AQA A-level Biology: Nutrient Cycles and Fertilisers (7402)

This guide teaches section 3.5.4 Nutrient cycles in the AQA AS and A-level Biology specification (7402 is the A-level), for first teaching 2015. Section 3.5.4 is A-level only, so it is not part of the AS course. The specification lists no required practical for this section. Its skills column suggests that you could plan investigations into the effect of named minerals on plant growth, and that idea is used in the worked examples below.

Use it with the revision notes and the practice questions. Course hub: AQA A-level Biology. Checklist: AQA A-level Biology checklist. To find weak spots first, try the free diagnostics.

What the section covers

Specification point	What you must be able to do
Nutrient recycling	Explain that nutrients are recycled in natural ecosystems, using the nitrogen cycle and the phosphorus cycle
Saprobionts	Describe their role in decomposition
Mycorrhizae	Describe how they help plants take up water and inorganic ions
Bacteria and nitrogen	Describe saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification (species names are not required)
Fertilisers	Explain the use of natural and artificial fertilisers to replace nitrates and phosphates lost when plants are harvested and livestock removed
Environmental issues	Explain leaching and eutrophication

Why nutrients must be recycled

Plants build proteins, nucleic acids, ATP and phospholipids. These need nitrogen and phosphorus, which they take up as inorganic ions: nitrate (NO₃⁻), ammonium (NH₄⁺) and phosphate (PO₄³⁻). An ecosystem has a limited store of these ions. If they were locked in dead bodies for ever, growth would stop. Microorganisms release them again, so the same atoms pass round the system many times. Energy, by contrast, flows through an ecosystem once and leaves as heat.

Saprobionts and decomposition

A saprobiont is an organism, usually a fungus or a bacterium, that feeds on dead organic matter such as dead plants, dead animals, faeces and urea. Saprobiotic nutrition works in three steps.

1. The saprobiont secretes digestive enzymes onto the dead material (extracellular digestion).
2. The enzymes hydrolyse large insoluble molecules, such as proteins, into small soluble ones, such as amino acids.
3. The saprobiont absorbs the soluble products it needs. It uses some for respiration and growth.

Two points matter for recycling. Some of the products leak into the soil and are available to other organisms. And when the saprobiont respires, it releases carbon dioxide and mineral ions such as ammonium and phosphate. This is decomposition: dead organic compounds are broken down and their elements return to the environment as inorganic ions.

Mycorrhizae

A mycorrhiza is a mutualistic association between a fungus and the root of a plant. Fungal hyphae wrap around, or grow into, the root.

• The hyphae are very thin, so they spread through a large volume of soil and increase the surface area for absorption far beyond that of the root alone.
• They reach water and ions that the root hairs cannot reach, and they take up ions from soil where those ions are present at very low concentration, notably phosphate.
• The fungus passes water and inorganic ions to the plant. The plant supplies the fungus with organic compounds, such as sugars, from photosynthesis. Both partners benefit.

Worked example 1: mycorrhizae and growth

Seedlings of the same species were grown for 12 weeks in sterile soil. Five seedlings had roots infected with a mycorrhizal fungus and five did not. The mean dry mass per seedling was 2.4 g with the fungus and 1.5 g without. The mean mass of phosphorus taken up per seedling was 4.2 mg with the fungus and 1.4 mg without.

Percentage increase in dry mass: (2.4 − 1.5) ÷ 1.5 × 100 = 60%.

Phosphorus taken up: 4.2 ÷ 1.4 = 3 times as much, an increase of 200%.

Phosphorus uptake rose much more than dry mass. Phosphorus per gram of dry mass was 4.2 ÷ 2.4 = 1.75 mg g⁻¹ with the fungus and 1.4 ÷ 1.5 = 0.93 mg g⁻¹ without. The infected plants take up more phosphate than their growth needs, so they are better supplied. A correct conclusion is that the fungus increases phosphate uptake and this is associated with greater growth. A causal claim needs the sterile soil, the equal ages and the same conditions as controls.

The nitrogen cycle

Most of the atmosphere is nitrogen gas, yet neither plants nor animals can use it in that form. Bacteria carry out the steps that link the gas, the soil and living things. You do not need the names of species, only the process and the role.

Process	What happens	Organisms	Conditions
Nitrogen fixation	Nitrogen gas (N₂) is converted to ammonia, which forms ammonium ions	Free-living nitrogen-fixing bacteria in soil; mutualistic nitrogen-fixing bacteria in root nodules of legumes	Needs energy; the nodule bacteria receive carbohydrate from the plant
Ammonification	Nitrogen-containing organic compounds (proteins, urea, nucleic acids) are converted to ammonia, which forms ammonium ions	Saprobionts	Decomposing dead matter and waste
Nitrification	Ammonium ions are oxidised to nitrite ions, then nitrite ions are oxidised to nitrate ions	Two groups of nitrifying bacteria	Aerobic: needs oxygen, so well-aerated soil helps
Denitrification	Nitrate ions are converted to nitrogen gas	Denitrifying bacteria	Anaerobic: waterlogged soil

Around the loop, plants absorb nitrate ions through their roots, often with the help of mycorrhizae, and use the nitrogen to make amino acids, proteins and nucleic acids. Animals obtain nitrogen compounds by eating plants or other animals. Dead organisms, faeces and urea go to saprobionts, and the cycle repeats.

Nitrification gives the form of nitrogen that plants use most easily. Denitrification removes it from the soil, so it reduces fertility. Farmers who drain waterlogged fields and plough to aerate soil limit this loss.

Worked example 2: a nitrogen budget

A field of wheat has these yearly nitrogen flows in kg per hectare (kg ha⁻¹ year⁻¹).

• Inputs: nitrogen fixation by bacteria 40; nitrate fertiliser 120.
• Outputs: removed in the harvested grain and straw 110; leaching 35; denitrification 15.

Total input = 40 + 120 = 160. Total output = 110 + 35 + 15 = 160. The budget is balanced, so the soil nitrogen store stays the same.

Harvest removes 110 ÷ 160 × 100 = 68.75%, or 69%, of the input. Leaching equals 35 ÷ 120 × 100 = 29% of the fertiliser applied. If the farmer stopped using fertiliser, the input would fall to 40 while the crop still removed up to 110, so the soil store would run down and yield would fall.

The phosphorus cycle

The phosphorus cycle has no gas phase. The main store is phosphate in rocks and sediments.

1. Weathering and erosion of rock release phosphate ions into soil and water.
2. Plants absorb phosphate ions through roots and root hairs, and through mycorrhizae. They use it in ATP, phospholipids, DNA and RNA.
3. Phosphorus passes to animals in food, then to saprobionts when organisms die or produce waste.
4. Saprobionts decompose organic phosphorus compounds and release phosphate ions back into soil. This is the phosphorus equivalent of ammonification.
5. Some phosphate is washed to the sea, where it forms sediment. It returns to land only over geological time, when rocks are uplifted and weathered.

Because the cycle is slow and lacks a gas phase, phosphate is lost from a farm when crops and livestock are removed, and natural replacement is slow.

Natural and artificial fertilisers

Harvesting plants and removing livestock takes nitrogen and phosphorus from the system without returning it through death and decay. Fertilisers replace the lost nitrate and phosphate.

	Natural (organic) fertiliser	Artificial (inorganic) fertiliser
Examples	Manure, slurry, compost, crop residues	Ammonium nitrate, ammonium phosphate and mixed N, P, K products
Form of nutrients	Mostly organic compounds; saprobionts must decompose them to release ions	Soluble inorganic salts, already as ions
Release	Slow and steady	Fast; dissolves in soil water
Composition	Variable, not exactly known	Known and can be matched to the crop
Other effects	Adds organic matter, which improves soil structure and water retention	No organic matter

Worked example 3: how much fertiliser

Ammonium nitrate is NH₄NO₃. Using Ar values N = 14, H = 1, O = 16, its Mr is 80 and the nitrogen in it is 2 × 14 = 28.

Percentage nitrogen = 28 ÷ 80 × 100 = 35%.

A crop on 12 hectares removes 98 kg of nitrogen per hectare at harvest. Mass of ammonium nitrate to replace this = 98 ÷ 0.35 = 280 kg per hectare, which is 280 × 12 = 3 360 kg for the field.

Suppose 30% of the nitrogen applied is lost by leaching. Only 70% reaches the crop. Nitrogen to apply = 98 ÷ 0.70 = 140 kg per hectare. Ammonium nitrate = 140 ÷ 0.35 = 400 kg per hectare, or 4 800 kg for the field.

Leaching and eutrophication

Leaching is the removal of soluble ions, here nitrate and phosphate, from soil when water drains through it. Leaching is greater when:

• heavy rain falls soon after fertiliser is applied;
• the fertiliser is a soluble artificial type, and more is applied than the crop can take up;
• the soil is sandy or bare, with no roots to absorb the ions.

Leached ions enter ditches, rivers and lakes. Eutrophication is the enrichment of water with nitrate and phosphate that leads to the sequence below.

1. Nitrate and phosphate concentrations rise in the water.
2. Algae and other surface plants grow rapidly, because the mineral that limited their growth is no longer limiting. A dense algal bloom forms.
3. The bloom blocks light, so submerged plants cannot photosynthesise and die.
4. The dead algae and plants are food for saprobionts, which multiply.
5. Saprobionts use oxygen for aerobic respiration, so dissolved oxygen falls and the biochemical oxygen demand rises.
6. Fish and other aerobic organisms suffocate, which gives the saprobionts still more dead matter.

Worked example 4: reading eutrophication data

A river had a nitrate concentration of 4.0 mg dm⁻³ upstream of a farm. After heavy rain, downstream it was 14.0 mg dm⁻³. Dissolved oxygen fell from 9.2 to 2.1 mg dm⁻³ over the following three weeks.

Nitrate rise: (14.0 − 4.0) ÷ 4.0 × 100 = 250%.

Oxygen fall: 9.2 − 2.1 = 7.1 mg dm⁻³, which is 7.1 ÷ 9.2 × 100 = 77%.

The oxygen fall lags behind the nitrate rise because the algae must grow, die and then be decomposed before the oxygen is used up.

Worked example 5: planning a mineral investigation

To test the effect of nitrate on plant growth you would grow identical seedlings in culture solutions that differ only in nitrate concentration. Control the volume of solution, light, temperature and seedling mass. Include a solution with no nitrate. Dry the plants to constant mass and compare. With five plants in each group, the mean dry mass was 3.1 g with nitrate and 1.9 g without. The difference of 1.2 g is 63% of the no-nitrate mean. A statistical test would show whether it is significant.

Common errors

• Writing that bacteria “make” nitrogen. Fixation converts nitrogen gas to ammonia or ammonium; it does not create nitrogen.
• Mixing up ammonification (organic compounds to ammonium, by saprobionts) with nitrification (ammonium to nitrite to nitrate).
• Saying that denitrification occurs in well-aerated soil. It needs anaerobic conditions.
• Blaming the fertiliser in the field. The harm comes from the leached ions reaching water, and then from oxygen use by saprobionts.
• Saying algae “use up the oxygen”. The main loss is from aerobic respiration of saprobionts feeding on dead material.
• Stating that mycorrhizae “absorb water” as a general rule, without saying that the hyphae raise the surface area and reach ions at low concentration.

Official syllabus

This guide follows section 3.5.4 Nutrient cycles in the AQA AS and A-level Biology specification (7401 and 7402), first teaching 2015, published by AQA.