Definition. Decomposition is the process in which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients.

The raw material. Dead plant remains such as leaves, bark and flowers, and the dead remains of animals including fecal matter, constitute detritus, which is the raw material for decomposition.

The processes.

Fragmentation. Detritivores, such as the earthworm, break down detritus into smaller particles.

Leaching. Water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.

Catabolism. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.

Humification. Leads to the accumulation of humus, a dark coloured amorphous substance highly resistant to microbial action, which decomposes extremely slowly and, being colloidal, serves as a reservoir of nutrients.

Mineralisation. The humus is further degraded by some microbes, releasing inorganic nutrients.

All of these steps operate simultaneously on the detritus, and humification and mineralisation occur during decomposition in the soil.

The products. Carbon dioxide, water, inorganic nutrients — and humus.

What controls the rate. Decomposition is largely an oxygen-requiring process, and its rate is controlled by the chemical composition of the detritus and by climatic factors. It is slower if detritus is rich in lignin and chitin, quicker if rich in nitrogen and water-soluble substances like sugars. Temperature and soil moisture are the most important climatic factors; warm and moist conditions favour decomposition, while low temperature and anaerobiosis inhibit it and lead to the build-up of organic materials.

In what sense opposites. Chemically they are reverses of each other: one assembles what the other dismantles, one consumes carbon dioxide and releases oxygen while the other does the opposite, and one stores energy while the other releases it.

In what sense mutually dependent. Decomposition has no raw material without production: detritus is the residue of what producers made. And production cannot continue without decomposition, because decomposers release the inorganic nutrients back for reuse by the autotrophs. Without mineralisation, every nutrient in an ecosystem would eventually be locked inside dead bodies and the producers would starve.

The larger point. They are not two processes but two halves of one cycle — which is why they appear together among the four functional aspects of an ecosystem, alongside energy flow and nutrient cycling.

Detritus is the whole category of dead organic matter that serves as the raw material for decomposition: dead plant remains such as leaves, bark and flowers, and the dead remains of animals, including fecal matter. It includes material of both plant and animal origin, of any size, and in any stage of breakdown, above ground or within the soil.

Litter is the layer of largely plant debris lying on the surface of the soil — fallen leaves, twigs, bark and flowers — still recognisable and not yet appreciably broken down. It is therefore a subset of detritus, distinguished by its position and its freshness.

Why the difference matters.

(i) They decompose at different rates. The rate of decomposition is controlled by the chemical composition of the detritus, and litter is typically richer in lignin, which makes decomposition slower, than fecal matter or soft animal remains, which are rich in nitrogen and decompose quickly.

(ii) Different steps act on them. Surface litter is where fragmentation by detritivores such as earthworms chiefly occurs, whereas humification and mineralisation occur during decomposition in the soil.

(iii) They are measured differently in the field. Litter can be collected from a marked quadrat and weighed; total detritus cannot, since much of it is dispersed through the soil.

In short: all litter is detritus, but not all detritus is litter.

The two properties, stated. Humification leads to the accumulation of humus, a dark coloured amorphous substance that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature, it serves as a reservoir of nutrients.

They are not in conflict — the first causes the second. A reservoir is useful precisely because it does not empty quickly. If humus decomposed as readily as fresh litter, its nutrients would all be mineralised in a season, taken up or leached away, and nothing would be held in store. Resistance to microbial attack is what allows a stock to accumulate at all.

How the reservoir actually works. Humus is colloidal, which means an enormous surface area carrying charged sites. Nutrient ions are held on those sites — available to roots, but not free to be washed away by the next rain. This is a real protection: recall that leaching carries water-soluble inorganic nutrients down into the soil horizon where they are precipitated as unavailable salts. Nutrients held on humus escape that fate.

And the slow release continues. The humus is further degraded by some microbes, and inorganic nutrients are released by mineralisation — slowly, and therefore steadily, over years.

The practical consequence. Soil fertility is largely a measure of humus content. It also explains an apparent paradox: tropical rainforest soils are often poor, because warm moist conditions favour decomposition so strongly that little humus accumulates — the nutrients are all in the living vegetation, not in the soil bank.

The farmer's field. Every control favours rapid decomposition. Ploughing achieves fragmentation mechanically and mixes the residue into the soil where humification and mineralisation occur. The soil is warm and moist, and temperature and soil moisture are the most important climatic factors regulating decomposition through their effects on the activities of soil microbes. Ploughing also aerates the soil, and decomposition is largely an oxygen-requiring process. Crop residues are comparatively rich in nitrogen and water-soluble sugars, which makes decomposition quicker. Result: the residue is mineralised within a season and its nutrients return to the next crop.

The peat bog. Nearly every control opposes decomposition. The bog is cold, and low temperature inhibits decomposition. It is waterlogged, so oxygen cannot reach the microbes — anaerobiosis inhibits decomposition, resulting in build-up of organic materials. And bog vegetation is typically rich in lignin, which makes decomposition slower still. Result: organic material accumulates for millennia, sometimes to a depth of several metres.

The single factor that differs most: oxygen availability, through waterlogging. Temperature slows decomposition, and tough chemistry slows it; but anaerobiosis comes closest to stopping it, because it removes the requirement of the process itself rather than merely reducing its rate. A cold, dry, lignin-rich site decomposes slowly; a waterlogged one barely decomposes at all.

A wider observation. The world's coal and peat are the accumulated failure of decomposition. Where the process runs to completion, carbon returns to the atmosphere as carbon dioxide; where it is blocked, carbon is stored — which is why draining a peatland releases carbon that has been locked away for thousands of years.