Most carbon on Earth is locked up as carbon dioxide (CO₂) in the atmosphere, carbonates in rocks, and hydrogencarbonate in water. Carbon enters the biotic environment through photosynthesis in which photosynthetic organisms use sunlight energy to transform CO₂ into carbohydrates. When other organisms eat the plants they take up this carbon. Both plants and animals respire which releases carbon back into the atmosphere as CO₂. Combustion also releases CO₂.

Organic material is returned to the soil through excretion, death and decay. Decomposers then break this down into humus. Detritivores and saprophytes feed on this humus and as they feed they respire the carbon as CO₂ back into the atmosphere.

Different ecosystems have a different carbon balance:

• A carbon source is an ecosystem which gives out more carbon as carbon dioxide than is takes in to make its biomass over the long term. For example, farmland and areas of deforestation are carbon sources.
• A carbon neutral ecosystem is one where carbon is released and fixed at a balanced rate, like in a mature forest.
• A carbon sink is an ecosystem which takes in more carbon as biomass than it releases as carbon dioxide. This happens in ecosystems where decomposer can’t survive, like if the habitat’s too hot or too acidic. Examples include peat bogs and the ocean floor. Carbon remains fixed in a organic form and can eventually form into fossil fuels over a long enough period.