Explain how water content and oxygen availability affect the rate of decomposition of dead plant material in a compost heap.

Edexcel GCSE Biology (1BI0) — 9.3 Material cycles · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

A gardener adds dead plant material to a compost heap. Over several weeks, the material breaks down into compost that can be used as a fertiliser.

Model answer (4 marks)

Higher water content increases the rate of decomposition because decomposers need water for survival and to carry out enzyme‑catalysed reactions.
Higher oxygen availability increases the rate of decomposition because decomposers use oxygen for aerobic respiration, which releases energy to break down dead material.

Examiner tips

Use the exact phrases ‘higher water content’ and ‘higher oxygen availability’. Explain the reason for each using the words ‘decomposers’, ‘microorganisms’, ‘enzyme‑catalysed reactions’ and ‘aerobic respiration’.
Show the cause–effect relationship clearly – water → survival/enzyme activity, oxygen → energy release.

Common mistakes

Confusing water with moisture or humidity; students say ‘more moisture’ instead of ‘higher water content’. Using ‘oxygen’ but not linking it to aerobic respiration. Omitting the role of decomposers/microorganisms in the explanation.

Mark scheme (4 marks)

Higher water content increases the rate of decomposition
Because decomposers / microorganisms need water to survive / carry out enzyme-catalysed reactions
Higher oxygen availability increases the rate of decomposition
Because decomposers / microorganisms use oxygen for aerobic respiration to release energy to break down dead material

Key terms in this question

decomposition · water content · oxygen availability