Natural carbon cycle. Diagram illustrating the natural processes that cycle carbon through different regions of the Earth. Reservoirs of carbon, which form the global carbon budget, are termed the biosphere, geosphere, hydrosphere, and atmosphere. Carbon dioxide is released during the metamorphosis of carbonate rocks when they are subducted into the earth's mantle (lower right). This carbon dioxide can be released into the atmosphere and ocean through volcanoes and hotspots. Sunlight is used by plants to incorporate atmospheric carbon dioxide into plant sugars and tissues during photosynthesis. Carbon dioxide is released during plant and animal respiration, and the decay of organic matter in the soil. Some carbon dioxide is absorbed by the oceans.