The effects of land use changes on soil carbon stocks are a matter of concern stated in international policy agendas on the
mitigation of greenhouse emissions. Afforestation is increasingly viewed as an environmental restorative land use change prescription
and is considered one of the most efficient carbon sequestration strategies currently available. Given the large quantity
of CO2 that soils release annually, it is important to understand disturbances in vegetation and soil resulting from land
use changes. The main objective of this study is to assess the effects of land abandonment, land use change and afforestation
practices on soil organic carbon (SOC) dynamics. For this aim, five different land covers (bare soil, permanent pastureland,
secondary succession, Pinus sylvestris (PS) and Pinus nigra (PN) afforestation), in the Central Spanish Pyrenees, were analysed.
SOC dynamics have been studied in the bulk soil, and in the fractions separated according to two methodologies: (i) aggregate
size distribution, and (ii) density fractionation, and rates of carbon mineralization have been determined by measuring CO2
evolution using an automated respirometer. The results showed that: (i) SOC contents were higher in the PN sites in the topsoil
(10 cm), (ii) when all the profiles were considered no significant differences were observed between pastureland and PN, (iii)
SOC accumulation under secondary succession is a slow process, and (iv) pastureland should also be considered due to the relative
importance in SOC stocks. The first step of SOC stabilization after afforestation is the formation of macro-aggregates promoted
by large inputs of SOC, with a high contribution of labile organic matter. However, our respiration experiments did not show
evidence of SOC stabilization. SOC mineralization was higher in the top layers and values decreased with depth. These results
gain insights into which type of land management is most appropriate after land abandonment for SOC.