Oxygen cycle refers to the movement of oxygen through the atmosphere (air), biosphere (plants and animals) and the lithosphere (the Earth’s crust). The oxygen cycle demonstrates how free oxygen is made available in each of these regions, as well as how it is used. The oxygen cycle is the biogeochemical cycle of oxygen atoms between different oxidation states in ions, oxides, and molecules through redox reactions within and between the spheres/reservoirs of the planet Earth. The word oxygen in the literature typically refers to the most common oxygen allotrope, elemental/diatomic oxygen (O), as it is a common product or reactant of many biogeochemical redox reactions within the cycle. Processes within the oxygen cycle are considered to be biological or geological and are evaluated as either a source (O production) or sink (O consumption).
Oxygen is one of the most common elements on Earth and represents a large portion of each main reservoir. By far the largest reservoir of Earths oxygen is within the silicate and oxide minerals of the crust and mantle (99.5% by weight). The Earths atmosphere, hydrosphere, and biosphere together hold less than 0.05% of the Earth's total mass of oxygen. Besides O, additional oxygen atoms are present in various forms spread throughout the surface reservoirs in the molecules of biomass, HO, CO, HNO, NO, NO, CO, HO, O, SO, HSO, MgO, CaO, Al2O3, SiO, and PO.