A farmer wants to improve the fertility of their soil by making compost from dead plant material. Explain why the rate of decomposition in the compost heap is affected by water availability, oxygen availability and temperature.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Decomposers such as bacteria and fungi break down dead plant material in a compost heap. The farmer notices that compost made in warm, moist and well-aerated conditions is produced much more quickly than compost made in cold, waterlogged or dry conditions.
Model answer (5 marks)
Decomposers need water to survive and to carry out enzyme‑controlled reactions, so a compost heap with good moisture supplies the water required for microbial activity and increases the rate of decomposition.
In a waterlogged heap the excess water displaces air, reducing the amount of oxygen available. Without sufficient oxygen the decomposers must switch to anaerobic metabolism, which is slower and produces less energy, so the decomposition rate falls.
When oxygen is plentiful, aerobic decomposition can occur. Aerobic reactions are more efficient and produce more energy, allowing the microbes to work faster and the material to break down more quickly.
Temperature also controls the rate of decomposition. At higher temperatures the enzymes in decomposer cells work more rapidly, increasing microbial activity and the speed of breakdown.
Conversely, cold temperatures reduce enzyme activity and microbial metabolism, so decomposition proceeds more slowly.
In a waterlogged heap the excess water displaces air, reducing the amount of oxygen available. Without sufficient oxygen the decomposers must switch to anaerobic metabolism, which is slower and produces less energy, so the decomposition rate falls.
When oxygen is plentiful, aerobic decomposition can occur. Aerobic reactions are more efficient and produce more energy, allowing the microbes to work faster and the material to break down more quickly.
Temperature also controls the rate of decomposition. At higher temperatures the enzymes in decomposer cells work more rapidly, increasing microbial activity and the speed of breakdown.
Conversely, cold temperatures reduce enzyme activity and microbial metabolism, so decomposition proceeds more slowly.
Examiner tips
- Use the exact terms: water availability, oxygen availability, aerobic/anaerobic, temperature effects on enzymes.
- Show the cause–effect chain: water → enzyme activity; oxygen → aerobic vs anaerobic; temperature → enzyme kinetics.
- Keep each point brief but complete; use bullet‑style sentences if allowed.
Common mistakes
- Confusing water with oxygen – remember water is needed for enzymes, oxygen for aerobic metabolism.
- Saying ‘more water always speeds up decomposition’ without noting that too much water can create anaerobic conditions.
- Ignoring the temperature effect or stating it in vague terms like ‘hot is better’ without linking to enzyme activity.
Mark scheme (5 marks)
- Decomposers need water to survive / carry out enzyme-controlled reactions, so greater water availability increases decomposition rate
- Waterlogged conditions reduce oxygen availability, leading to anaerobic decomposition which is slower than aerobic decomposition
- Good oxygen availability allows aerobic decomposition, which is faster / more efficient than anaerobic decomposition
- Higher temperature increases the rate of decomposition because decomposers are more active / enzyme-controlled reactions occur faster
- Cold temperatures slow decomposition because decomposers / enzymes are less active at low temperatures
Key terms in this question
Related
- All OCR A-Level Biology B — Advancing Biology (H422) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Energy budgets and ecosystems questions
- A peat bog is a waterlogged ecosystem where dead plant material accumulates over…
- A group of students is investigating the decomposition of leaf litter in two for…
- A marine ecosystem contains phytoplankton (producers), krill (primary consumers)…
- A wildlife organisation is investigating the carbon cycle in a woodland ecosyste…