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The sun is the source of all energy on earth. This energy initiates life processes in the biosphere through photosynthesis, the main source of food and energy for green plants. A very small fraction of about 0.1 percent of solar energy reaching earth is fixed in photosynthesis. More than half is used for plant respiration and the remaining part is temporarily stored or is shifted to other portions of the plant. During photosynthesis carbon dioxide is converted into organic compounds and oxygen.
The balance of the chemical elements is maintained by a cyclic passage through the tissues of plants and animals. The cycle starts by absorbing the chemical elements by the organism and is returned to the air, water and soil through decomposition. These cyclic movement of chemical elements of the biosphere between the organism and the environment are referred to as bio-geochemical cycles.
Fixing of nitrogen in the atmosphere Action of soil micro-organisms and associated plant roots on atmospheric nitrogen found in pore spaces of the soil comprise the principal source of free nitrogen. Lighting and cosmic radiation can also fix nitrogen in the atmosphere.