Decomposition

Without decomposers, the earth would be piled with dead plants and animals, and nutrients would be locked up in them forever. Decomposition breaks down complex organic matter into simple inorganic substances such as CO₂, water and nutrients.

The raw material is detritus: dead plant remains (leaves, bark, flowers), dead animals, and animal faeces. Decomposition happens in steps:

• Fragmentation: detritivores such as earthworms break the detritus into small pieces.
• Leaching: water-soluble nutrients seep down into the soil, where they may form unavailable salts.
• Catabolism: enzymes from bacteria and fungi break the detritus down into simpler substances.
• Humification: a dark, amorphous substance called humus builds up. It is highly resistant to microbes, so it breaks down extremely slowly. Being colloidal, it holds nutrients well, acting as a reservoir of nutrients in the soil.
• Mineralisation: the humus is slowly broken down by some microbes, and inorganic nutrients are released.

These steps all go on at the same time on different bits of detritus. Decomposition needs oxygen: it is largely an aerobic process.

Speed. Decomposition is controlled by the chemical makeup of the detritus and by climate:

• It is fast when the detritus is rich in nitrogen and sugars, and when it is warm and moist.
• It is slow when the detritus is rich in lignin (in wood) and chitin (in insect shells), and when it is cold, dry, or waterlogged (with little oxygen: anaerobiosis). Organic matter therefore piles up in cold, waterlogged soils.

Temperature and soil moisture are the most important climatic factors.