A student investigates the rate of decomposition of dead leaves in two different compost heaps. One compost heap is kept moist and regularly turned to allow air in. The other compost heap is waterlogged and never turned. Explain why the rate of decomposition differs between the two compost heaps.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Decomposition is the process of breaking down dead material into simple organic matter. Decomposers, such as microorganisms, carry out this process. The rate of decomposition is influenced by water availability, oxygen availability and temperature.
Model answer (5 marks)
The moist, turned heap supplies sufficient oxygen, so decomposers respire aerobically and release more energy, breaking down the leaves quickly. The waterlogged heap lacks oxygen, so decomposition is anaerobic, slower and less efficient. Hence the moist, turned heap decomposes faster than the waterlogged heap.
Examiner tips
- Use the word ‘aerobic’ and ‘anaerobic’ to show understanding of oxygen availability.
- Explain how energy release differs between aerobic and anaerobic respiration.
- Link the faster decomposition to the higher energy yield of aerobic processes.
- Mention turning as a way to introduce air.
Common mistakes
- Confusing ‘aerobic’ with ‘anaerobic’ or vice‑versa.
- Failing to mention that turning provides air and therefore oxygen.
- Giving a generic explanation of decomposition without linking it to oxygen availability and energy release.
Mark scheme (5 marks)
- The moist, turned compost heap has sufficient oxygen available (for aerobic decomposition)
- Decomposers / microorganisms respire aerobically in the moist, turned heap, releasing more energy to break down dead material faster
- The waterlogged compost heap has little or no oxygen available (anaerobic conditions)
- Anaerobic decomposition is slower / less efficient than aerobic decomposition
- Therefore the moist, turned compost heap decomposes faster / produces compost more quickly than the waterlogged heap
Key terms in this question
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