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The chemical elements tend to circulate in the biosphere in characteristic paths from environment to organisms and back to the environment.
These more or less circular paths of the chemical elements are known as the Biogeochemical cycles.
These cycles of chemical elements involve biological organisms and their geological environment. Hence, they are collectively referred to as biogeochemical In this term “bio” refers to living organisms and “geo” to the rocks, soil, air, and water of the earth (Odum, 1963).
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For each cycle two compartments or pools may be recognized. These are: (a) the nutrient pool or the reservoir pool, which is the large, slow-moving, generally non-biological component; and (b) the exchange or cycling pool, which is a small but more active portion that is exchanged (i.e., moved back and forth). There are three basic types of biogeochemical cycles.
(a) Gaseous type:
In this type of biogeochemical cycles, the atmosphere constitutes the major reservoir of the element that exists there in gaseous phase. Such cycles show little or no permanent change in the distribution and abundance of the element. The Carbon, Oxygen and Nitrogen cycles are good examples of biogeochemical cycles with prominent gaseous phase.
(b) Sedimentary type:
In the sedimentary type of cycle major reservoir is the lithosphere, from which the elements are released by weathering. The sedimentary types are best exemplified by Phosphorus, Sulphur, and Iodine cycles. In these cycles a little portion of the supply may get lost, as in the deep ocean sediments, and thereby becomes inaccessible to organisms and to continual cycling.
(c) Water cycle:
The hydrologic (water) cycle has been included in gaseous types of cycles by Odum (1963). But Kormondy (1969) considers it to be a separate major cycle, involving the movement of a compound, while the others involve the movement of elements.
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The movement of those elements and inorganic compounds that are essential to life can be conveniently designated as the nutrient cycling. The latter may include all the three major types of cycles i.e., hydrologic cycles, gaseous nutrient cycles, and sedimentary nutrient cycles.
