Carbon Stocks in Natural Ecosystems and C Loss with Land Conversion in Africa

Total C stocks in Southern and Eastern Africa have been estimated to be 55.4 to 275 metric tons per hectare, respectively, in the humid forest and miombo woodland (a savannahlike deciduous woodland), and varies with precipitation (White, 1983; Woomer et al., 1998). Generally, the dead to live biomass and belowground to aboveground C ratios increase with aridity (White, 1983; Woomer et al., 1998). Of the total system C, 23%, 50%, and 65%, respectively, occurs below ground in the humid forest, Guinea savanna, and broadleaf (woody) savanna (Woomer, 1993). Although the drier areas tend to be more susceptible to burning, the amount of system C lost from burning is less because a greater proportion of biomass is present as coarse roots and in soil. Although there is biomass destruction prior to cultivation, the total soil C pools remain relatively stable throughout the slash-and-burn cycle, with only a slight decrease in the initial 4 years, and then increase to initial levels within 13 additional years of natural re-growth (Woomer et al., 1998). In Northern Zambia, where the practice of slash-and-burn agriculture is on the increase due to expensive mechanical clearing costs, the loss of C, biota, and crop biomass has accelerated in the past decade (Goma, 1999). SOM loss may be small, but the system C loss may be massive under slash-and-burn and continuous cultivation. Hence, as sometimes practiced in East and Southern Africa, continuous cultivation has caused large and more permanent changes in soil C, ranging from 0.9 metric tons ha-1 year-1 to 10 metric tons ha-1 year-1. Lands that have been recently converted to agriculture show the greatest decline (Wissen, 1974; Woomer, 1993).

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