Decomposition

Decomposition is the process by which the complex organic compounds present in detritus -- the dead remains of plants and animals, together with fecal matter and other organic waste -- are progressively broken down into simpler inorganic substances such as carbon dioxide, water, and mineral nutrient ions. It is carried out chiefly by decomposer organisms, principally bacteria and fungi, and it is what keeps an ecosystem's nutrients cycling rather than becoming permanently locked up in accumulating dead organic material.

Although textbooks generally describe decomposition as a sequence of named steps for clarity, in a real ecosystem several of these steps occur simultaneously and continuously, acting together on the same body of detritus rather than strictly one after another.

Fragmentation is usually the first step to act on freshly fallen detritus. Detritivorous animals -- earthworms and termites on land, and various small invertebrates in aquatic sediments -- physically tear, chew and grind larger pieces of detritus into much smaller fragments. This step is important less for the small amount of nutrient it directly releases and more because it dramatically increases the total surface area of the detritus that is exposed to further chemical and microbial attack, greatly speeding up every step that follows.

Leaching takes place alongside fragmentation, particularly wherever detritus lies in contact with rainwater or soil moisture. Water percolating through the fragmented material dissolves a portion of the water-soluble inorganic nutrients present in it and carries them downward through the soil profile, where they may become bound up again as soil salts that are not immediately available for plant root uptake, at least until later chemical or biological processes free them once more.

Catabolism is the step in which bacterial and fungal decomposers secrete extracellular digestive enzymes directly onto the detritus, chemically breaking down its complex organic molecules -- proteins, carbohydrates and lipids -- into progressively simpler organic and eventually inorganic compounds. The decomposer microbes then absorb many of these simpler products directly into their own cells for their own nutrition and growth.

Humification is a step that operates more specifically in soil systems, producing a dark-coloured, structurally amorphous, colloidal substance called humus. Humus is highly resistant to further rapid microbial breakdown, decomposes only very slowly compared to fresh detritus, and is ecologically valuable because it significantly improves soil structure, aeration and water-holding capacity, indirectly benefiting the plants growing in that soil quite apart from any nutrients it may still contain.

Mineralization is the final named step, in which the accumulated humus is itself very gradually broken down further by continued microbial action, releasing simple inorganic nutrients -- forms such as nitrate, phosphate and other mineral ions -- back into the surrounding soil solution, from where plant roots can once again take them up. Mineralization is therefore the step that most directly completes the nutrient cycle, returning elements that began as living tissue back to a chemical form that producers can reuse to build new biomass. …