# A compost heap contains kitchen waste such as vegetable peelings and fruit scraps. A gardener notices that the compost heap at the bottom, which is several months old, has been fully broken down into a dark, crumbly material, whilst the freshly added material at the top has not yet decomposed. Explain why the rate of decomposition is greater in the lower part of the compost heap than in the freshly added material at the top.

> Pearson Edexcel A-Level Biology A (9BI0) — 5.5 Decomposition and decay · Explain · 5 marks

> Compost heaps are used by gardeners to recycle organic waste. Decomposers, including bacteria and fungi, break down dead organic material. The lower layers of a compost heap are older and more compact, whilst the top layer consists of freshly added material exposed to the open air.

## Mark scheme (5 marks)

1. Decomposers (bacteria and/or fungi) break down the organic material by secreting enzymes
2. The lower layer has had longer to be acted upon by decomposers, so more decomposition has occurred over time
3. The lower layer is warmer due to heat released by microbial respiration, increasing the rate of enzyme activity
4. The lower layer has less oxygen / is more anaerobic because it is compacted, whereas the top layer has more oxygen available
5. Moisture / water is retained in the lower layer, providing suitable conditions for decomposer activity

## Key terms

- [decomposition](https://www.gradenine.co.uk/glossary/decomposition)

## Related

- [Revision notes for Pearson Edexcel A-Level Biology A (9BI0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-compost-heap-contains-kitchen-waste-233adebf) · Published by Druglandscape Ltd.