Bio-geochemical cycles: A biogeochemical cycle is a pathway by which a chemical substance moves through both biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. The sun is the source of all energy on earth. 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 chemical elements in the atmosphere and hydrosphere is maintained for the last one billion years. These cycles are energized by solar Insolation. These cyclic movements are referred to as biogeochemical cycles.
Fixing of Nitrogen in the Atmosphere: Nitrogen is a major constituent of the atmosphere comprising about seventy-nine percent of the atmospheric gases. 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. Lightning and cosmic radiation can also fix nitrogen in the atmosphere.