9.18B Explain the effects of temperature, water content and oxygen…

The idea

Decomposition in composting is carried out mainly by microorganisms and detritivores that break down dead organic matter, releasing nutrients. The rate depends on three conditions. Temperature: warm conditions (but not too hot) increase microbial enzyme activity and so speed decomposition; very high temperatures can kill many microorganisms and slow the process, while cold slows it. Water content: microorganisms need moisture for growth and enzyme action, so damp material decomposes faster, but waterlogged conditions exclude oxygen and slow aerobic decay. Oxygen availability: most composting microorganisms are aerobic, so good aeration (turning the heap, mixing in air spaces) increases the rate; if oxygen is used up, anaerobic decay is slower and produces different, often smelly, products. Explain each factor by linking it to microbial activity and enzyme-controlled reactions.

What earns the marks

• States that decomposition is the breakdown of dead organic matter by microorganisms (and detritivores), releasing nutrients.
• Explains that increasing temperature (up to an optimum) increases microbial enzyme activity and so increases the rate of decomposition.
• Explains that very high temperatures can kill microorganisms and slow decomposition, and low temperatures slow enzyme activity.
• Explains that adequate water content is needed for microbial growth and enzyme action, so moist material decomposes faster than dry material.
• Explains that waterlogging reduces oxygen availability and so slows aerobic decomposition.
• Explains that most composting microorganisms are aerobic, so greater oxygen availability (e.g. turning the heap) increases the rate of decomposition.
• Explains that low oxygen leads to slower anaerobic decay with different products.

Common mistakes and how to put them right

• Thinking hotter is always faster.

  Put it right by noting there is an optimum temperature and very high temperatures can kill microorganisms.

• Assuming more water is always better.

  Put it right by explaining that waterlogging excludes oxygen and slows aerobic decay.

• Forgetting that composting microorganisms are mostly aerobic.

  Put it right by linking aeration and oxygen supply to a faster rate.

• Describing decomposition as purely physical breakdown.

  Put it right by emphasising microbial enzyme-controlled breakdown of organic matter.

Exam tips

• For each factor, state the direction of effect and the reason (link to enzyme activity or oxygen supply).
• Use the idea of an optimum for temperature rather than a simple 'more is better' rule.
• When discussing water, distinguish 'enough moisture' from 'waterlogged', and link waterlogging to reduced oxygen.