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Decomposition is the breakdown of dead matter, which is often called rotting. Decomposing bacteria and fungi are organisms that help the process of decomposition. Decomposition is crucial to the cycling of elements, such as carbon from one living organism to another. You can learn more about the carbon and water cycles here.

The rate of decay is the speed at which dead matter is broken down by decomposers. The rate can be estimated by measuring changes in pH, (for example in milk), change in mass (decaying fruit and vegetables) or change in temperature (grass cuttings). Rates of decay are affected by a number of key factors.

Temperature

At colder temperatures decomposing organisms will be less active, thus the rate of decomposition remains low. This is why we keep food in a fridge. As the temperature increases, decomposers become more active and the rate increases. At extremely high temperatures decomposers will be killed and decomposition will stop.

Oxygen

Many decomposers need oxygen to survive and without it there is little or no decomposition. Oxygen is needed for decomposers to respire, to enable them to grow and multiply. Therefore, we often seal food in bags or cling film before putting it in the fridge. As the volume of available oxygen increases, the rate of decomposition also increases. Some decomposers can survive without oxygen, getting their energy by anaerobic respiration. Instead of producing carbon dioxide this process results in the production of methane, which is a flammable gas. We use anaerobic decomposers in biogas generators.

Water

With little or no water there is less decomposition because decomposers cannot survive. As the volume of available water increases, the rate of decomposition also increases. Many decomposers secrete enzymes onto decaying matter and then absorb any dissolved molecules. Without water these reactions cannot occur.

The Egyptians mummified their dead kings and queens. This process removed all water from the mummy and so stopped decomposers from breaking down the dead tissue.

Archaeologists have found very old remains of people in peat bogs. A famous example of this was the Tollund Man. In peat bogs there is low oxygen, low temperature and low pH which can naturally mummify dead remains, similar to the process that the Egyptians undertook.

Learn more about decomposition with Dr Alex Lathbridge.

Listen to the full series on 91�ȱ� Sounds.