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Soils play an important role on the global carbon cycle, but conventional land use practices generate negative impact by reducing soil organic carbon (SOC) content. Studies regarding the use of mathematical models on the magnitude of such impacts are scarce in semi-arid regions, but they are essential to broaden the understanding of the effects of cropping systems and help in proposing more rational land use alternatives. However, mathematical models (e.g. Century) must be previously calibrated and validated to present satisfactory results. Therefore, the aim of the present study was to validate the Century model for simulating SOC dynamics in areas of native vegetation and estimating SOC stocks on the adoption of agricultural systems in the semi-arid region of Brazil with a previous calibrated model. The study was carried out in three soil types (Quartzipsamments, Psamments and Ultisol), located in the state of Alagoas, north-eastern Brazil, adopting conventional (with minimal use of machinery) agricultural land-use practices under rainfed conditions. Simulations consisted of validating the model with previously calibrated parameters from another area of the Brazilian semi-arid region. The results showed that the model proved to be effective in simulating SOC dynamics in areas of native vegetation in the semi-arid region of Alagoas (PBIAS ranging from 20 to −9%); however, it needs further adjustment for satisfactory SOC simulation in agricultural/pasture systems. The results showed that conventional systems of agricultural crops and pasture in the semi-arid region of Brazil lead to losses (2673 g C m−2 in native vegetation to 1960 g C m−2 in conventional systems) between 22% and 35%, (PBIAS variating −9 to 49%) and that SOC stocks reach a new steady state between 30 and 40 years after conversion from native vegetation into agriculture (ranging from 24 to 42% in different soil types). [ABSTRACT FROM AUTHOR]