2.4.3.4. Horizontal Fluxes of Carbon
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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