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The largest flux in the global carbon cycle is the uptake of CO2 by plants as photosynthesis. Estimates of gross primary production (GPP), or total amount of CO2 assimilated by terrestrial plants, range between about 109 and 120 Pg C per year at the global scale (Schlesinger, 1997; Zhao et al., 2005). Except for carbon that remains stored in passive organic matter pools, as fossil fuel, and an estimated 0.2 Pg C per year sedimenting on the ocean floors, assimilated carbon is eventually returned to the atmosphere by respiration, either by plants or by heterotrophic organisms. The time between the fixation of a carbon atom by the plant and its conversion back to CO2 is extremely variable, ranging between a few hours and thousands of years. How long it remains part of organic compounds will depend on its turnover within the plant and, eventually, as part of soil organic matter.
Carbon assimilated by plants is translocated to plant organs where it can be used as building material for structural biomass, for storage or as substrate for respiration. Carbon imported into roots can also be exudated or transferred to symbionts such as mycorrhizal fungi (Farrar, 1999). The amount of assimilated carbon used for each purpose will depend on the plant's requirements, which are further determined by plant and environmental factors.
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