Decomposition — Ecology & Food Chains | IGCSE Biology

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 12% of your exam marks.

stable

High

Stable12%

Food chains, energy transfer, and ecological definitions are regularly tested, often as short-answer questions.

Decomposition is the breakdown of dead organic matter and waste by decomposer organisms (mainly bacteria and fungi). Decomposition is what recycles the carbon and nitrogen tied up in dead bodies back into the soil and atmosphere, where they can be used by new organisms.

Without decomposers:

Dead bodies would pile up
The minerals (especially nitrogen) inside them would never be released back to the soil
Plants would run out of nitrate within a few years and growth would collapse
The entire ecosystem would grind to a halt within a generation

Factors that affect the rate of decomposition

Like all enzyme-controlled processes (decomposers digest their food using extracellular enzymes), decomposition is sensitive to environmental conditions:

Factor	Effect	Why
Temperature	Optimum around 25 °C. Faster as temperature rises towards this; slower in cold; stops below freezing. Denatures decomposer enzymes above ~45 °C	Higher kinetic energy speeds up enzyme-controlled digestion. Outside the optimum range enzymes work slowly or denature
Water availability	Faster in moist conditions	Decomposers need water for their enzymes to work, and for soluble products to be absorbed. Bone-dry conditions stop decomposition. Mummified bodies in deserts can stay intact for thousands of years
Oxygen availability	Faster with oxygen present	Most decomposers respire aerobically. In waterlogged or compacted soils, decomposition is slow and incomplete. This is how peat bogs preserve dead bodies for thousands of years
pH	Optimum near pH 7 (neutral)	Decomposer enzymes work best at neutral pH. Acidic soils (e.g. peat) slow decomposition
Decomposer abundance	More decomposers = faster rate	A compost heap with plenty of bacteria and fungi works much faster than a pile of leaves on a forest floor

Designing an investigation of decomposition

A typical experiment to investigate factors affecting decomposition rate:

Choose an organic substance to decompose (leaf litter, apple slices, slices of bread, cotton fabric). Make sure each replicate has the same starting mass.
Set up several samples in identical containers, each with different conditions for the variable being tested:
- Different temperatures (fridge at 4 °C, room temperature at 20 °C, incubator at 30 °C)
- Different moisture levels (dry, damp, soaked)
- Different oxygen levels (open container vs sealed container, optionally with water cover to exclude air)
Leave for a fixed time (a few days for fast-decomposing substances, longer for slower ones).
Re-weigh each sample. The greater the mass loss, the faster the decomposition.
Repeat several times and take a mean to reduce error.

Applying CORMS

For an investigation into the effect of temperature on decomposition:

C (change): the temperature of incubation
O (organism): same species and amount of dead organic matter
R (repeat): at least three replicates at each temperature
M (measure): the change in dry mass of the sample after a fixed time
S (same): control moisture, light, container size, and how the samples are handled

Real-world applications

Compost heaps are designed to maximise decomposition: garden waste is mixed with kitchen scraps, kept moist, and turned regularly to introduce oxygen. A well-managed heap reaches 60 °C and breaks down material in weeks.
Sewage treatment uses controlled decomposition by microbes in aeration tanks to break down organic waste into safe products.
Refrigeration slows decomposition by lowering temperature, which is why food keeps longer in a fridge.
Freezing stops decomposition entirely (water is needed for enzymes; ice is not available water).
Drying and salting stop decomposition by removing water. This is why beef jerky and salted fish keep without refrigeration.