In ecology and Earth science, a biogeochemical cycle or nutrient cycle is a pathway by which a chemical element or molecule moves through both biotic (biosphere) and abiotic (lithosphere, atmosphere, and hydrosphere) compartments of Earth. In effect, the element is recycled, although in some cycles there may be places (called reservoirs) where the element is accumulated or held for a long period of time. Elements, chemical compounds, and other forms of matter are passed from one organism to another and from one part of the biosphere to another through the biogeochemical cycles.
All chemical elements occurring in organisms are part of biogeochemical cycles. In addition to being a part of living organisms, these chemical elements also cycle through abiotic factors of ecosystems such as water (hydrosphere), land (lithosphere), and the air (atmosphere). The living factors of the planet can be referred to collectively as the biosphere. All the nutrients — such as carbon, nitrogen, oxygen, phosphorus — used in ecosystems by living organisms operate on a closed system, which refers to the fact that these chemicals are recycled instead of being lost and replenished constantly such as in an open system. The energy of an ecosystem occurs on an open system; the sun constantly gives the planet energy in the form of light while it is eventually used and lost in the form of heat throughout the trophic levels of a food web.
The Earth does not constantly receive more chemicals as it receives light; it has only those from which it formed. The only way for Earth to obtain more nutrients is from occasional meteorites from outer space. Because chemicals operate on a closed system and cannot be lost and replenished like energy can, these chemicals must be recycled throughout all of Earth’s processes that use those chemicals or elements. These cycles include both the living biosphere and the nonliving lithosphere, atmosphere, and hydrosphere.
The chemicals are sometimes held for long periods of time in one place. This place is called a reservoir, which, for example, includes such things as coal deposits that are storing carbon for a long period of time. When chemicals are held for only short periods of time, they are being held in exchange pools. Examples of exchange pools include plants and animals, which temporarily use carbon in their systems and release it back into the air or surrounding medium. Generally, reservoirs are abiotic factors while exchange pools are biotic factors. Carbon is held for a relatively short time in plants and animals when compared to coal deposits. The amount of time that a chemical is held in one place is called its residence.
The most well-known and important biogeochemical cycles, for example, include the carbon cycle, the nitrogen cycle, the oxygen cycle, the phosphorus cycle, and the water cycle. There are many biogeochemical cycles that are currently being studied for the first time as climate change and human impacts are drastically changing the speed, intensity, and balance of these relatively unknown cycles. These newly studied biogeochemical cycles include the mercury cycle and the human caused cycle of atrazine.
Biogeochemical cycles always involve equilibrium states: a balance in the cycling of the element between compartments. However, overall balance may involve compartments distributed on a global scale.
As biogeochemical cycles describe the movements of substances on the entire globe, the study of these is inherently multidiciplinary. Hydrology, Geology, Pedology (soil study), atmospheric sciences and ecology, need to be addressed in a full description of a particular cycle.