The term biogeochemical cycle refers to any set of changes that occur as a particular element passes back and forth between the living and non-living worlds. For example, carbon occurs sometimes in the form of an atmospheric gas (carbon dioxide), sometimes in rocks and minerals (limestone and marble), and sometimes as the key element of which all living organisms are made. Over time, chemical changes occur that convert one form of carbon to another form. At various points in the carbon cycle, the element occurs in living organisms and at other points it occurs in the Earth's atmosphere, lithosphere, or hydrosphere.

The universe contains about ninety different naturally occurring elements. Six elements, carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus, make up over 95% of the mass of all living organisms on Earth. Because the total amount of each element is essentially constant, some cycling process must take...