Nutrient-depleted fields have little biomass carbon stock; a time-averaged modal figure is on the order of 23 t C ha-1, virtually all below ground (Table 4-8). Soil fertility replenishment practices that are based on improved fallows, rock phosphate, and biomass transfers of Tithonia diversifolia for 25 years are estimated to result in time-averaged carbon stocks of 35 t C ha-1. Such stocks are virtually all in the soil; crop and fallow accumulation may account for only 1 t C ha-1 above ground. The increase in soil carbon at equilibrium (8 t C ha-1) reflects 80 percent replenishment of the lost soil carbon. When trees are incorporated after fertility replenishment, total time-averaged stocks reach 70 t C ha-1, which includes 34 t C ha-1 in aboveground biomass and 36 t C ha-1 below ground (Table 4-8). It should be noted that all the foregoing figures are estimates rather than hard data as in the case of the humid tropical forest margins.
The transformation of low-productivity croplands to sequential agroforestry in subhumid smallholder Africa can triple carbon stocks (from 23 to 70 t C ha-1) in a 25-year period (Figure 4-11). This transformation consists of a two-stage process: first fertility replenishment, then more trees on the farm. The first stage increases carbon stocks by 9 t C ha-1-all but 1 t C ha-1 as soil carbon. The second step increases carbon stocks by an additional 28 t C ha-1-5 t C ha-1 as soil carbon and 23 t C ha-1 as aboveground biomass. Assuming such increases can take place in 46 percent (37.5 Mha) of the smallholder farms of subhumid tropical Africa (Sanchez et al. 1997b), during the next 25 years-after which we assume equilibrium is reached-this practice would provide a global contribution of 0.045-0.191 Gt C yr-1, with a modal value of 0.116 Gt C yr-1, in subhumid tropical Africa alone.
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| Figure 4-11: Project time course and system carbon stocks (biomass and soil) and time-averaged carbon stocks in sequential agroforestry systems based on soil fertility replenishment and intensification with high-value trees in subhumid tropical Africa. Based on Table 4-7 and additional data from Woomer et al. (1997). |
Carbon sequestration in subhumid Africa may therefore be considerable with land conversion to agroforestry systems that involve soil fertility replenishment and intensification and diversification of farming with the use of high-value domesticated trees. Lesser amounts can be expected in semiarid tropical areas with similar practices. The widespread use of green manure cover crops in subhumid Latin America (Bunch, 1999) and subhumid West Africa (Buckles et al., 1998) suggest considerable carbon sequestration potentials, but the rates have yet to be estimated.
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