The amount of carbon dioxide in the atmosphere is maintained by a balance between the process (photosynthesis) which withdraws carbon dioxide (CO2) from it and those (respiration and combustion) which add carbon dioxide to it. The carbon cycle is mainly the processes, which bring about the circulation of carbon compounds in nature. The air around us contains a “pool of carbon dioxide about 0.03%. Green plants are continuously removing it from the atmosphere for building up sugar (carbohydrate) and other complex compounds during photosynthesis. When the plant is eaten by an animal (primary consumer), the materials are digested, absorbed and built up into compounds which form the animal tissues. In this way, the carbon atoms are passed along the food chain. Some of the carbon atoms pass out into the environment as faeces and urine.
When organisms respire, the sugars are broken down to liberate carbon dioxide (CO2) and water as waste products to the atmosphere. Eventually, plants and animals must die, decomposers feed on their remains and bring about decay. The complex carbon compounds that are incorporated into their tissues are broken down and set free as carbon dioxide. The bacteria and fungi too, respire and again release carbon dioxide into the atmosphere. When fuels like wood, coal, petroleum and natural gas are burnt, the carbon in them is released into the atmosphere. When aquatic organisms respire, carbon dioxide is dissolved in water. As it evaporates, a small quantity is liberated into the atmosphere. Another carbon source that recycles slowly is calcium carbonate (CaCO3). It is what builds up the shell of molluscs and other shell organisms.
Importance of Carbon in Nature
The importance of carbon in nature cannot be overemphasised. This is because all living organisms are composed of various forms of carbon compounds. The processes which bring about the recycling of carbon compounds between the biotic and abiotic components of an ecosystem, such as photosynthesis, respiration and decay are all carried out by living organisms. Inorganic carbon dioxide (which has poor energy) and complex organic compound, carbohydrate (with rich energy) maintain a functioning ecosystem by enabling a linear flow of energy. Carbon compounds are the vehicles, through which the trapped solar energy from the sun passes from organism to organism, in the food chain of an ecosystem.
Carbon – Oxygen Balance
Oxygen makes up about one-fifth of the earth’s atmosphere. This is fairly maintained in the oxygen cycle.
Respiration, decay and combustion are the processes, which remove oxygen from the atmosphere, while photosynthesis releases oxygen into the atmosphere. In fact, we can say that carbon is the oxygen cycle reversed.
Deforestation and combustion of fossil fuels are the two human activities which affect the carbon-oxygen balance. A small decrease in the atmospheric oxygen level about 3 to 8% does not cause any significant effect. But a slight increase in atmospheric carbon dioxide may lead to an increase in the retention of the sun’s radiant (heat) energy, i.e., ‘green house effect’. This could lead to warming of the earth’ s atmosphere (global warming), it could also cause the melting of the polar ice, with a related rise in sea levels. Climatic changes also could affect the important food growing areas of the world. Pollution of aquatic bodies will increase the activities of decomposers. This will as well result in a drop of the oxygen level, threatening the survival of many aquatic organisms, especially fish.