A group of students is investigating the decomposition of leaf litter in two forest areas. In one area, the soil is well-drained and rich in oxygen. In the other area, the soil is waterlogged. Explain why the rate of decomposition differs between the two areas, and describe what happens to the carbon stored in the leaf litter in each area.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Leaf litter is dead plant material that falls to the forest floor and is broken down by decomposers. The conditions in the soil affect how quickly decomposition occurs and which gases are released.
Model answer (5 marks)
In the well‑drained area the soil contains enough oxygen for aerobic decomposers to work. Aerobic decomposition is fast and produces mainly CO₂, which is released into the atmosphere. In the waterlogged area the soil is oxygen‑poor, so decomposers must use anaerobic pathways. Anaerobic decomposition is slower and produces CH₄ (and some CO₂), which is released. Because the anaerobic process is less efficient, carbon remains stored in the leaf litter for a longer time.
Examiner tips
- Use the terms aerobic and anaerobic decomposition; link oxygen availability to rate and gas produced.
- Show the sequence: oxygen → aerobic → CO₂; no oxygen → anaerobic → CH₄/CO₂ and slower rate.
- Mention that carbon accumulates in the litter in the waterlogged site.
Common mistakes
- Confusing the gases released (e.g. saying only CO₂ in both sites).
- Forgetting that the anaerobic process is slower and leads to carbon storage.
Mark scheme (5 marks)
- The well-drained soil has sufficient oxygen, allowing aerobic decomposition to occur.
- Aerobic decomposition produces carbon dioxide, which is released into the atmosphere.
- The waterlogged soil lacks oxygen, so anaerobic decomposition occurs.
- Anaerobic decomposition produces methane (and carbon dioxide), which is released.
- The rate of decomposition is slower in the waterlogged soil because decomposers work less efficiently without oxygen, so carbon accumulates / is stored in the leaf litter.
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
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