Carbon Sequestration and Turnover in Semiarid Savannas and Dry Forest

Data on carbon and biomass budgets under different land use in tropical savannas and some dry forests are reviewed. Global data show wide ranges of biomass carbon stocks (20-150 Mg C ha-1), net primary production (2-15 Mg C ha-1y-1) and litter production (2-10 Mg C ha-1y-1) for the semiarid tropics. Although ranges for soil carbon are also 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 for NE Brazil and W. Africa.In NE Brazil approximately 40% of the lands have "near-climax" native vegetation. Less than 10% of the area is planted annually, but about 3-4 times that area is affected by shifting cultivation which has an average cycle of 5 y arable use followed by 20 y or more recovery. Standing biomass of native caatinga shows nearly the full global range with 2-50 Mg C ha-1. Litter fall around 1-2 Mg C ha-1y-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 ha-1. Under cultivation, C sequestration is decreased, and soils lose approximately half their C stocks before being abandoned.In W. Africa between 50-70% of the land is under a management regime with minimal C returns to the soils. Overgrazing and over-exploitation for fuel wood has resulted in land degradation. Short fallow periods on cultivated lands have caused serious declines in soil C stocks. Both C sequestration and stocks are therefore lower in W. Africa than in NE Brazil.Improvements in the C sequestration in these semi arid regions depend on an increase in crop production under suitable rotations, improved fallow and animal husbandry, and a limitation on biomnass burning. Use of fertilizer is required for improved productivities but socioeconomic constraints largely prevent such improvements, resulting in a very limited scope for changes in soil C management.