The carbon cycle tracks movement of carbon among biological and nonbiological reservoirs.

Atmospheric and dissolved carbon dioxide ($CO_2$) is converted into organic carbon by autotrophic carbon fixation. In photosynthetic ecosystems, this is driven by light.

Organic carbon then moves through food webs as organisms consume producers and one another. Cellular respiration returns much of that carbon to $CO_2$:

$$\text{organic carbon} + O_2 \rightarrow CO_2 + H_2O + \text{released energy}. $$

Decomposition also returns carbon from dead organic matter to $CO_2$ through decomposer metabolism.

Not all carbon cycles rapidly. Some enters longer-lived reservoirs:

• dissolved inorganic carbon in oceans;
• organic matter stored in soils and sediments;
• carbonate minerals;
• fossil organic carbon formed over geological timescales.

Combustion transfers reduced carbon from biomass or fossil fuels back to atmospheric $CO_2$. Exchange between atmosphere and ocean also moves large amounts of carbon in both directions.

The carbon cycle therefore contains fast biological fluxes and slow geological fluxes. A reservoir can be large even if its turnover is slow, and a small reservoir can exchange carbon rapidly.

The key biological loop is

$$CO_2 \xrightarrow{\text{carbon fixation}} \text{organic carbon} \xrightarrow{\text{respiration/decomposition}} CO_2,$$

embedded within a larger Earth system containing oceans, sediments and rocks.