A biogeochemical cycle is a set of processes by which an element passes from one environment to another and then returns to its original environment, following an endless recycling loop.
Living organisms, to ensure their existence, need a constant supply of certain essential elements, such as carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. These elements are found in the soil, atmosphere, water, and in living tissue. The elements circulate continuously from one environment to another, from one form of life to another. This continual circulation is called a biogeochemical cycle.
All biogeochemical cycles, regardless of the element considered, are made up of biological processes, chemical processes and geological processes. Biological processes, such as respiration or digestion, as well as chemical processes, such as combustion or synthesis reactions, take place over a short period of time. The elements circulate rapidly in this part of a cycle, in a few hours or in a few days. On the other hand, geological processes, such as erosion or sedimentation, take place on a much larger time scale, ranging from months to several years.
The circulation of each element listed above can be explained using a biogeochemical cycle. The following links describe three such cycles: