Decomposition and the carbon cycle

Nutrient cycles

Within an ecosystem, nutrients begin as simple inorganic compounds (carbon dioxide and nitrate ions), that are taken in by a plant and used to make more complex biological molecules.

These molecules are passed along the food chains.

When plants and animals die, the complex biological molecules are decomposed.


Dead organisms are broken down into smaller pieces by the process of decay. Organisms such as earthworms are involved in this process.

Worms eating rotten food in compost.

Decomposition is the process by which bacteria and fungi break dead organisms into their simple compounds.

Plants can absorb and use these compounds again, completing the cycle.

Decomposing bacteria and fungi are described as saprophytic because of the way they break down dead organic matter.

Saprophytic nutrition involves:

  1. Bacteria/fungi secreting enzymes out of their cells into the soil or dead organism.
  2. The enzymes digest the organic material. This is known as extracellular digestion as it happens outside the cells.
  3. The products of digestion are absorbed by the bacteria/fungi.

Humus is the organic content of the soil formed from decomposing plants and animals.

Decomposition happens quickly when:

  • The temperature is warm
  • There is enough moisture
  • The decomposing organism has a large surface area

Anaerobic conditions (little/no oxygen available - for example, waterlogged soil) cause the rate of decomposition to decrease.