BIOGEOCHEMICAL CYCLE

any of the natural circulation pathways of the essential elements of living matter. These elements in various forms flow from the nonliving (abiotic) to the living (biotic) components of the biosphere and back to the nonliving again. In order for the living components of a major ecosystem (e.g., a lake or forest) to survive, all the chemical elements that make up living cells must be recycled continuously. Each cycle can be considered as having a reservoir (nutrient) poola larger, slow-moving, usually abiotic portionand an exchange (cycling) poola smaller but more active portion concerned with the rapid exchange between the biotic and abiotic aspects of an ecosystem. Biogeochemical cycles can be classed as gaseous, in which the reservoir is the air or the oceans (via evaporation), and sedimentary, in which the reservoir is the Earth's crust. Gaseous cycles include those of nitrogen, oxygen, carbon, and water; sedimentary cycles include those of iron, calcium, phosphorus, and other more earthbound elements. Gaseous cycles tend to move more rapidly than do the sedimentary ones and to adjust more readily to changes in the biosphere because of the large atmospheric reservoir. Local accumulations of carbon dioxide, for example, are soon dissipated by winds or taken up by plants. Extraordinary and more frequent local disturbances can, however, seriously affect the capacity for self-adjustment. Sedimentary cycles vary from one element to another, but each cycle consists fundamentally of a solution phase and a rock (or sediment) phase. Weathering releases minerals from the Earth's crust in the form of salts, some of which dissolve in water, pass through a series of organisms, and ultimately reach the deep seas, where they settle out of circulation indefinitely. Other salts deposit out as sediment and rock in shallow seas, eventually to be weathered and recycled. Plants and some animals obtain their nutrient needs from solutions in the environment. Other animals acquire the bulk of their needs from the plants and animals that they consume. After the death of an organism, the elements fixed in its body are returned to the environment through the action of decay organisms and become available to other living organisms again.