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How does the carbon cycle move carbon between organisms, the atmosphere, soil, and oceans?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — Biosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

The carbon cycle describes the continuous movement of carbon through the biosphere, atmosphere, soil, and oceans. This process is essential for life, as carbon serves as a fundamental building block for all organic compounds.12

Biological and Atmospheric Exchange

The cycle is driven primarily by photosynthesis, where terrestrial plants and aquatic algae (producers) fix atmospheric or dissolved carbon dioxide (CO₂) into carbohydrates. These organic compounds move through the food chain as consumers eat producers. Carbon is returned to the atmosphere or water through respiration by organisms and the decomposition of organic matter by bacteria and fungi.12

Soil and Ocean Reservoirs

Soil: Soils act as significant carbon reservoirs, storing carbon in humus and other organic matter. Microbes decompose this matter, releasing CO₂ back into the atmosphere through soil respiration.3
Oceans: The oceans are the largest carbon reservoirs. Atmospheric CO₂ dissolves in water, reacting to form carbonic acid, which dissociates into bicarbonate and carbonate ions. This inorganic carbon is used by aquatic organisms and can precipitate as limestone sediment.24

Human activities, such as burning fossil fuels and deforestation, have disrupted these natural balances, leading to increased atmospheric CO₂ levels and global warming.52