Decomposition

Where it happens

Decomposition happens wherever dead organisms, fallen leaves, faeces and other organic waste are found, and is carried out mainly by bacteria and fungi acting as decomposers.

The main sites are the soil surface and leaf litter of forests, the mud at the bottom of ponds and rivers, compost heaps, and the gut and skin of dead animals. Fungi dominate on tough plant material rich in cellulose and lignin, sending thread-like hyphae through the material; bacteria dominate on soft, protein-rich remains.

Detritivores such as earthworms, millipedes and woodlice are not decomposers in the strict sense, because they ingest solid food, but by shredding leaf litter they greatly increase the surface area available to bacteria and fungi.

Inputs and outputs

• Input: dead plants and animals, fallen leaves, faeces and urine, all rich in complex organic compounds such as cellulose, protein and lipid.
• Input: digestive enzymes secreted by bacteria and fungi, including cellulase, protease and lipase.
• Input: oxygen, used by aerobic decomposers to respire the substances they absorb.
• Input: water, needed for enzymes to work and for the soluble products to be absorbed.
• Output: carbon dioxide and water from decomposer respiration, plus heat, which is why a compost heap feels warm.
• Output: ammonia and other nitrogen compounds, mineral ions such as phosphate and potassium, and humus that improves soil structure.

The steps

1. An organism dies or releases waste, and the organic matter starts to be colonised by bacteria and the spores of fungi already present in soil, water and air.
2. Detritivores such as earthworms and millipedes shred and partly digest the material, increasing the surface area on which decomposers can act.
3. Bacteria and fungi secrete digestive enzymes onto the material: cellulase acts on plant cell walls, protease on protein and lipase on fat.
4. The enzymes hydrolyse the complex organic compounds outside the decomposer's body into simple soluble substances such as glucose, amino acids, fatty acids and glycerol.
5. The decomposer absorbs these soluble products across its cell wall and membrane, a mode of feeding called saprophytic nutrition.
6. The absorbed substances are used in aerobic respiration, releasing carbon dioxide, water and heat into the surroundings.
7. Nitrogen from amino acids is released as ammonia, which nitrifying bacteria in the soil convert to nitrate; phosphate and other mineral ions are returned to the soil water.
8. Producers absorb the released carbon dioxide by photosynthesis and the mineral ions through their roots, completing the nutrient cycle.

Factors affecting the rate

The rate of decomposition is not constant; 3 environmental factors speed it up or slow it down.

Why it matters and how it is controlled

Decomposition prevents the permanent loss of nutrients as organic matter accumulates. By breaking down dead organisms and waste, decomposers keep essential nutrients such as nitrogen and carbon cycling through an ecosystem, making them available again for producers.

The rate is controlled by the same factors that control any enzyme-driven process, because decomposition is carried out by enzymes. Warm, moist, well-aerated conditions with a near-neutral pH allow decomposers to grow and respire fastest, which is why leaf litter disappears within weeks in a tropical rainforest but persists for years in cold or waterlogged ground.

Where oxygen is absent, only anaerobic bacteria can act, decomposition is slow and incomplete, and material may be preserved as peat or eventually as fossil fuel.

Humans manage decomposition on purpose. Composting piles organic waste so that heat, moisture and air speed up decay and produce fertiliser.

Refrigeration, drying, salting and pickling preserve food by removing one of the conditions decomposers need: low temperature slows enzyme activity, drying removes water, and salt or acid creates an environment in which most decomposers cannot survive.

How it is examined

Decomposition is examined mainly through the nutrient cycles and through experiment-style items. In cycle diagrams, the arrow from dead organisms to soil nitrate or to atmospheric carbon dioxide passes through decomposers, and you are asked to name the organisms and the process.

Experiment items describe bread, leaves or fruit kept under different temperatures, moisture levels or oxygen availability and ask you to predict which sample decays fastest and explain why, using the ideas of enzyme activity and respiration. Essay items link decomposition to the greenhouse effect, food preservation or the role of microorganisms in the ecosystem, and marks depend on stating that decomposers release nutrients for reuse by producers.

Common misconceptions

Worked exam-style question

Question. A student placed equal masses of fresh bread in 4 sealed transparent containers. Container A was kept moist at room temperature, container B was kept moist in a refrigerator, container C was kept dry at room temperature, and container D was kept moist at room temperature after the bread was soaked in strong salt solution.

After 10 days, visible fungal growth was greatest in A and least in C and D. (a) Name the type of nutrition shown by the fungi on the bread.

(b) Explain why fungal growth was greater in A than in B. (c) Explain why very little growth occurred in C.

(d) Suggest why the bread in D did not decay, and relate this to a method of food preservation.

Model answer. (a) Saprophytic nutrition: the fungi secrete enzymes onto the bread, digest it externally and absorb the soluble products. (b) Container A was at a higher temperature, so the fungal enzymes worked faster and the fungi respired and grew faster.

In the refrigerator the low temperature slowed enzyme activity, so decomposition was slower. (c) The bread in C was dry.

Fungi need water for their enzymes to act and to absorb the dissolved products, and the spores need moisture to germinate, so growth was minimal. (d) The strong salt solution draws water out of fungal cells by osmosis, so the fungi cannot grow; this is the principle behind salting of fish and meat to preserve food.

Source:SRC-DSKP-EN

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