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General Aspects of BiosphereThe energy required to drive nutrient cycling is obtained by processes occurring in the biosphere. The primary source of energy for ecosystem processes is photosynthesis. Directly or indirectly, photosynthesis provides the energy for all forms of life in the biosphere. Photosynthesis utilizes energy from the sun to reduce CO2 into high energy organic compounds. Subsequently, the energy contained in these organic compounds is released by the process of respiration. The respiration process releases energy by oxidizing the high energy bonds of the organic compounds, the energy released being used to perform all the metabolic processes of biota.To maintain all the biochemical reactions necessary for plant growth, at least 14 essential mineral nutrients are required from the geosphere. These nutrients are extracted from the soil by plant uptake and returned to the soil directly or indirectly as organic matter. The organic matter in turn is a source of energy for heterotrophs which further oxidize the organic compounds by decomposition processes (i.e., by respiration), simultaneously releasing the mineral nutrients back to the soil. Thus, the biosphere processes of nutrient uptake, incorporation of mineral nutrients into biological tissues, litterfall, and the decomposition of organic matter with the concomitant release of nutrients by microorganisms are essential to the cyclic flow of nutrients in terrestrial ecosystems.Biosphere processes are intimately linked with processes in the atmosphere, hydrosphere, and geosphere. Biological processes play a major role in regulating atmospheric CO2 concentrations through the balance between photosynthesis and respiration. Microorganisms are responsible for several other oxidation/reduction reactions consuming (e.g., N2 by N fixation) and producing (e.g., N2O and N2 by denitrification) atmospheric gases. The hydrologic cycle is strongly influenced by transpiration, consuming water from the soil and returning it directly to the atmosphere. Biological processes have a very important influence on the geosphere, especially on weathering rates which are enhanced 3-10 times by the action of biota. The action of roots breaking apart rocks is an important agent of physical weathering. Chemical weathering is greatly enhanced through the production of acids (e.g., carbonic acid - CO2 released by root and microbial respiration and organic acids) and chelating agents. Accumulation of organic matter/humus in soils is a major soil forming processes by which plants are able to alter the soil environment in which they live. In turn, the storage of organic matter in soils is a major sink for atmospheric CO2, thus having a feedback on atmospheric processes.


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Biological reactions in terrestrial ecosystems add a tremendous amount of complexity that does not exist in an abiotic environment. As a result, attempts to produce simulation models that accurately capture the detailed complexity of biogeochemical processes in terrestrial ecosystems are very difficult due to all the linkages and feedbacks occurring in these systems.
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