Nutrient Cycles: AQA A Level Biology Exam Questions by Topic

AQANutrient CyclesExam Questions
Written by Tom Whitburn

Nutrient cycles (AQA 3.5.4) is an A-level only topic, so you'll meet it in Year 13 on Papers 2 and 3. You need to know how nitrogen and phosphorus are recycled within ecosystems, the role of saprobionts in decomposition, how mycorrhizae help plants take up water and inorganic ions, and what bacteria do in saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification. You also need to explain why farmers add natural and artificial fertilisers, and how fertilisers cause leaching and eutrophication.

The examiners' reports find the same gaps every year. Students write 'nitrogen-containing compounds' without naming a protein or DNA, convert ammonia into nitrate and then nitrite, mix up nitrifying and nitrogen-fixing bacteria, and give nitrate ions when asked for biological molecules. In eutrophication answers they leave out the saprobionts using up oxygen in respiration, and in evaluations they describe the data without questioning the method.

Start with the nitrogen and phosphorus cycle questions, then mycorrhizae, then fertilisers and eutrophication. Answer each question on paper before you open the mark scheme. If you'd like help with this topic, my AQA masterclasses run online on Tuesdays at 5.15pm for Year 12 and on Wednesdays or Thursdays at 6.30pm for Year 13.

What you need from GCSE and Year 12

Each question on this page assumes you're secure on these GCSE and Year 12 ideas. If any of them feels shaky, revise it before you start.

  • GCSE: decomposers such as bacteria and fungi break down dead organisms and waste, returning mineral ions to the soil; decay is faster when it's warm, moist and there's oxygen.
  • GCSE: plants absorb nitrate ions from the soil to make amino acids and proteins, and fertilisers replace the mineral ions removed when crops are harvested.
  • Year 12 (AQA 3.1.4.1, 3.1.5.1 and 3.1.6): amino acids, proteins, DNA, RNA and ATP all contain nitrogen, and DNA, RNA, ATP and phospholipids contain phosphate. See the Proteins, DNA and RNA structure and ATP pages.
  • Year 12 (AQA 3.2.3): root cells take up mineral ions by active transport and water by osmosis. See the Transport across cell membranes page.
  • Year 12 (AQA 3.1.4.2): enzymes work fastest at an optimum temperature and pH. See the Enzymes page.

Key terms

These words come up in the questions and mark schemes. Examiners expect you to use them exactly.

  • Saprobiont: a microorganism that feeds by secreting enzymes onto dead organic matter and absorbing the products (saprobiotic nutrition); saprophyte and saprotroph are accepted.
  • Ammonification: saprobionts break down nitrogen-containing compounds such as proteins, DNA and urea, releasing ammonia or ammonium ions.
  • Nitrification: nitrifying bacteria oxidise ammonium ions to nitrite ions and then nitrite to nitrate ions; it needs oxygen.
  • Nitrogen fixation: nitrogen-fixing bacteria convert nitrogen gas into ammonia, using the enzyme nitrogenase.
  • Denitrification: denitrifying bacteria convert nitrate ions back to nitrogen gas in anaerobic conditions, such as waterlogged soil.
  • Mycorrhizae: associations between fungi and plant roots; the fungal hyphae increase the surface area for uptake of water and inorganic ions, and the fungus gets carbohydrates from the plant.
  • Natural fertiliser: organic matter such as manure or compost, broken down by saprobionts to release ions.
  • Artificial fertiliser: inorganic chemicals, such as potassium nitrate or ammonium sulfate, containing ions like nitrate and phosphate.
  • Leaching: water-soluble ions such as nitrate are washed out of the soil by rain into rivers and lakes.
  • Eutrophication: extra nutrients cause algae to grow, blocking light so submerged plants die; saprobionts decomposing them use up oxygen in respiration, so fish die.

What these questions cover (AQA 3.5.4)

The questions test these statements from the AQA AS and A-level Biology specification (7401/7402):

  • Nutrient recycling: Nutrients are recycled within natural ecosystems, exemplified by the nitrogen cycle and the phosphorus cycle. (the 2023 calculation uses the phosphorus cycle; the other questions use nitrogen)
  • Microorganisms: Microorganisms play a vital role in recycling chemical elements such as phosphorus and nitrogen.
  • Saprobionts: The role of saprobionts in decomposition.
  • Mycorrhizae: The role of mycorrhizae in facilitating the uptake of water and inorganic ions by plants.
  • Bacteria in the nitrogen cycle: The role of bacteria in the nitrogen cycle in sufficient detail to illustrate the processes of saprobiotic nutrition, ammonification, nitrification, nitrogen fixation and denitrification.
  • Names of bacteria: (The names of individual species of bacteria are not required). (no question tests this; it means you won't be asked to name a species, but you must say which group of bacteria carries out each process)
  • Fertilisers: The use of natural and artificial fertilisers to replace the nitrates and phosphates lost by harvesting plants and removing livestock.
  • Leaching and eutrophication: The environmental issues arising from the use of fertilisers including leaching and eutrophication. (the questions here are about eutrophication; none asks about leaching on its own)

How to use this page

Write your answer on paper before you open the mark scheme. Allow about 1.3 minutes per mark: the 18 questions are worth 47 marks, so about 61 minutes in total. Then mark your answer strictly against the mark scheme and read the examiners' tip underneath it. If you'd rather print the questions, download the PDF pack.

The nitrogen and phosphorus cycles (3.5.4)

Question 1

A level Paper 3 · June 2019 · Q02.1 · 2 marks · Medium

Describe the role of saprobionts in the nitrogen cycle.

Question text: describe the role of saprobionts in the nitrogen cycle, 2 marks.
Show the mark scheme Mark scheme: they use enzymes to decompose proteins, DNA, RNA or urea (any named nitrogen-containing molecule; 'nitrogen-containing compounds' alone is ignored); producing or releasing ammonia or ammonium ions (ammonification accepted).

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From the examiners' report: Two-thirds got at least one mark. Name a compound such as protein or DNA, and only describe the part of the cycle that saprobionts do, not the whole cycle.

Question 2

A level Paper 2 · June 2025 · Q10.2 · 2 marks · Hard

Explain how two abiotic factors in tropical rainforests speed up nutrient recycling.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Passage: tropical rainforests have high annual rainfall and high mean temperatures providing moist, warm conditions; nutrient recycling occurs rapidly on the ground, and recycling of nitrogen-containing compounds from dead organic matter sustains plant growth. Question: explain how two abiotic factors in these rainforests enable the rapid rate of recycling (lines 4–5), 2 marks.
Show the mark scheme Mark scheme, any two: high or optimum temperature for enzymes, respiration, nitrification or ammonification; water for metabolic or enzyme reactions, or for hydrolysis; optimum pH for enzymes; high oxygen for respiration by saprobionts or nitrifying bacteria. Reject water for photosynthesis.

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From the examiners' report: Less than 10% got both marks. Pick abiotic factors, not organisms, and link each to enzymes, hydrolysis or respiration rather than just 'faster decomposition'.

Question 3

A level Paper 2 · June 2025 · Q10.3 · 4 marks · Medium

Describe the roles of bacteria in recycling nitrogen from dead organic matter.

Question text: the recycling of nitrogen-containing compounds from dead organic matter sustains the growth of plants (lines 5–7); describe the roles of bacteria in this recycling, 4 marks.
Show the mark scheme Mark scheme: protein, amino acids or DNA into ammonium or ammonia; by saprobionts; ammonium into nitrite and then into nitrate; by nitrifying bacteria. Ignore nitrogen fixation and denitrification.

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From the examiners' report: Less than 20% got all four. Start with a named compound such as protein, get the order right (ammonia, nitrite, nitrate) and don't add nitrogen fixation, which isn't recycling from dead matter.

Question 4

A level Paper 3 · November 2021 · Q04.2 · 2 marks · Medium

Suggest why waterlogged marsh soils are high in ammonium and low in nitrite and nitrate.

Question text: freshwater marsh soils are normally waterlogged, creating anaerobic conditions; use knowledge of the nitrogen cycle to suggest why these soils contain relatively high concentrations of ammonium compounds and low concentrations of nitrite and nitrate ions, 2 marks.
Show the mark scheme Mark scheme, any two: less nitrification, as nitrifying bacteria need oxygen; less oxidation of ammonium to nitrite and then nitrate; more denitrification, as denitrifying bacteria need anaerobic conditions; so more nitrate converted to nitrogen gas.

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From the examiners' report: 25% got both marks. Use the right part of the cycle: nitrifying bacteria need oxygen, and denitrifying bacteria turn nitrate into nitrogen gas. Saprobionts aren't anaerobic.

Question 5

A level Paper 2 · June 2019 · Q09.2 · 2 marks · Hard

Suggest how digesting insects helps the sundew grow in nutrient-poor soil.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Question text: the sundew is a small flowering plant of acidic bogs and marshes with very low concentrations of some nutrients; it can trap and digest insects; suggest and explain how digesting insects helps the sundew to grow in this soil, 2 marks.
Show the mark scheme Mark scheme, in pairs: digestion of proteins, providing amino acids or a named nitrogen-containing compound; or digestion of a named phosphate-containing compound such as DNA, providing nucleotides or phosphate. Ignore carbohydrates and lipids; nitrate or ammonium as products not credited.

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From the examiners' report: Almost 70% scored nothing and only 5% got both. 'Nutrients for growth' is too vague: say proteins are digested to amino acids. The sundew digests the insects, not saprobionts.

Question 6

A level Paper 3 · June 2018 · Q06.3 · 3 marks · Hard

Use the ammonia graph to support the idea that soils A and B contain different communities of nitrifying bacteria.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 6: line graph of the concentration of ammonia in soil (µg g⁻¹) over 20 days for soil A at pH 6.9, soil B at pH 4.3 and a mixture of A and B adjusted to pH 6.9; all start at about 20; B falls fastest, A falls more slowly and the mixture falls most slowly. Question: what evidence from Figure 6 supports the conclusion that there were different communities of bacteria in soils A and B, with reasons, 3 marks.
Show the mark scheme Mark scheme: B at pH 4.3 has the fastest rate of ammonia breakdown; the mixture at pH 6.9 is slowest; suggests the bacteria from B don't work well at pH 6.9, or only the bacteria from A work in the mixture.

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From the examiners' report: Only 3% got all three. Ammonia is being broken down, not made, so the steepest fall is the fastest rate. Then reason that B's bacteria don't work at the mixture's pH.

Question 7

A level Paper 2 · June 2022 · Q03.2 · 3 marks · Medium

Evaluate the conclusion that ammonia inhibits nitrogenase in nitrogen-fixing bacteria.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Table 2: for Azotobacter grown in 0 to 120 µg cm⁻³ ammonium chloride, nitrogenase activity falls from 45 to 30, 17, 7 and then 0 arbitrary units from 80 µg cm⁻³ upwards, while ammonium chloride remaining in the medium is 0 up to 60 and then 6, 14 and 20. Question: a student concluded that ammonia inhibits nitrogenase activity in nitrogen-fixing bacteria; use all the information to evaluate this conclusion, 3 marks.
Show the mark scheme Mark scheme, any three: nitrogenase activity decreases as ammonium chloride increases; activity is zero at high concentrations, or ammonium chloride remains when activity is zero; only one species used; the inhibition may be due to chloride ions, or ammonium chloride was used, not ammonia. Ignore statistical tests.

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From the examiners' report: 92% got at least one mark but only 8% got three. After the trend, question the method: one species only, and ammonium chloride isn't ammonia. A statistical test wasn't needed for such large differences.

Question 8

A level Paper 2 · June 2023 · Q03.1 · 2 marks · Medium

Calculate the difference in phosphorus supplied to the soil by two stores, in teragrams.

Figure 1: part of the phosphorus cycle; a store in the biotic component on land of 2.1 × 10³ Tg supplies 0.05% to the soil by decomposition, and a store in rocks of 1 × 10¹³ Tg supplies 1.3 × 10⁻¹⁰% by weathering and erosion; one teragram is 1 × 10¹² g. Question: calculate in teragrams the difference in the supply of phosphorus to the soil from the two stores, showing working, 2 marks.
Show the mark scheme Mark scheme: 12 or 11.95 for 2 marks; 1 mark for 1195 (wrong order of magnitude), or for 1.05 or 13 Tg.

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From the examiners' report: 40% got both marks. Convert each percentage to a mass first (1.05 and 13 Tg), then subtract. Adding them or dividing them was a common slip.

Mycorrhizae (3.5.4)

Question 9

A level Paper 2 · June 2017 · Q05.1 · 1 mark · Easy

Suggest how taking up more phosphate could increase plant growth.

Question text: arbuscular mycorrhiza fungi (AMF) grow on and into plant roots and can increase the uptake of inorganic ions such as phosphate; suggest one way an increase in the uptake of phosphate could increase plant growth, 1 mark.
Show the mark scheme Mark scheme: used to produce a named phosphate compound in cells, such as ATP, ADP, phospholipids, DNA, RNA, RuBP, TP or GP.

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From the examiners' report: Over 80% got this, often for ATP or DNA. Proteins don't contain phosphate, so they didn't score.

Question 10

A level Paper 2 · June 2017 · Q05.2 · 1 mark · Easy

Suggest how the fungi benefit from their association with plants.

Question text: suggest one way in which AMF may benefit from their association with plants, 1 mark.
Show the mark scheme Mark scheme: an example of a carbon-containing biological compound, such as carbohydrate, amino acid or vitamin; accept sugars or organic compounds. Ignore products of photosynthesis and starch.

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From the examiners' report: Almost 60% got this, usually for carbohydrate or sugars. 'Nutrients', water and oxygen didn't score: name the organic compound the fungus gets.

Question 11

A level Paper 2 · June 2023 · Q03.2 · 5 marks · Medium

Evaluate using mycorrhizae to increase tomato production with and without a water shortage.

Figure 2: bar chart of mean mass of tomatoes (kg m⁻²) with error bars of ± 2 standard deviations for greenhouses P (untreated, water shortage) about 0.95, Q (treated with G. intraradices, water shortage) about 1.65, R (untreated, no shortage) about 2.05 and S (treated, no shortage) about 2.6; P and Q bars don't overlap, R and S bars overlap. Question: using all the information, evaluate the effectiveness of using mycorrhizae to increase crop production with and without water shortage, 5 marks.
Show the mark scheme Mark scheme, any five (max 3 from points 5 to 9): no significant difference between R and S; significantly greater production in Q than P; significant difference between S and Q; overlap of SDs means no significant difference, no overlap means significant; only one mycorrhizal species; only tomatoes; large sample increases reliability; done in sterile soil; done in greenhouses, not a field.

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From the examiners' report: About 21% scored nothing, often by describing results without saying whether differences were significant. Say whether the SD bars overlap, then give limits such as one crop and sterile soil.

Question 12

A level Paper 2 · June 2023 · Q03.3 · 2 marks · Hard

Suggest two reasons why the tomato seeds were planted in sterilised soil.

Question text: the tomato seeds were planted into sterilised soil; suggest two reasons why the soil was sterilised, 2 marks.
Show the mark scheme Mark scheme, any two: kill other mycorrhizae or fungi; kill nitrogen-fixing, nitrifying, denitrifying or saprobiotic bacteria; kill pathogens; kill pests; remove competitors; kill seeds or spores; it is a controlled variable. 'Harmful bacteria' alone isn't enough.

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From the examiners' report: Only 12% got both. Sterilising kills organisms, not nutrients or pesticides. Think of the microorganisms in soil that would affect the result, such as other fungi or nitrifying bacteria.

Fertilisers and eutrophication (3.5.4)

Question 13

A level Paper 3 · June 2017 · Q03.1 · 2 marks · Easy

Give two nitrogen-containing biological molecules removed when a crop is harvested.

Question text: farmers use artificial fertilisers to maintain or increase yield of grain crops such as wheat; artificial fertiliser replaces mineral ions, including nitrate, removed when crops are harvested; give two examples of biological molecules containing nitrogen that would be removed when a crop is harvested, 2 marks.
Show the mark scheme Mark scheme, any two: amino acid or protein; nucleic acid, nucleotide or base; DNA; RNA; ATP or ADP; NAD or NADP; cyclic AMP; chlorophyll. Reject nitrates, nitrites, ammonia, ammonium and urea.

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From the examiners' report: Just over half got both marks. The question asks for biological molecules in the crop, so nitrate and ammonium ions don't count.

Question 14

A level Paper 3 · June 2017 · Q03.2 · 2 marks

Use a two-axis graph to calculate the change in grain produced per hectare between 1970 and 2005.

Figure 4: graph for India, 1970 to 2005, with a line for kg of artificial fertiliser used per hectare per year (left axis, rising from about 14 to 75) and bars for the fertiliser response ratio, kg grain per kg fertiliser (right axis, falling from about 12.4 to 3.8). Question: use these data to calculate the difference in the mass of grain produced per hectare in 1970 compared with 2005, showing working, 2 marks.
Show the mark scheme Mark scheme: an answer from 90 to 133.2 for 2 marks; 1 mark for a correct yield for 1970 (170.8 to 176.4) or 2005 (266.4 to 304.0).

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From the examiners' report: About 30% got an answer in range. Multiply fertiliser used by the response ratio for each year, reading each from its own axis, then subtract.

Question 15

A level Paper 3 · June 2017 · Q03.3 · 2 marks · Hard

Use the graph to evaluate the use of artificial fertilisers on grain crops in India.

Question text: use the data in Figure 4, fertiliser used per hectare and fertiliser response ratio in India from 1970 to 2005, to evaluate the use of artificial fertilisers on grain-producing crops in India, 2 marks.
Show the mark scheme Mark scheme, any two: using more fertiliser but getting less response over time; the graph shows correlation but doesn't prove yield changes are due to fertiliser, as there could be other factors; it becomes less cost-effective over time.

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From the examiners' report: Many read the graph as different amounts of fertiliser at the same time and wrote 'more fertiliser produces less crops'. The x-axis is years: describe change over time.

Question 16

A level Paper 2 · November 2020 · Q07.2 · 5 marks · Medium

Evaluate the effect on spinach growth of adding three fertilisers to the soil.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Figure 4: line graph of mean spinach biomass per pot (g) after 20 days against mass of fertiliser added (0 to 60 g) for potassium nitrate (rising to about 4.9 at 30 g, then level), ammonium sulfate (about 3.7 at 30 g, rising slowly to 4.0), chicken manure (about 1.6 until 30 g, peaking at 3.3 at 45 g, then 2.4) and a control line at 1.5. Question: using all the information, evaluate the effect on plant growth of adding the different fertilisers, 5 marks.
Show the mark scheme Mark scheme, any five: potassium nitrate most effective and chicken manure least; all more effective than the control; no increase with potassium nitrate above 30 g; small gradual increase with ammonium sulfate above 30 g; chicken manure less effective above 45 g; fertilisers provide nitrogen for protein; no statistical test or SDs; only spinach.

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From the examiners' report: 92% got a mark but only 6% got five. Use precise wording, 'no increase above 30 g' rather than 'no effect at 30 g', then add the limits: no statistical test and only spinach.

Question 17

A level Paper 2 · November 2021 · Q10.3 · 4 marks · Hard

Explain why loss of nutrients into Lake Malawi reduced some fish populations.

Information from earlier in this question:

Earlier part of the same exam question, giving the information this part uses. Passage: Lake Malawi has more species of fish than any other lake; forest cover in Malawi fell from 41.4% in 1990 to 26.4% in 2016; deforestation and farming along the shores have caused increased soil erosion and loss of nutrients into the lake, resulting in a decrease in some fish populations. Question: loss of nutrients into Lake Malawi has resulted in a decrease in some fish populations (lines 12–13); explain why, 4 marks.
Show the mark scheme Mark scheme: growth of algae or an algal bloom blocks light; less photosynthesis so submerged plants die; saprobionts respire aerobically, using oxygen; less oxygen for fish to respire. Decomposer is neutral.

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From the examiners' report: Half misread 'into' and wrote about nutrients lost from the lake. Extra nutrients mean eutrophication: algae block light, plants die, saprobionts respire and use up the oxygen.

Question 18

A level Paper 3 · June 2019 · Q02.2 · 3 marks · Medium

Describe how to measure the cloudiness of water from three rivers quantitatively.

Question text: one environmental issue arising from fertiliser use is eutrophication, which can make water cloudy; you are given samples of water from three rivers; describe how you would obtain a quantitative measurement of their cloudiness, 3 marks.
Show the mark scheme Mark scheme: use a colorimeter (reject calorimeter); measure the absorbance or transmission of light; a way to standardise the method, such as the same volume, zeroing the colorimeter, the same wavelength or shaking the sample. Ignore calibration curves.

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From the examiners' report: Only about 15% scored nothing. Spell colorimeter correctly, say you measure absorbance, and skip calibration curves and Benedict's, which this question doesn't need.

Where students lose marks (from AQA examiners' reports and mark schemes)

Each point below is followed by wording that gained marks.

  • Name the nitrogen compound that saprobionts decompose: In 'describe the role of saprobionts in the nitrogen cycle', very few named a nitrogen-containing compound, and some confused ammonification with nitrogen fixation. On the rainforest question most responses missed the same mark point.

    ✔ Gains credit: "(They use enzymes to) decompose proteins/DNA/RNA/urea"; "Producing/releasing ammonia/ammonium compounds/ammonium ions." 'Nitrogen-containing compounds' unqualified is ignored.

  • Nitrite comes before nitrate: Less than 20% got all four marks on the roles of bacteria in recycling. A significant number said ammonia was converted into nitrates and then nitrites, some described nitrification as reduction, and many confused nitrifying with nitrogen-fixing bacteria.

    ✔ Gains credit: "Ammonium/ammonia into nitrite and (then) into nitrate"; "(By) nitrifying bacteria/microorganisms."

  • Link each abiotic factor to enzymes or respiration: Less than 10% got both marks for two abiotic factors that speed up recycling in rainforests. Temperature was often linked to 'faster decomposition' rather than enzymes, and water to waterlogging or photosynthesis.

    ✔ Gains credit: "(High/optimum) temperature for enzymes/respiration/nitrification/ammonification"; "Water/moisture for metabolic/enzyme reactions OR Water for hydrolysis."

  • Biological molecules, not nitrate ions: Just over half got both marks for two nitrogen-containing biological molecules removed at harvest. Those who didn't usually gave inorganic ions, and on the sundew question nitrate and ammonium as products of digestion weren't credited.

    ✔ Gains credit: "amino acid/protein/polypeptide/peptide"; "DNA"; "RNA"; "ATP/ADP"; "Chlorophyll" (nitrates, nitrites, ammonia, ammonium and urea rejected).

  • The fungus gets carbohydrate from the plant: Almost 60% got this. Incorrect answers often said the fungus gains nutrients, water or oxygen from the plant.

    ✔ Gains credit: "Example of a carbon-containing biological compound e.g. carbohydrate/amino acid/vitamin" (sugars or organic compounds accepted).

  • Saprobionts use up the oxygen by respiring: 50% misread 'loss of nutrients into' Lake Malawi and scored nothing. Only 6% got full marks by including saprobionts using oxygen in respiration; some only said 'bacteria' or 'decomposers', and many left out respiration.

    ✔ Gains credit: "Saprobiotic (microorganisms) use oxygen in respiration"; "Less oxygen for fish to respire" (decomposer is neutral).

  • Ammonium chloride isn't ammonia: 92% got at least one mark, but only 8% got all three, by noting that only one species was used or that chloride ions might cause the inhibition. 'No statistical test' and 'correlation does not mean causation' weren't credited.

    ✔ Gains credit: "Only used one species"; "(Inhibition/results) may be due to chloride (ions) OR (Investigation) uses ammonium chloride not ammonia."

  • The fertiliser data run over time, not dose: Very many students read Figure 4 as crops given different amounts of fertiliser at the same time, giving statements such as 'more fertiliser produces less crops', instead of changes over time.

    ✔ Gains credit: "Using more but getting less response over time"; "The graph shows correlation but doesn't prove changes in yield due to fertiliser."

Download the questions

Download the PDF pack (18 questions, mark schemes at the end)

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Questions, mark schemes and examiners' report extracts © AQA.

Tom Whitburn
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