Nutrient circuits involve both biotic and abiotic

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Nutrient circuits involve both biotic and abiotic components of ecosystems and are called biogeochemical cycles. There are two general categories of biogeochemical cycles: global and regional. Gaseous forms of carbon, oxygen, sulfur, and nitrogen occur in the atmosphere, and cycles of these elements are global. Elements that are less mobile in the environment, such as phosphorus, potassium, calcium, and trace elements generally cycle on a more localized scale in the short term. Soil is the main abiotic reservoir for these elements. We will consider a general model of chemical cycling that includes the main reservoirs of elements and the processes that transfer elements between reservoirs. Each reservoir is defined by two characteristics: whether it contains organic or inorganic materials and whether or not the materials are directly available for use by organisms. Reservoir a. The nutrients in living organisms and in detritus are available to other organisms when consumers feed and when detritivores consume nonliving organic material. Reservoir b. Some materials move to the fossilized organic reservoir as dead organisms and are buried by sedimentation over millions of years. Nutrients in fossilized deposits cannot be assimilated directly. Reservoir c. Inorganic elements and compounds that are dissolved in water or present in soil or air are available for use by organisms. Reservoir d. Inorganic elements present in rocks are not directly available for use by organisms. These nutrients may gradually become available through erosion and weathering. Describing biogeochemical cycles in general terms is much simpler than trying to trace elements through these cycles. Ecologists study chemical cycling by adding tiny amounts of radioactive isotopes to the elements they are tracing. There are a number of important biogeochemical cycles. We will consider the cycling of water, carbon, nitrogen, and phosphorus. The water cycle Biological importance Water is essential to all organisms and its availability influences rates of ecosystem processes. Biologically available forms Liquid water is the primary form in which water is used. Reservoirs The oceans contain 97% of the water in the biosphere.
2% is bound as ice, and 1% is in lakes, rivers, and groundwater. A negligible amount is in the atmosphere. Key processes The main processes driving the water cycle are evaporation of liquid water by solar energy, condensation of water vapor into clouds, and precipitation. Transpiration by terrestrial plants moves significant amounts of water. Surface and groundwater flow returns water to the oceans. The carbon cycle Biological importance Organic molecules have a carbon framework.

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