Biogeochemical cycles are a set of natural processes that recycle essential elements such as carbon, nitrogen, phosphorus, and water between living organisms and the environment. These cycles involve biotic (living) and abiotic (non-living) components, including plants, animals, atmosphere, soil, and water. For example, plants use carbon dioxide from the air to make food through a process called photosynthesis, and animals release carbon dioxide back into the air when they breathe. The nutrients in the soil are used by plants and other organisms, and returned to the soil when these organisms decay.