The five sources of carbon in the carbon cycle
1. Biological respiration
Biological respiration is the source of carbon. All animals, plants and microorganisms perform respiration. Animals inhale oxygen, decompose organic matter in the body, such as glucose, to obtain energy, and exhale carbon dioxide. When plants cannot perform photosynthesis at night, they also use respiration to maintain life activities and release carbon dioxide into the atmosphere. Microorganisms decompose organic residues in the soil. During respiration, carbon is also returned to the atmosphere in the form of carbon dioxide, which is an important way for biological activities to contribute carbon in the carbon cycle.

II. Combustion of Fossil Fuels
Since ancient geological times, a large number of biological remains have been buried deep in the ground. After a long period of time and complex geological processes, they have been converted into fossil fuels such as coal, oil, and natural gas. With the development of human society, the demand for energy is increasing, and fossil fuels are exploited and burned on a large scale. During combustion, carbon rapidly combines with oxygen to generate carbon dioxide, which floods into the atmosphere in large quantities. This is the main reason for the sharp increase in the concentration of carbon dioxide in the atmosphere since modern times, and has a profound impact on the global carbon cycle balance.

III. Oceans Release
Oceans are huge carbon reservoirs, dissolving a large amount of carbon dioxide. However, there is a dynamic balance between the ocean and the atmosphere. When ocean water temperature increases, salinity changes, or is affected by other physical factors, part of the carbon dioxide dissolved in seawater will escape to the atmosphere. For example, in tropical waters, the seawater temperature is higher and the solubility of carbon dioxide decreases, releasing carbon into the atmosphere. And marine biological activities, such as the decomposition of some plankton after respiration and death, also promote carbon from the ocean to the atmosphere in the form of carbon dioxide, participating in the carbon cycle.

4. Soil organic carbon decomposition
Soil is rich in a large amount of organic matter, such as animal and plant residues, humus, etc. Soil microorganisms are active and decompose and transform these organic carbons. Under aerobic conditions, microorganisms completely oxidize organic carbon into carbon dioxide and release it; in anaerobic environments, although the decomposition process is different, some organic carbon will still be converted into carbon dioxide or methane and other carbon-containing gases into the atmosphere. Soil organic carbon decomposition rate is restricted by many factors such as temperature, humidity, soil texture and microbial community structure, and is an important link in the carbon cycle of terrestrial ecosystems.

5. Volcanic eruptions
Volcanoes are the phenomenon in which hot magma and other substances inside the earth are ejected from the surface. When a volcano erupts, not only magma, volcanic ash, etc. are ejected, but also a large amount of gases, including carbon dioxide. Magma deep in the earth contains carbon compounds, which are released into the atmosphere with high-temperature substances during eruptions. Although volcanic eruptions do not occur every day, they can inject a large amount of carbon into the atmosphere in the short term. During geological history, they have had an important impact on the global carbon cycle, altering the concentration of carbon dioxide in the atmosphere, which in turn affects the climate and ecosystems.