CARBON SEQUESTRATION AND TURNOVER IN SEMIARID SAVANNAS AND DRY FOREST

Data on carbon and biomass budgets under different land use in tropica l savannas and some dry forests are reviewed. Global data show wide ra nges of biomass carbon stocks (20-150 Mg C ha(-1)), net primary produc tion (2-15 Mg C ha(-1)y(-1)) and litter production (2-10 Mg C ha(-1)y( -1)) for the semiarid tropics. Although ranges for soil carbon are als o wide, an average figure for the top 20 cm is probably 10 g C kg(-1), or about 25 Mg C ha(-1). In order to arrive at a better understanding of C budgets and their controls, two regional reviews are presented f or NE Brazil and W. Africa. In NE Brazil approximately 40% of the land s have ''near-climax'' native vegetation. Less than 10% of the area is planted annually, but about 3-4 times that area is affected by shifti ng cultivation which has an average cycle of 5 y arable use followed b y 20 y or more recovery. Standing biomass of native caatinga shows nea rly the full global range with 2-50 Mg C ha(-1), Litter fall around 1- 2 Mg C ha(-1)y(-1) is partly decomposed and partly consumed by animals , resulting in low average soil C levels near 8 g kg(-1), or 20 Mg C h a(-1). Under cultivation, C sequestration is decreased, and soils lose approximately half their C stocks before being abandoned. In W. Afric a between 50-70% of the land is under a management regime with minimal C returns to the soils. Overgrazing and over-exploitation for fuel wo od has resulted in land degradation. Short fallow periods on cultivate d lands have caused serious declines in soil C stocks. Both C sequestr ation and stocks are therefore lower in W. Africa than in NE Brazil. I mprovements in the C sequestration in these semi arid regions depend o n an increase in crop production under suitable rotations, improved fa llow and animal husbandry, and a limitation on biomnass burning. Use o f fertilizer is required for improved productivities but socioeconomic constraints largely prevent such improvements, resulting in a very li mited scope for changes in soil C management.