Materials Used by Organisms Are Recycled

The Earth is essentially a closed chemical system through which the elements necessary for life are reused and move from abiotic reservoirs to the biota and back in global biogeochemical cycles. Some elements are held as gases in the atmosphere, others are components of the lithosphere (rocks and soil of the Earth's crust), many move through the hydrosphere (marine and freshwaters) before or after their sojourn in the biosphere (the living components).

The cycles through the lithosphere are said to be sedimentary cycles (from the sedimentary rocks in which the elements reside) and are of such long duration that the elements are essentially removed from further cycling until tectonic (mountain building) or volcanic eruptions expose the rock layers to new weathering. Elements have shorter residence times in the air in the atmospheric cycles and generally the least of all in the biota. A surprisingly small amount of the world's matter is held in living organisms at any time; the reservoirs for the elements of concern to life are almost entirely abiotic ones.

Plants are more than merely users of the chemicals of the Earth; through their metabolic processes they exert a considerable influence on the cycling of the major chemicals. Plants have been indispensable through deep (geologic) time in maintaining the steady‐state condition of most of the biogeochemical cycles. All of the ecologically significant chemical elements have both an abiotic and a biotic component. Carbon, hydrogen, and oxygen enter plants from the air and from the decomposition of organic matter, but the other 14 essential nutrients are taken from the soil, as are the miscellaneous other elements used in small amounts by a variety of organisms. Nutrients released from weathered rocks enter the soil solution and move by diffusion and mass flow to the sites of biological activity. Rock weathering is a long‐term process that adds small quantities of minerals slowly, over time to the ecosystem. Plants and other organisms, therefore, obtain most of the minerals they need by recycling existing organic matter.