The spatial distribution of soil organic carbon (SOC) can be affected by environmental factors such as land use change, type of vegetation, soil redistribution processes and soil management practices. Because data are scarce in mountain agroecosystems, improving knowledge on the relationships between land use, soil redistribution processes and SOC fractions is of interest, especially in rapidly changing Mediterranean landscapes. Typically, SOC is divided into two distinct carbon fractions: the active and decomposable fraction (ACF) with rapid turnover rates, which acts as a short-term carbon reservoir, and the stable carbon fraction (SCF) with lower turnover rates that acts as a long-term reservoir. In the present study SOC, ACF and SCF contents were measured by the dry combustion method and converted to inventories expressed as mass per unit surface area (kg/m2). The SOC distribution patterns were related to land use and soil redistribution processes in soil profiles along a representative mountain agroecosystem toposequence in northeast Spain. The soil depth profiles were identified as stable, eroded and depositional sites using fallout 137caesium (Cs). Significantly higher amounts of SOC were found in forest soils (36 ± 20·2 g/kg) compared to abandoned (21 ± 14·3 g/kg) and cultivated arable land (11 ± 6·3 g/kg), suggesting that cultivation decreases SOC content. In addition, stable soil profiles had significantly higher SOC content (42 ± 24·3 g/kg) than at depositional and eroded profiles (18 ± 14·5 and 17 ± 13·1 g/kg, respectively). A positive and statistically significant relationship between SOC and 137Cs inventories suggested that both are moved and associated with similar soil redistribution processes.