Organisms are built up of chemical substances. They require certain chemicals like N2,O2,H2,P,C etc, continuously for their survival. These chemicals enter the organisms from the environment and come out after undergoing changes or without changes. Thus, these elements tend to circulate in a characteristic path from the environment to the organism and back to the environment. This cyclical path of the element from the abiotic system to biotic system and back is called biogeochemical cycle.