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

These notes condense section 3.5.4 Nutrient cycles of the AQA AS and A-level Biology specification (7402 is the A-level), for first teaching 2015. The section is A-level only. The study guide has the full explanations and worked examples. Then test yourself with the practice questions.

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Key terms

Term	Meaning
Saprobiont	Organism (fungus or bacterium) that feeds on dead organic matter by secreting enzymes and absorbing the soluble products
Saprobiotic nutrition	Extracellular digestion, then absorption of the soluble products
Decomposition	Breakdown of dead organic matter by saprobionts, releasing inorganic ions
Mycorrhiza	Mutualistic association between a fungus and a plant root
Ammonification	Organic nitrogen compounds converted to ammonia, forming ammonium ions
Nitrification	Ammonium ions oxidised to nitrite, then to nitrate
Nitrogen fixation	Nitrogen gas converted to ammonia, forming ammonium ions
Denitrification	Nitrate converted to nitrogen gas
Leaching	Soluble ions washed out of soil by draining water
Eutrophication	Enrichment of water by nitrate and phosphate, followed by algal bloom and oxygen loss

Saprobionts: method in steps

1. Secrete digestive enzymes onto dead material.
2. Enzymes hydrolyse large molecules to small soluble ones.
3. Absorb the soluble products.
4. Respire some of them. Ammonium and phosphate ions are released into the soil.

Say “extracellular” digestion and say that the products are absorbed. Both are easy marks.

Mycorrhizae

• Fungal hyphae grow around or into plant roots.
• Hyphae are thin and extensive, so the surface area for absorption rises and a larger volume of soil is reached.
• The plant gains more water and more inorganic ions, particularly phosphate, which moves slowly in soil.
• The fungus gains organic compounds from the plant.
• The association benefits both partners, so it is mutualistic.

The nitrogen cycle at a glance

Process	From	To	Organisms	Oxygen?
Fixation	N₂	NH₃ then NH₄⁺	Free-living soil bacteria; bacteria in legume root nodules	Not the key condition
Ammonification	Proteins, urea, nucleic acids	NH₄⁺	Saprobionts	Not the key condition
Nitrification (step 1)	NH₄⁺	NO₂⁻	Nitrifying bacteria	Aerobic
Nitrification (step 2)	NO₂⁻	NO₃⁻	Nitrifying bacteria	Aerobic
Denitrification	NO₃⁻	N₂	Denitrifying bacteria	Anaerobic, e.g. waterlogged soil

Other facts to link in:

• Plants absorb nitrate and use it to make amino acids, proteins and nucleic acids.
• Animals obtain nitrogen compounds by feeding.
• Nodule bacteria receive carbohydrate from the legume. The legume receives fixed nitrogen. This is why legumes can be grown to improve soil.
• Species names of bacteria are not required.

The phosphorus cycle at a glance

Stage	Detail
Store	Phosphate in rocks and sediments; there is no gas phase
Release	Weathering and erosion dissolve phosphate into soil water
Uptake	Plant roots, helped by mycorrhizae, absorb phosphate ions
Use	ATP, phospholipids, DNA, RNA
Transfer	Food chains: plant to animal
Return	Saprobionts decompose dead matter and waste and release phosphate
Loss	Washing to sea, forming sediment; very slow return

Nitrogen cycle versus phosphorus cycle: must-know distinctions

Point	Nitrogen	Phosphorus
Atmospheric store	Yes, nitrogen gas	No
Fixation step	Yes, by bacteria	No equivalent
Denitrification	Yes	No equivalent
Role of saprobionts	Ammonification	Release of phosphate from organic compounds
Ion taken up by plants	Nitrate (and ammonium)	Phosphate

Ammonification and nitrification are different: ammonification starts from organic compounds and is carried out by saprobionts. Nitrification starts from ammonium ions and is oxidation.

Fertilisers

Natural	Artificial
Manure, slurry, compost	Inorganic salts, such as ammonium nitrate or ammonium phosphate
Organic; must be decomposed to release ions	Already soluble ions
Slow release	Fast release
Variable composition	Known composition
Adds organic matter to the soil	Does not add organic matter

Reason for use: harvesting plants and removing livestock takes nitrate and phosphate out of the system. Fertiliser replaces them so that yield is maintained.

Percentage by mass and mass needed

1. Work out the mass of the element in one formula unit.
2. Divide by the Mr and multiply by 100 for the percentage.
3. Mass of fertiliser = mass of nutrient needed ÷ the fraction of the nutrient in the fertiliser.
4. If a fraction is lost by leaching, divide by the fraction that remains.

Example: 28 ÷ 80 = 35% nitrogen in ammonium nitrate. To supply 70 kg of nitrogen, 70 ÷ 0.35 = 200 kg of fertiliser.

Leaching and eutrophication

Leaching is greatest when heavy rain follows application, when more is applied than the crop needs, when the ions are soluble, and when soil is sandy or bare.

Eutrophication, in order:

1. Nitrate and phosphate enter the water.
2. Algae grow rapidly and form a bloom.
3. The bloom blocks light.
4. Submerged plants die.
5. Saprobionts feed on the dead matter and multiply.
6. Saprobionts respire aerobically and oxygen falls.
7. Fish and other aerobic organisms suffocate.

Ways to reduce the problem: apply the right amount at the right time, avoid spreading before heavy rain, keep strips of vegetation along waterways, and use slow-release or natural fertilisers where suitable.

Small worked reminders

Percentage change = (new − old) ÷ old × 100. If nitrate in a stream goes from 2.5 to 6.0 mg dm⁻³, the change is 3.5 ÷ 2.5 × 100 = 140% increase.

Nitrogen budget: inputs minus outputs gives the change in the soil store. Inputs of 30 (fixation) and 90 (fertiliser) kg ha⁻¹ against outputs of 85 (harvest), 25 (leaching) and 10 (denitrification) give 120 − 120 = 0, so the store is unchanged.

Investigating minerals

Skills note in the specification: devise investigations into the effect of named minerals on plant growth.

1. Change only one variable, such as nitrate concentration in a culture solution.
2. Keep the volume of solution, light, temperature, species, age and starting mass the same.
3. Include a solution lacking the mineral as the control.
4. Use several plants per group and calculate a mean.
5. Measure dry mass after drying to constant mass. Fresh mass varies with water content.
6. Look for patterns, such as stunted growth or yellow leaves in the absence of nitrate, and say that a statistical test is needed to judge significance.

Describing eutrophication data

When a graph or table shows nitrate, algae and oxygen over time, take these steps.

1. Quote figures with units for the rise or fall.
2. Say which variable changes first. Nitrate rises, then algal numbers rise, then oxygen falls later.
3. Link each change to its cause: more nitrate allows more growth, a bloom shades the plants below, decomposition by saprobionts uses oxygen.
4. Explain a later fall in nitrate by uptake by the algae.

Quick self-test

1. What is a saprobiont?
2. Give two ways in which mycorrhizae help a plant.
3. Which process converts ammonium ions to nitrite ions, and what condition does it need?
4. Which process do bacteria in legume root nodules carry out?
5. In which conditions does denitrification occur?
6. A fertiliser is 20% nitrogen by mass. What mass is needed to supply 50 kg of nitrogen?
7. A river has 500 units of nitrate before a farm and 350 units after a drought. Calculate the percentage decrease.
8. A farmer applies 80 kg of nitrogen per hectare and 24 kg is leached. Calculate the percentage leached.
9. Name the main store in the phosphorus cycle.
10. Why does oxygen concentration fall in a eutrophic lake?
11. Give one advantage of an artificial fertiliser over a natural one.

Answers

1. An organism, usually a fungus or bacterium, that feeds on dead organic matter by secreting enzymes and absorbing the products.
2. Any two: hyphae increase the surface area for absorption; they reach a larger volume of soil; they supply more water; they supply more phosphate and other ions.
3. Nitrification (the first step). It is aerobic, so it needs oxygen.
4. Nitrogen fixation: nitrogen gas is converted to ammonia, which forms ammonium ions.
5. Anaerobic conditions, such as waterlogged soil.
6. 50 ÷ 0.20 = 250 kg.
7. (500 − 350) ÷ 500 × 100 = 30%.
8. 24 ÷ 80 × 100 = 30%.
9. Phosphate in rocks and sediments.
10. Saprobionts feed on the dead algae and plants, multiply, and use oxygen in aerobic respiration.
11. Any one: the nutrients are already ions, so they act quickly; the composition is known and can be matched to the crop.

Where marks are usually lost

• Writing “bacteria break down dead things” without naming ammonification and saying that ammonium ions are formed.
• Writing nitrification when ammonification is meant, or the reverse.
• Describing denitrification as helpful to soil fertility. It removes nitrate from the soil.
• Omitting the role of saprobionts in the oxygen fall in eutrophication, and blaming only the algae.
• Forgetting the light-blocking step, which explains why submerged plants die.
• Writing “nutrients” instead of nitrate or phosphate ions when asked what the plant takes up.
• Not showing the working for a percentage by mass: set out each stage so that your method can be followed.
• Describing mycorrhizae as parasitic. They are mutualistic.

Official syllabus

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