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Cycles of Matter

 Very much connected with environmental chemistry, cycles of matter, often based on elemental cycles, are of utmost importance in the environment. Global geochemical cycles can be regarded from the viewpoint of various reservoirs, such as oceans, sediments, and the atmosphere, connected by conduits through which matter moves continuously among the hydrosphere, atmosphere, geo- sphere, biosphere, and, increasingly, the anthrosphere. The movement of a specific kind of matter between two particular reservoirs may be reversible or irreversible. The fluxes of movement for particular kinds of matter vary greatly as do the contents of such matter in a specified reservoir. Most cycles of matter have a strong biotic component, especially through the biochemical processesof plants and microorganisms. The cycles in which organisms participate are called biogeochemical cycles, which describe the circulation of matter, particularly plant and animal nutrients, through ecosystems. Most biogeochemical cycles can be described as elemental cycles involving nutrient elements such as carbon, nitrogen, oxygen, phosphorus, and sulfur. As part of the carbon cycle, atmospheric carbon in CO, is fixed as biomass; as part of the nitrogen cycle, atmospheric N, is fixed in organic matter. The reverse of these kinds of processes is mineralization, in which biologically bound elements are returned to inorganic states. Biogeochemical cycles are ultimately powered by solar energy, which is fine-tuned and directed by energy expended by organisms. In a sense, the solar-energy-powered hydrological cycle acts as an endless conveyer belt to move mate- rials essential for life through ecosystems. Cycles of matter are also very much involved with the chemical fate and transport of pollutans. 



Cycles of matter may be divided into two major categories. Exogenic cycles occur largely on the Earth's surface and are those in which the element in question spends part of the cycle in the atmosphere-O, for oxygen, N, for nitrogen, and CO, for carbon. Endogenic cycles, notably the sulfur cycle, predominantly involve subsurface rocks of various kinds and are without a gaseous component. In general, sediment and soil can be viewed as being shared between the two cycles and constitute the predominant interface between them. All sedimentary cycles involve salt solutions or soil solutions that contain dissolved substances leached from weathered minerals; these substances may be deposited as mineral formations, or they may be taken up by organisms as nutrients.

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