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

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.5.4 Nutrient cycles of the AQA AS and A-level Biology specification (7402 is the A-level), for first teaching 2015. This section is A-level only. Values such as relative atomic masses are given in the question. Learn the content in the study guide and recall it with the revision notes. Course hub: AQA A-level Biology. Checklist: AQA A-level Biology checklist.

Questions

1. Describe how saprobionts bring about the decomposition of a dead leaf, and state what is released into the soil as a result. [4]

2. Pine seedlings were grown in the same soil. The roots of some were infected with a mycorrhizal fungus. The mean mass of phosphorus taken up per seedling was 5.4 mg with the fungus and 3.0 mg without.

(a) Calculate the percentage increase in phosphorus uptake caused by the fungus. [2] (b) Explain how the fungus increases the uptake of phosphate and water by the seedling. [2]

3. Name the process described in each case.

(a) Nitrogen gas converted into ammonium ions by bacteria in swellings on the roots of a pea plant. [1] (b) Urea in urine converted into ammonium ions by saprobionts. [1] (c) Ammonium ions converted into nitrate ions by bacteria in aerated soil. [1] (d) Nitrate ions converted into nitrogen gas in a waterlogged field. [1]

4. After flooding, a farmer found that the soil in one field contained much less nitrate than before. Explain why. [3]

5. The table shows measurements from a lake near a farm, taken from May to August.

Month	Nitrate (mg dm⁻³)	Algae (cells per cm³)	Dissolved oxygen (mg dm⁻³)
May	3.2	40	10.4
June	11.6	1480	9.8
July	7.5	620	5.2
August	3.0	90	3.1

(a) Calculate the percentage increase in nitrate from May to June. [2] (b) Explain the change in the number of algae between May and June. [2] (c) Explain why dissolved oxygen was lower in August than in June, even though algae numbers fell. [3]

6. Ammonium nitrate (NH₄NO₃) is used as an artificial fertiliser. Ar values: N = 14, H = 1, O = 16.

(a) Calculate the percentage of nitrogen by mass in ammonium nitrate. [2] (b) Calculate the mass of ammonium nitrate needed to supply 56 kg of nitrogen. [1] (c) A mixed fertiliser contains 15% nitrogen by mass. Calculate the mass of nitrogen in 200 kg of it. [1] (d) Explain why a crop needs nitrogen to be replaced after harvest. [1]

7. A farmer compared three treatments on plots of wheat. Each treatment had four plots. Yield is in tonnes per hectare.

Treatment	Plot 1	Plot 2	Plot 3	Plot 4
No fertiliser	4.4	4.0	4.2	4.6
Manure	6.2	5.8	6.0	6.4
Artificial fertiliser	7.1	6.9	7.3	6.7

(a) Calculate the mean yield for each treatment. [3] (b) Calculate the percentage increase in yield caused by the artificial fertiliser compared with no fertiliser. [1] (c) Suggest why manure gave a smaller increase than artificial fertiliser in the first season. [2]

8. The nitrogen flows for a field in one year are in kg ha⁻¹ year⁻¹. Inputs: fixation by bacteria 25; fertiliser added x. Outputs: harvested crop 72; leaching 21; denitrification 7. The soil store of nitrogen did not change.

(a) Calculate x. [2] (b) Calculate the percentage of the fertiliser nitrogen that was leached. [1] (c) State two ways in which the farmer could reduce leaching. [2]

9. Explain why heavy rain soon after spreading an artificial fertiliser is more likely to cause problems in a river than the same rain falling on a field that has not been fertilised. [4]

10. Compare the phosphorus cycle with the nitrogen cycle. [5]

11. A farmer plans to apply 135 kg of nitrogen per hectare to 8 hectares using a fertiliser that contains 27% nitrogen by mass.

(a) Calculate the mass of fertiliser needed. [2] (b) The farmer is advised to grow a crop of peas in alternate years. Explain how this could reduce the use of artificial fertiliser. [3] (c) Evaluate the use of natural and artificial fertilisers on this farm, referring to the effects on a nearby lake. [4]

Answers

1. Saprobionts secrete digestive enzymes onto the leaf [1]; the enzymes hydrolyse large insoluble molecules, such as proteins, to small soluble ones [1]; the saprobionts absorb the soluble products [1]; ammonium ions and phosphate ions are released into the soil, as well as carbon dioxide from respiration [1]. Examiner insight: “Extracellular” digestion and absorption are two separate steps, and answers that mention only one are usually incomplete. Name the ions released rather than writing “nutrients”.

2. (a) Increase = 5.4 − 3.0 = 2.4 mg [1]; 2.4 ÷ 3.0 × 100 = 80% [1]. (b) The hyphae are very thin and spread through a large volume of soil, so they increase the surface area for absorption [1]; they supply more water and phosphate ions to the plant, including ions at low concentration that the roots alone cannot reach [1]. Examiner insight: A percentage change is divided by the original value, the one without the fungus. For the explanation, link the structure (hyphae) to a function (surface area, reach), not just “helps absorb”.

3. (a) Nitrogen fixation [1]. (b) Ammonification [1]. (c) Nitrification [1]. (d) Denitrification [1]. Examiner insight: Give the process name asked for. Describing what happens without naming it does not answer “name”. Spell the terms correctly, because nitrification and denitrification are easy to confuse.

4. The flooded soil was waterlogged and anaerobic [1]; denitrifying bacteria converted nitrate to nitrogen gas [1]; some nitrate may also have been leached out in the draining water [1]. Examiner insight: “Explain” needs the condition (anaerobic) and the process, in that order. Say nitrate is converted to nitrogen gas rather than “lost”.

5. (a) Increase = 11.6 − 3.2 = 8.4 [1]; 8.4 ÷ 3.2 × 100 = 262.5, which is 263% to three significant figures [1]. (b) Algae rose from 40 to 1480 cells per cm³ [1]; the extra nitrate (and phosphate) removed the mineral limit on growth, so the algae reproduced rapidly [1]. (c) Dead algae and plants were decomposed by saprobionts [1]; the saprobionts multiplied and used oxygen in aerobic respiration [1]; the algae were no longer producing as much oxygen by photosynthesis, so oxygen was used faster than it was replaced [1]. Examiner insight: Quote the units and take the figures from the table with the sign of the change. In part (c), the saprobionts are the main cause. An answer that blames only the algae “using up oxygen” misses the point.

6. (a) Mr = 14 + 4 + 14 + 48 = 80 [1]; nitrogen = 28 ÷ 80 × 100 = 35% [1]. (b) 56 ÷ 0.35 = 160 kg [1]. (c) 200 × 0.15 = 30 kg [1]. (d) Harvest removes the nitrogen in the plants, which is not returned through death and decay [1]. Examiner insight: Show the Mr and the nitrogen mass within it, so that each stage of your working is visible. Both nitrogen atoms count, not one.

7. (a) No fertiliser: 17.2 ÷ 4 = 4.3 [1]; manure: 24.4 ÷ 4 = 6.1 [1]; artificial: 28.0 ÷ 4 = 7.0 [1]. (b) (7.0 − 4.3) ÷ 4.3 × 100 = 63% [1]. (c) Manure is organic, so saprobionts must decompose it before nitrate and phosphate ions are released [1]; artificial fertiliser is already soluble ions that dissolve at once, so the crop can use them sooner [1]. Examiner insight: Write the unit with each mean (tonnes per hectare). In part (c), a bare “manure is slower” is weaker than saying why it is slower.

8. (a) Inputs = outputs = 72 + 21 + 7 = 100 [1]; x = 100 − 25 = 75 kg ha⁻¹ year⁻¹ [1]. (b) 21 ÷ 75 × 100 = 28% [1]. (c) Any two: apply smaller amounts that match crop needs; apply when the crop is growing and rain is unlikely; use slow-release or natural fertiliser; keep ground covered with plants or a strip of vegetation by the drainage ditch [2]. Examiner insight: A balanced budget means inputs equal outputs, so state this equation first. Suggestions must be specific actions, and a vague “use less fertiliser” is weaker than a link to timing or crop need.

9. The artificial fertiliser is made of soluble nitrate and phosphate ions [1]; the rain dissolves them and drains through the soil, a process called leaching [1]; the ions are carried into the river [1]; the unfertilised field has little extra nitrate to leach, so the river would receive far fewer ions, and eutrophication would be less likely [1]. Examiner insight: Use the term leaching and name the ions. A good answer ends with the consequence for the river, since the question is about problems.

10. Any five from: both cycles include uptake by plants and transfer along food chains [1]; in both, saprobionts decompose dead matter and release ions (ammonium, phosphate) [1]; the nitrogen cycle has an atmospheric store of nitrogen gas, but the phosphorus cycle has no gas phase [1]; the nitrogen cycle includes nitrogen fixation and denitrification by bacteria, and phosphorus has no equivalent [1]; phosphate comes mainly from weathering of rocks and, once lost to sea sediment, returns only over a very long time [1]. Examiner insight: A comparison needs a similarity and a difference, stated for both cycles in the same answer. Write both sides of each difference.

11. (a) Nitrogen needed = 135 × 8 = 1080 kg [1]; fertiliser = 1080 ÷ 0.27 = 4000 kg [1]. (b) Peas are legumes with nitrogen-fixing bacteria in root nodules [1]; the bacteria convert nitrogen gas to ammonium, which is nitrified to nitrate and taken up by the next crop when the plant residues decay [1]; so less nitrate needs to be added as fertiliser [1]. (c) Natural fertiliser: adds organic matter and releases ions slowly, so less leaching [1]; but its composition is variable and release may not match the needs of the crop [1]; artificial fertiliser: gives a quick, known supply, but if excess nitrate and phosphate are leached into the lake, eutrophication follows [1]; a reasoned judgement, such as using the right amount at the right time or a mix of both, to protect the lake and keep yield [1]. Examiner insight: For “evaluate”, weigh a benefit against a drawback for each option, then end with a conclusion that the evidence supports. Listing facts without a judgement is not an evaluation.

Where marks are usually lost

• Using “nutrients” or “minerals” when the question asks for named ions such as nitrate, ammonium or phosphate.
• Confusing ammonification, nitrification, nitrogen fixation and denitrification, and not naming the product of each.
• Forgetting that nitrification needs oxygen and denitrification occurs without it.
• Explaining the oxygen fall in eutrophication with algae only, and not with the aerobic respiration of saprobionts.
• Calculating a percentage change with the new value as the divisor.
• Stating that fertiliser “causes” eutrophication without mentioning leaching.
• Treating all hyphae as roots, or not linking mycorrhizae to surface area.
• Writing a one-sided answer to an evaluation question.

Next steps

• Review the revision notes and re-read the study guide.
• Course hub: AQA A-level Biology. Checklist: AQA A-level Biology checklist.
• Use the free diagnostics to pick your next topic.
• Book a free trial class.

Official syllabus

These questions 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.