Decomposition is the process by which dead organic matter (detritus) is broken down into simpler inorganic substances like carbon dioxide, water, mineral salts, and other nutrients by microorganisms (bacteria and fungi). It is fundamentally the opposite of photosynthesis in the carbon cycle — while photosynthesis converts inorganic carbon (CO₂) into organic biomass, decomposition converts organic biomass back to inorganic forms. Without decomposition, all nutrients would remain locked in dead organic matter, making them unavailable for new plant growth, and ecosystems would collapse. Decomposers are therefore critical functional components of every ecosystem. The rate of decomposition is a key determinant of nutrient cycling rate and ecosystem productivity. Tropical ecosystems have rapid decomposition (warm, moist) while polar/arid ecosystems have very slow decomposition.

Detritus consists of all dead organic matter that serves as the substrate for decomposition. It includes dead plant material (leaf litter, fallen logs, dead roots, bark), dead animal bodies, faecal matter (dung, urine), shed skin/exoskeletons, hair, feathers, and any other organic material from living organisms. Detritus is rich in complex organic molecules: cellulose, lignin, chitin, proteins, fats. Some components decompose rapidly (sugars, proteins) while others are extremely resistant (lignin, suberin, sporopollenin, cutin). Detritivores are animals that feed directly on detritus: earthworms, millipedes, woodlice, dung beetles, certain fly larvae, marine polychaetes. They fragment detritus into smaller pieces, increasing surface area for microbial attack. The quality of detritus (C:N ratio) determines how quickly it decomposes.

Decomposition proceeds through multiple interconnected stages. Fragmentation: physical breakdown of detritus into smaller pieces by detritivores (earthworms, millipedes, woodlice, mites). This increases the surface area available for microbial attack. Leaching: water-soluble compounds (sugars, amino acids, mineral salts) are washed down into the soil by rainwater. This process removes the most easily decomposable compounds first. Catabolism: extracellular enzymes secreted by bacteria and fungi chemically degrade complex macromolecules — cellulases break down cellulose, proteases break down proteins, lipases break down fats, ligninases break down lignin (very slowly). The products are absorbed by the microorganisms. Humification: formation of a dark-coloured, amorphous, colloidal substance called humus from partially decomposed organic matter. Humus is resistant to further rapid decomposition (recalcitrant). Mineralisation: gradual mineralisation of humus by microorganisms releases inorganic nutrients — N (as NH₄⁺/NO₃⁻), P (as H₂PO₄⁻), S (as SO₄²⁻), K⁺, Ca²⁺ into soil solution for plant uptake.

Humification is the process by which partially decomposed organic matter is transformed into humus. Humus is a complex, dark-coloured, amorphous, colloidal organic substance that is resistant to rapid further decomposition. It consists of high-molecular-weight aromatic polymers derived from lignin breakdown and microbial metabolic products. Humus provides many benefits to soil: improves soil structure (aggregation), increases water-holding capacity, provides cation exchange capacity (binds and releases nutrients), slowly releases nutrients through mineralisation, provides a buffer against pH changes, and supports diverse soil microbial communities. Humus formation is faster in cool, moderately moist conditions. Peat (in bogs): partially decomposed organic matter where decomposition is slow due to anaerobic, acidic conditions.

Mineralisation is the final stage of decomposition where organic nutrients are converted to inorganic forms that can be directly absorbed by plant roots. Nitrogen mineralisation: organic N (proteins, nucleic acids) → ammonification (NH₄⁺ by bacteria like Bacillus, Clostridium) → nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻ by Nitrosomonas and Nitrobacter). Phosphorus mineralisation: organic P (phospholipids, nucleic acids) → phosphatase enzymes → H₂PO₄⁻ (inorganic phosphate). Sulphur mineralisation: organic S (proteins with cysteine, methionine) → sulphate (SO₄²⁻). All these inorganic ions are water-soluble and plant-available. The rate of mineralisation affects the fertility of soil and the productivity of the ecosystem.

Temperature: warm temperatures (25-35°C) accelerate decomposition (optimal enzyme activity). Cold temperatures slow decomposition — tropical ecosystems decompose much faster than arctic tundra. Moisture: optimal moisture accelerates decomposition. Waterlogged conditions create anaerobic environment → only anaerobic decomposers work → much slower decomposition → peat formation. Very dry conditions: desiccates microorganisms → slow decomposition. Oxygen: aerobic decomposition is 20-30× faster than anaerobic. Flooded soils = anaerobic = slower nutrient release. Quality of detritus: High C:N ratio (lignin-rich wood, straw) → slow decomposition. Low C:N ratio (protein-rich material, legume leaves) → rapid decomposition. Lignin content: most resistant to decomposition (only certain fungi like white-rot fungi can effectively break it down). Soil pH: most decomposers work best at neutral pH (6.5-7.5). Acidic soils (bogs, conifer forests) have slow decomposition.

Decomposers (saprotrophs): microorganisms (bacteria and fungi) that decompose dead organic matter by secreting extracellular enzymes and absorbing the products. They work at the microscopic level. Key decomposers: Bacteria — Bacillus, Pseudomonas, Clostridium (anaerobic), Nitrosomonas, Nitrobacter. Fungi — Trichoderma (cellulose decomposer), Aspergillus, Penicillium, Basidiomycetes (white-rot fungi decompose lignin). Detritivores: animals that feed on detritus, physically breaking it into smaller pieces. Examples: Earthworms (called ecosystem engineers — mix organic matter into soil, improve aeration), millipedes, woodlice (pill bugs), dung beetles, isopods, certain beetles, marine polychaete worms, some flies and their larvae (maggots), crabs, shrimps in aquatic systems. Detritivores do not directly release inorganic nutrients — they prepare detritus for microbial decomposition by increasing surface area.

Decomposition is the primary pathway by which carbon stored in organic matter (biomass) is returned to the atmosphere as CO₂. In a balanced ecosystem, the CO₂ released by decomposition equals the CO₂ fixed by photosynthesis — the system is carbon-neutral. However, human activities have disturbed this balance. Burning fossil fuels releases carbon that was stored for millions of years. Deforestation reduces photosynthesis while increasing decomposition of exposed organic matter. Peatland drainage and fires release enormous amounts of stored carbon (peatlands store 30% of all soil carbon despite covering only 3% of land). Conversely, soil carbon (humus) is a massive carbon sink — improving soil management to increase humus content is being studied as a carbon sequestration strategy. Methane (CH₄) from anaerobic decomposition in wetlands and livestock is another major greenhouse gas.