In smallholder agriculture, a three-step process drives C changes. Initially, in shifting cultivation, C and nutrient stocks accumulated in natural vegetation are mobilized through land conversion (Nye and Greenland, 1960; Sanchez and van Houten, 1994). Clearing land for agriculture involves removing large volumes of vegetation, often followed by burning to enhance access to the land for cultivation. This causes considerable loss of above-ground biomass C to the atmosphere. The nonvolatile plant nutrients are concentrated in the ash, and incompletely combusted materials cover or enter the soil (Ewel et al., 1981; Araki, 1993; Kauffman, et al., 1995). The soil resource base is utilized for crop production for a number of years while nutrient-rich mineralized organic matter and root residues decompose. As the SOM declines, yields also decline. The land is then abandoned to fallow for reaccumulation of C and nutrients in vegetation and soils. With increasing population pressure, the fallow period is curtailed almost to continuous cropping. As a final step in soil C dynamics, the labile SOM becomes mineralized as the lower-level equilibrium of the SOM. External inputs become increasingly crucial in crop productivity (Swift et al., 1994), or else the farmers abandon the land.
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