All of these parts are reservoirs of carbon. The cycle is usually thought of as four main reservoirs of carbon interconnected by pathways of exchange. The reservoirs are the atmosphere, terrestrial biosphere (usually includes freshwater systems), oceans, and sediments (includes fossil fuels). The annual movements of carbon, the carbon exchanges between reservoirs, occur because of various chemical, physical, geological, and biological processes. The ocean contains the largest pool of carbon near the surface of the Earth, but most of that pool is not involved with rapid exchange with the atmosphere.
The global carbon budget is the balance of the exchanges (incomes and losses) of carbon between the carbon reservoirs or between one specific loop (e.g., atmosphere - biosphere) of the carbon cycle. An examination of the carbon budget of a pool or reservoir can provide information about whether the pool or reservoir is functioning as a source or sink for carbon dioxide.
Through the decay of animal and plant matter. Fungi and bacteria break down the carbon compounds in dead animals and plants and convert the carbon to carbon dioxide.
Through combustion. When the bodies of dead plants and animals are compressed for a long period of time, they eventually form peat, lignite or black coal. These are then burnt and carbon dioxide is realeased into the atmosphere.
Through the erosion of limestone. Limestone, marble and chalk are composed mainly of calcium carbonite. These can react in water to eventually form water, carbon dioxide and carbon trioxide gas.
Volcanic eruptions can also release carbon back into the atmosphere.
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