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Plants absorb carbon dioxide from the atmosphere during photosynthesis and convert it into organic compounds like glucose. Heterotrophs consume these plants to obtain carbon for growth and energy. Both organisms return carbon to the atmosphere through cellular respiration, which releases carbon dioxide as a byproduct, completing the biological exchange.

Decomposers such as bacteria and fungi break down dead organisms and animal waste into carbon dioxide and organic compounds. This decomposition process releases carbon back into the atmosphere and soil, allowing it to re-enter the ecosystem and be used by other organisms for growth and energy.

Carbon dioxide is highly soluble in water and reacts with water molecules to form carbonic acid and carbonate ions. These carbonates combine with calcium ions to create calcium carbonate, which marine organisms like coral and oysters use to build their shells and skeletons, making oceans a major carbon reservoir.

Carbon is stored long-term in fossil fuels like coal, petroleum, and natural gas deep within the Earth, formed from organic remains over millions of years. Additionally, limestone in ocean sediments and large, long-lived trees store carbon for extended periods. These geological reservoirs represent the largest carbon sinks on Earth.

Carbon stored deep under the Earth's surface in sediments and soils can be melted into magma. During volcanic eruptions, this process called degassing vents carbon dioxide directly into the atmosphere, returning carbon that has been stored in the Earth's crust for extended periods.

Burning fossil fuels and wood releases substantial amounts of carbon dioxide into the atmosphere. Deforestation and increased agricultural practices reduce organic matter in soil, decreasing carbon storage and increasing greenhouse gases like methane, fundamentally altering the balance of what are biogeochemical cycles.

Oceans absorb carbon dioxide from the atmosphere, which dissolves and reacts with water to form carbonates. When marine organisms die, their calcium carbonate shells and skeletons sink to the ocean floor and gradually accumulate as sediment, eventually forming limestone, the largest carbon reservoir on Earth.