A biogeochemical cycle is the continuous recycling of water or an element (mainly carbon, hydrogen, nitrogen, oxygen, phosphorus and sulfur) between organisms (animals, plants and other forms of life) and the abiotic components of the ecosystem (atmosphere, hydrosphere and lithosphere). These elements form a wide variety of compounds and may exist for long periods in the atmosphere, on land, in water, or beneath the earth's surface. The geologic processes playing a role in these cycles include weathering, erosion, drainage, and even the subduction of the continental plates.
For example, the carbon cycle consists of carbon being converted into organic matter and then back into inorganic molecules. Plants require carbon, in the form of carbon dioxide, for their respiration during photosynthesis and to build tissue. Animals acquire carbon directly through carbohydrates by eating plants or indirectly through consuming plant-eating animals, and they release the carbon into the atmosphere in the form of carbon dioxide when they breath. When plants and animals die, their carbon is released back into the atmosphere from oxidation of their tissues.