OxfordAQA International AS and A-level Biology: Nutrient cycles: carbon and nitrogen (9610) – 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 3.3.5 Nutrient cycles (3.3.5.1 to 3.3.5.3) of the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1. The section is in Unit 3, which the specification lists as International A-level only. Relative atomic masses are given in the questions. Learn the content in the study guide and recall it with the revision notes. Course hub: OxfordAQA A-level Biology. Checklist: OxfordAQA A-level Biology checklist.

Questions

1. A fallen branch is colonised by a fungus that feeds as a saprophyte. Describe how the fungus obtains its food, and state what is released that producers can take up. [4]

2. Explain how the nitrogen in a plant protein becomes part of the protein of a caterpillar that eats the leaf. Use the terms digestion and assimilation. [3]

3. The table shows carbon dioxide concentration (ppm) at a station in the northern hemisphere.

Month	Year 1	Year 2
January	422.0	424.6
April	424.4	427.0
July	421.5	424.1
October	419.9	422.5

(a) Calculate the difference between the highest and lowest values in year 1. [1] (b) Calculate the percentage increase in the January value from year 1 to year 2, to 2 significant figures. [2] (c) Explain the fall from April to October. [2] (d) Explain the rise from year 1 to year 2. [2]

4. Mean carbon dioxide concentrations at a site were 317 ppm in 1960, 339 ppm in 1980, 370 ppm in 2000 and 413 ppm in 2020.

(a) Calculate the mean rate of increase per year between 1980 and 2000. [2] (b) Describe the change in the rate of increase over the 60 years. [2] (c) Name two human activities that have contributed to the increase. [2]

5. Explain how carbon dioxide and methane enhance the greenhouse effect, and give one source of methane. [5]

6. A farm recorded the data below for two growing seasons.

	Cool season	Warm season
Mean temperature (°C)	15.3	17.1
Generations of an aphid pest	9	13
Mean wheat yield (t ha⁻¹)	7.8	6.9

(a) Calculate the percentage change in yield from the cool to the warm season. [2] (b) Explain why more generations of aphid occurred in the warm season. [3] (c) Evaluate whether the data show that climate change reduces yield. [2]

7. Name the process in each case and the type of bacteria involved.

(a) Ammonium ions converted to nitrite ions then nitrate ions. [2] (b) Nitrate ions converted to nitrogen gas in waterlogged soil. [2] (c) State the condition that decides which of these two processes occurs. [1]

8. In a pasture each year, nitrogen enters as fertiliser (90 kg ha⁻¹) and by fixation (38 kg ha⁻¹). Nitrogen leaves in milk and meat (64), by leaching (29) and by denitrification (11), all in kg ha⁻¹ year⁻¹.

(a) Calculate the change in the soil nitrogen store each year. [2] (b) Leaching removed some of the fertiliser nitrogen. Give this loss as a percentage of the fertiliser nitrogen, to 3 significant figures. [1] (c) Explain why the farmer must replace the nitrogen removed in milk and meat. [1]

9. Ammonium sulfate is (NH₄)₂SO₄. Ar values: N = 14, H = 1, S = 32, O = 16.

(a) Show that nitrogen makes up 21.2% of the mass of ammonium sulfate (3 significant figures). [2] (b) Calculate the mass of ammonium sulfate that contains 63 kg of nitrogen. [1] (c) Only 70% of the nitrogen applied is taken up by the crop. Calculate the mass of ammonium sulfate needed to supply 63 kg of nitrogen to the crop, to the nearest kg. [1]

10. After heavy rain, the nitrate concentration in a pond rose from 1.8 to 7.4 mg dm⁻³. Over the next month, algae first multiplied, then many died, and dissolved oxygen fell.

(a) Calculate the percentage increase in nitrate, to the nearest whole number. [2] (b) Explain how the nitrate reached the pond from a fertilised field. [2] (c) Explain why dissolved oxygen fell. [2]

11. A farmer plans to grow clover in alternate years instead of adding all the nitrogen as artificial fertiliser, and to spread manure on the fields. The farm drains to a lake.

Evaluate this plan, using your knowledge of the nitrogen cycle. [9]

Answers

1. The fungus secretes digestive enzymes onto the wood [1]; large insoluble molecules are hydrolysed to small soluble ones [1]; the fungus absorbs the soluble products [1]; respiration releases carbon dioxide and decay releases ammonium ions [1]. Examiner insight: Describe digestion outside the cell and absorption as two separate steps. Answers are stronger when they name products such as carbon dioxide and ammonium ions, and not just “nutrients”.

2. The caterpillar digests the protein, hydrolysing it to amino acids [1]; the amino acids are absorbed into its blood or body fluid [1]; assimilation: they are used to build the caterpillar’s own proteins [1]. Examiner insight: Keep the two terms apart. Digestion is breakdown and absorption, and assimilation is the building of the absorbed molecules into the organism.

3. (a) 424.4 − 419.9 = 4.5 ppm [1]. (b) Increase = 424.6 − 422.0 = 2.6 [1]; 2.6 ÷ 422.0 × 100 = 0.62% [1]. (c) In the growing season, photosynthesis by plants in leaf is faster than respiration [1]; so more carbon dioxide is removed than released [1]. (d) Burning of fossil fuels and wood releases carbon dioxide stored for a long time [1]; this adds more than photosynthesis and the oceans remove, so the average rises [1]. Examiner insight: Divide by the year 1 value, the starting value, for a percentage increase. For explanations, name both processes and say which one is faster.

4. (a) 370 − 339 = 31 ppm [1]; 31 ÷ 20 = 1.55 ppm per year [1]. (b) The rate rises: 1.1 ppm per year from 1960 to 1980, 1.55 from 1980 to 2000 and 2.15 from 2000 to 2020 [1]; so the increase is accelerating, with each period faster than the one before [1]. (c) Any two: burning fossil fuels; deforestation; burning wood [2]. Examiner insight: A good description quotes values with units. Writing “it goes up” without figures is a weak answer to a question on data.

5. Incoming short-wave radiation from the Sun mostly reaches the ground and heats it [1]; the surface emits longer-wave infrared radiation [1]; carbon dioxide and methane absorb some of it [1]; they re-radiate it, including back towards the surface, so the lower atmosphere warms [1]; any source of methane: cattle or sheep digestion, rice paddies, wetlands, landfill, leaks from natural gas supplies [1]. Examiner insight: Mention the change in wavelength of the radiation. Saying that gases “trap heat” without absorbing and re-radiating infrared is a vague answer.

6. (a) 6.9 − 7.8 = −0.9 [1]; −0.9 ÷ 7.8 × 100 = −12% (a fall of 12%) [1]. (b) Insects are ectotherms, so their enzymes work faster at higher temperature and they develop faster [1]; the life cycle is shorter [1]; so more generations fit into the same season [1]. (c) Only two seasons, so it is not enough to show a trend [1]; other variables, such as rainfall, disease or pests, may also differ, so the data show an association and not cause [1]. Examiner insight: Give a verdict and back it with a reason from the data. “The data show a link but do not prove cause” is correct only if you support it with sample size or uncontrolled variables.

7. (a) Nitrification [1]; nitrifying bacteria [1]. (b) Denitrification [1]; denitrifying bacteria [1]. (c) Whether oxygen is present: nitrification is aerobic and denitrification anaerobic [1]. Examiner insight: Spell the terms carefully, because nitrification and denitrification are easy to confuse and a reader cannot guess which you meant. Give the starting form and end form when you can.

8. (a) Input = 90 + 38 = 128 [1]; output = 64 + 29 + 11 = 104, so the store rises by 24 kg ha⁻¹ year⁻¹ [1]. (b) 29 ÷ 90 × 100 = 32.2% [1]. (c) The nitrogen in the milk and meat leaves the farm and is not returned through death and decay [1]. Examiner insight: Give units with the change and its direction. Divide leaching by the fertiliser input, not the total input.

9. (a) Mr = 2 × (14 + 4) + 32 + 64 = 132 [1]; 28 ÷ 132 × 100 = 21.2% [1]. (b) 63 ÷ (28 ÷ 132) = 297 kg [1]. (c) Nitrogen to apply = 63 ÷ 0.70 = 90 kg; 90 ÷ (28 ÷ 132) = 424.3, which is 424 kg [1]. Examiner insight: Count both nitrogen atoms in the formula unit. Show the Mr working so that a slip in the final figure can still be traced.

10. (a) 7.4 − 1.8 = 5.6 [1]; 5.6 ÷ 1.8 × 100 = 311% [1]. (b) The rain dissolved soluble nitrate from the fertiliser and carried it through the soil by leaching [1]; the drainage water ran into the pond [1]. (c) Dead algae and plants were decomposed by saprophytic bacteria, which multiplied [1]; their aerobic respiration used oxygen faster than it was replaced [1]. Examiner insight: Name leaching in part (b), and decomposer respiration in part (c). Blaming only the algae loses the second mark.

11. Credit any of the following points, up to nine in total. Clover is a legume with nitrogen-fixing bacteria in its root nodules [1]; the bacteria change nitrogen gas into ammonium, and nitrifying bacteria then produce nitrate for the following crop, so less artificial fertiliser is needed [1]; manure replaces nitrogen removed at harvest [1]; manure is organic, so saprophytic bacteria release ammonium slowly [1]; this reduces the risk of leaching compared with soluble artificial fertiliser [1]; manure has variable composition, so supply may not match crop need [1]; slow release may leave early growth short of nitrate [1]; heavy rain on manure or soil rich in nitrate can still cause leaching into the lake, and eutrophication follows [1]; a judgement supported by the points, such as that the plan reduces the risk if applications are timed with crop growth [1]. Examiner insight: Weigh both sides and finish with a conclusion that follows from the points. A list of facts with no judgement does not evaluate.

Where marks are usually lost

• Writing “nutrients” where nitrate, ammonium or carbon dioxide is wanted.
• Mixing up ammonification, nitrification, nitrogen fixation and denitrification.
• Forgetting that denitrification needs anaerobic conditions.
• Giving the oxygen fall in a lake as the work of algae only.
• Dividing a percentage change by the final value.
• Reading two data points as proof of cause.
• Missing the idea that insects are ectotherms in pest questions.
• Saying that gases “trap heat” without absorbing and re-radiating infrared.

Next steps

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

Official syllabus

These questions follow section 3.3.5 Nutrient cycles in the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, published by OxfordAQA.