17.4 Rates of Decomposition

A free GCSE Biology lesson for AQA (8461) from OBA School: how temperature, water and oxygen affect decay; compost and biogas. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain how temperature, water and oxygen affect the rate of decay
• Calculate rates of decay from changes in mass
• Explain how gardeners and farmers make compost quickly
• Describe how anaerobic decay in a biogas generator produces methane fuel
• Evaluate the recycling of kitchen and garden waste

Key words

• Rate of decay: How quickly dead material is broken down, e.g. mass lost per day or per week.
• Aerobic: Using oxygen. Aerobic decay is fast and releases carbon dioxide.
• Anaerobic: Without oxygen. Anaerobic decay is slower and produces methane.
• Compost: Decayed plant material used as a natural fertiliser for growing plants.
• Biogas: A fuel gas, mainly methane, made by anaerobic decay of waste in a biogas generator.
• Optimum: The condition (such as temperature) at which a process is fastest.

Factors that affect decay

Decomposers (bacteria and fungi) digest dead material with enzymes, then respire. Anything that affects enzymes, growth or respiration of the decomposers affects the rate of decay.

• Temperature; Effect on decay Faster when warm, up to an optimum. Very slow when cold; slows or stops when too hot.; Why Enzymes work faster as temperature rises; above the optimum they denature. Freezing stops decomposers growing.
• Water; Effect on decay Faster when moist. Very slow in dry conditions.; Why Decomposers need water for enzyme reactions, to absorb dissolved food and to grow.
• Oxygen; Effect on decay Faster when oxygen is available.; Why Most decomposers respire aerobically. Without oxygen, decay is slow and anaerobic.

Link: We use these factors to stop food decaying: fridges and freezers (cold), drying (no water), vacuum packing and canning (no oxygen, no microorganisms).

Calculating rates of decay

Worked example: mass of grass clippings

A bag of grass clippings has a mass of 200 g. After 4 weeks in a compost heap its mass is 148 g.
Mass lost = 200 − 148 = 52 g.
Rate of decay = 52 g ÷ 4 weeks = 13 g per week.
Percentage lost = 52 ÷ 200 × 100 = 26%.

Answer: 13 g per week (26% of the mass in 4 weeks)

Exam tip: Rate = change ÷ time. Always give units, such as g per week or g/day. To compare samples with different starting masses, use percentage change.

Making compost

Gardeners and farmers try to provide the best conditions for decay so they can make compost quickly. Compost returns mineral ions to the soil as a natural fertiliser.

• Chop or shred waste to give a larger surface area for decomposers.
• Keep it moist but not waterlogged.
• Turn the heap or use bins with air holes so oxygen can get in.
• Keep it warm: dark bins and large heaps hold the heat released by decomposers’ respiration.
• Add some soil or finished compost to supply decomposers.

Biogas generators

Anaerobic decay produces methane gas, which can be used as a fuel. In a biogas generator, animal manure, food waste and plant waste are sealed in a tank without oxygen. Anaerobic bacteria break it down.

• Biogas is mainly methane with some carbon dioxide. It is burned for cooking, heating or to generate electricity.
• It works best at about 30–40 °C, so generators are often buried or insulated to keep warm.
• The leftover digestate is a good fertiliser.
• Advantages: uses waste, renewable fuel, reduces methane escaping from rotting waste. Disadvantages: gas production slows in cold weather; needs a steady supply of waste.

Common mistakes

• Saying decomposers are “killed” by cold. Low temperatures slow their enzymes; they are only killed or denatured at high temperatures.
• Saying anaerobic decay produces carbon dioxide only. The key product for biogas is methane.
• Forgetting units when giving a rate of decay.
• Saying compost should be sealed. Aerobic composting needs oxygen, so it should be turned or aerated.

Revision questions

Try these, then check your answers below each one.

Question 1. Which three factors affect the rate of decay? (Choose from: Temperature, water and oxygen, Light, carbon dioxide and temperature, Wind, light and water, Oxygen, light and salt.)

Answer: Temperature, water and oxygen

Working: Temperature, water (moisture) and oxygen availability.

Question 2. True or false? Decay is fastest at very high temperatures, such as 80 °C.

Answer: False

Working: False. Above the optimum, enzymes in decomposers denature, so decay slows or stops.

Question 3. Why do gardeners turn their compost heaps? (Choose from: To let in oxygen so decomposers can respire aerobically, To cool the heap down to stop decay, To remove the decomposers, To let in light for photosynthesis.)

Answer: To let in oxygen so decomposers can respire aerobically

Working: Turning adds oxygen, which speeds up aerobic decay.

Related lessons

• Previous lesson: 17.3 The Carbon Cycle
• Next lesson: 17.5 Required Practicals: Organising an Ecosystem
• In this chapter: 17.1 Feeding Relationships
• In this chapter: 17.2 Materials Cycling

Continue in the full course

This lesson is section 17.4 (AGCBH1704) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.