Soil carbon sinks resulting from sequestration activities are not permanent and will continue only for as long as appropriate management practices are maintained. If a land management or land-use change is reversed, the carbon accumulated will be lost, usually more rapidly than it was accumulated (Smith et al., 1996). For the greatest potential of soil carbon sequestration to be realized, new carbon sinks, once established, need to be preserved in perpetuity. Within the Kyoto Protocol, mechanisms have been suggested to provide disincentives for sink reversal, i.e. when land is entered into the Kyoto process it has to continue to be accounted for and any sink reversal will result in a loss of carbon credits.
Soil carbon sinks increase most rapidly soon after a carbon-enhancing land management change has been implemented, but soil carbon levels may decrease initially if there is significant disturbance (e.g. when land is afforested). Sink strength (i.e. the rate at which carbon is removed from the atmosphere) in soil becomes lesser with time as the soil carbon stock approaches a new equilibrium. At equilibrium, the sink is saturated: the carbon stock may have increased, but the sink strength has decreased to zero.
The time taken for sink saturation (i.e. new equilibrium) to occur is highly variable. The period for soils in a temperate location to reach a new equilibrium after a land-use change is ~100 years (Jenkinson, 1988; Smith et al., 1996) but tropical soils may reach equilibrium faster. Soils in boreal regions may take centuries to approach a new equilibrium. As a compromise, good practice guidelines give 20 years for soil carbon to approach a new equilibrium (IPCC, 1997; Paustian et al., 1997).
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