Decomposition

Decomposition is the process by which the complex organic matter in dead plants and animals — leaves, roots, animal bodies, waste — is broken down into simpler inorganic substances. It is the opposite of production: where producers build organic matter, decomposers take it apart. Without decomposition, nutrients would remain locked in dead tissue and the ecosystem would run out of raw materials.

The raw material for decomposition is called detritus — dead plant remains like leaf litter, dead animal bodies, and faecal matter. Decomposers act on this detritus through five processes that all operate simultaneously, not as a strict one-after-another sequence.

The processes of decomposition

1. Fragmentation — Detritus is physically broken into smaller pieces. Earthworms, millipedes, and other detritivores chew and grind the material. This increases the surface area available for microbial action.

2. Leaching — Water-soluble inorganic nutrients (like calcium, potassium, and magnesium ions) dissolve out of the fragmented detritus and percolate into the soil. This is a passive, physical process driven by rainfall or soil moisture.

3. Catabolism — The actual chemical breakdown. Fungi and bacteria (the true decomposers) secrete enzymes that digest complex organic compounds — cellulose, lignin, proteins, fats — into simpler absorbable forms. This is a biochemical process.

4. Humification — The resistant, undigested organic matter accumulates as a dark, amorphous, colloidal substance called humus. Humus is highly resistant to further microbial action and can persist in the soil for centuries. It gives soil its dark colour and improves its water-holding capacity and structure.

5. Mineralisation — The final step. The humus itself is slowly degraded by microbes, releasing inorganic nutrients (nitrates, phosphates, sulphates, etc.) back into the soil, where they become available again for uptake by plants.

Factors that affect decomposition

Decomposition rate is controlled primarily by two abiotic factors:

• Temperature — Decomposition speeds up with warmth (within biological limits) and slows down in cold conditions.
• Soil moisture — Adequate moisture is essential for microbial activity. Waterlogged or extremely dry soils slow decomposition.

A third factor is the chemical composition of the detritus itself. Detritus rich in lignin and chitin decomposes very slowly because these substances are tough and resistant to microbial enzymes. Detritus rich in nitrogen and water-soluble sugars decomposes faster.

The decomposer community

The organisms that carry out decomposition are called decomposers (or saprotrophs). They are mostly:

• Fungi — the primary decomposers of lignin and cellulose in plant litter.
• Bacteria — especially important for breaking down animal proteins and for the final steps of mineralisation. …