recycled. Nitrogen is needed to make amino acids, proteins, nucleic acids and ATP. Phosphorus is needed for DNA, RNA, ATP and phospholipids. Each cycle has the same pattern:

1. An inorganic form of the nutrient (such as nitrate or phosphate ions) is taken up by plants.
2. The plant builds it into organic molecules, which pass along the food chain when animals eat.
3. Dead organisms, faeces and urine are broken down by microorganisms.
4. The microorganisms release the nutrient back into the soil or water in an inorganic form.

AQA A-level Biology says nutrients are recycled in natural ecosystems, using the nitrogen and phosphorus cycles as examples, and that microorganisms play a vital role. At GCSE, the carbon cycle and the water cycle are the ones you learn.

Saprobionts and decomposition

Saprobionts (also called saprotrophs or decomposers) are bacteria and fungi that feed on dead organic matter. They secrete enzymes onto the dead material, so digestion happens outside their cells, and then absorb the small soluble molecules that are made. This is called saprobiotic nutrition. As they break down proteins and nucleic acids, they release ammonium ions and phosphate ions into the soil. Without saprobionts, nutrients would stay locked in dead bodies and waste.

Mycorrhizae

Mycorrhizae are partnerships between fungi and the roots of plants. The fungus grows long, thin threads called hyphae through the soil, which act like an extension of the root system. Because the hyphae are so thin and so many, they give a far larger surface to take up water and inorganic ions, especially phosphate, which does not move easily through soil. In return, the plant gives the fungus carbohydrates made in photosynthesis. Both organisms benefit, so it is a mutualistic relationship.

The nitrogen cycle

Air is about 78% nitrogen gas, N₂, but plants and animals cannot use it directly. Bacteria do almost all of the work of turning it into forms plants can absorb, and back again.

1. Nitrogen fixation: nitrogen gas is turned into ammonia.
2. Ammonification: saprobionts turn the nitrogen compounds in dead matter, faeces and urine into ammonium ions.
3. Nitrification: ammonium ions become nitrite, then nitrate.
4. Denitrification: nitrate is turned back into nitrogen gas.

Between these, plant roots take up nitrate ions by active transport and use them to make proteins and nucleic acids, animals get their nitrogen by eating, and dead organisms and waste return it to the soil.

Nitrogen fixation

Nitrogen-fixing bacteria turn nitrogen gas into ammonia, which forms ammonium ions in the soil. Some live freely in the soil. Others live in nodules on the roots of legumes such as peas, beans and clover, where they give the plant nitrogen compounds and get carbohydrates in return. Lightning and the industrial Haber process also fix a small amount of nitrogen.

Ammonification

Saprobionts break down proteins, nucleic acids and urea in dead matter and waste, releasing ammonia, which forms ammonium ions in the soil.

Nitrification

Nitrifying bacteria oxidise ammonium ions to nitrite ions (NO₂⁻), and then nitrite ions to nitrate ions (NO₃⁻). They need oxygen, so nitrification happens in well aerated soil. The bacteria get energy from these reactions.

Denitrification

Denitrifying bacteria turn nitrate ions back into nitrogen gas. They work in anaerobic conditions, so denitrification is fastest in soil with little oxygen. It takes nitrate out of the soil, which is bad for crops.

AQA wants the role of bacteria in each process, but not the names of individual species. Check your understanding.

The phosphorus cycle

Phosphorus has no important gas stage, so the phosphorus cycle runs through rocks, soil, water and living things instead of the air.

1. Phosphate ions are locked up in rocks. Weathering and erosion slowly release them into soil and water.
2. Plants absorb phosphate ions through their roots, helped by mycorrhizae.
3. Animals get phosphorus by eating plants and other animals.
4. Saprobionts break down dead organisms and waste, releasing phosphate ions again.
5. Some phosphate is carried into rivers and seas, where it settles as sediment. Over millions of years the sediment becomes rock, and the cycle can start again.
6. Sea birds return some phosphate to land in their droppings, called guano.

Natural and artificial fertilisers

In a natural ecosystem, nutrients return to the soil where they came from. On a farm, crops are harvested and livestock are sold, so the nitrates and phosphates in them are taken away and never decompose back into that field. Farmers replace them with fertilisers:

• Natural (organic) fertilisers are dead and waste organic matter, such as manure, slurry, compost and crop remains. Saprobionts have to decompose them before the nutrients are released, so the nutrients come out slowly.
• Artificial (inorganic) fertilisers are mined or made in factories, such as ammonium nitrate and mixed NPK fertilisers (nitrogen, phosphorus, potassium). The ions are already soluble, so they act fast and the amounts are exact.

Planting legumes such as clover is a third way: their root nodule bacteria fix nitrogen, and ploughing the plants back in adds nitrogen compounds to the soil.

Leaching and eutrophication

Nitrate ions are very soluble. If more fertiliser is added than the crop can take up, or heavy rain falls soon after spreading, rainwater carries the ions down through the soil and into rivers and lakes. This is called leaching, and it is more likely with soluble artificial fertilisers. In the water it can cause eutrophication:

1. Nitrate (and phosphate) levels in the water rise.
2. Algae at the surface grow rapidly, forming an algal bloom.
3. The bloom blocks light from reaching plants lower down.
4. Those plants cannot photosynthesise enough, and they die.
5. Saprobiotic bacteria feed on the dead plants and algae, and their population grows.
6. The bacteria use up dissolved oxygen in aerobic respiration.
7. Fish and other aerobic organisms die from lack of oxygen.

Exam questions with answers

Why do farmers grow clover between cereal crops?

Clover is a legume with nitrogen-fixing bacteria in its root nodules. They turn nitrogen gas into ammonium compounds, so when the clover is ploughed in and decomposed, ammonification and nitrification add nitrate to the soil for the next crop. This means less artificial fertiliser is needed.

Why does harvesting crops reduce the nitrate in a field?

The nitrogen taken up by the crop is in its proteins and nucleic acids. When the crop is removed, it is not decomposed in that field, so ammonification cannot return that nitrogen to the soil.

Why is the phosphorus cycle so slow?

Most phosphorus is in rocks, and it is only released by weathering, which is very slow. Phosphate that settles as sediment in the sea takes millions of years to become rock and be lifted back above sea level. There is no fast route through the air, as there is for nitrogen.

Common mistakes

• Mixing up nitrification and nitrogen fixation. Fixation starts with nitrogen gas; nitrification starts with ammonium.
• Getting the conditions the wrong way round. Nitrifying bacteria need oxygen; denitrifying bacteria work without it.
• Saying plants absorb nitrogen gas. They absorb nitrate ions through their roots.
• Saying the algae use up the oxygen in eutrophication. It is mainly the saprobiotic bacteria decomposing dead matter.
• Using "decomposers" vaguely. Say saprobionts secrete enzymes and absorb the products, releasing ammonium or phosphate ions.
• Naming bacteria species you are not asked for. AQA does not require them.

Nutrient cycles in geography

If you met nutrient cycles in geography, you probably used Gersmehl's model. It shows an ecosystem as three nutrient stores, the living biomass, the litter (dead leaves and organic matter on the ground) and the soil, drawn as circles sized by how many nutrients they hold. Arrows show the flows between them: leaf fall from biomass to litter, decomposition from litter to soil, and uptake by plants from soil to biomass, plus inputs from rain and weathering and losses by runoff and leaching. The biology on this page explains what happens inside those arrows.

Practise with a short timed paper

Nutrient cycles come up in the second year of A-level Biology, and the carbon cycle and decomposition at GCSE. The daily A-level Biology: Year 2 and GCSE Biology Higher papers are short timed papers of 8 questions, with a new one every day.

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