Carbon may be lost from ecosystems in the form of dissolved or particulate organic compounds that are found in runoff waters and during the erosion of soils that often accompanies human land use. Some carbon may also be lost in the deep seepage of soil waters that enter groundwater. Typically, the loss of dissolved organic compounds is a small fraction of the pool of carbon in soils or the changes in soil carbon storage that result from human activities. Globally, about 1-10 g C m-2 yr-1 [0.01-0.10 t C ha-1 yr-1] are lost in surface runoff from natural ecosystems (Schlesinger and Melack, 198l; Kortelainen et al., 1997). Losses of soil carbon from agricultural soils are dominated by oxidation (Schlesinger, 1986). In the case of soil erosion, a large fraction of the organic matter that is lost may be deposited in floodplain and riparian habitats in downstream locations (Lal, 1995; Dean and Gorham, 1998; Stallard, 1998). The accumulation of soil carbon in these ecosystems can be assessed using sampling methods outlined above. It is important to recognize, however, that increments in these deposits do not necessarily represent net sequestration of atmospheric CO2.
Horizontal fluxes of carbon are generally missed by atmospheric measurements of flux, which points to the fact that exchanges of carbon between the atmosphere and the land surface may not be equivalent to changes in the terrestrial storage of carbon.
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