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Soils are a major location of carbon (C) storage in most terrestrial ecosystems and changes in soil C pools therefore play an important role in the C cycle. Human conversion of native ecosystems to agro-ecosystems, overgrazing and lack of agricultural management influence the amount, quality and turnover of soil organic carbon (SOC). This paper focuses on changes in reactivity, distribution and total stocks of SOC in the upper 1 m of soils representing savanna as well as groundnut (Arachis hypogaea L.) and millet (Pennisetum glaucum L.) fields cultivated for up to 40 years within the semi-arid part of Senegal. The soil C stock varied from 4.4 kg C m(-2) in open woodland savanna to 2.7 kg C m(-2) in grass savanna. Clearing and cultivation of savanna land has-resulted in a loss of SOC up to 24% in the upper 1 m equal to 1.1 kg C m(-2) over 40 years, or more than six times the short-term C loss associated with clearing of above-ground biomass in nowadays semi-arid savanna in Senegal. Observed SOC losses the first 4-8 years following cultivation were consistent for all sites, whereas observed long-term losses suggest important variations related to a north-south (N-S) climatic gradient across the region. Laboratory respiration measurements reveal similarity with respect to SOC reactivity, which is consistent with the observations of significant higher Soil CO2 effluxes from savanna soils than cultivated soils. The results call for further attention on region-specific changes in SOC stocks (time scales) due to cultivation and improved understanding of the long-term controls of SOC cycling after clearing of tropical savanna. (C) 2003 Elsevier Science B.V. All rights reserved.