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Dynamics of soil organic carbon (SOC) in chronosequences of soils below forests that had been replaced by grazed pastures 3-25 years ago,were investigated for two contrasting soil types (Andic Humitropept and Eutric Hapludand) in the Atlantic Zone of Costa Rica. By forest clearing and subsequent establishment of pastures, photosynthesis changes from a C-3 to a C-4 pathway. The accompanying changes in C-input and its delta13C and 14C signals, were used to quantify SOC dynamics. C-input from root turnover at a pasture site was measured by sequential harvesting and 14C-pulse labelling. With a spatial resolution of 5 cm, data on total SOC, delta13C and delta14C of soil profiles were interpreted with a model that distinguishes three pools of SOC: 'active' C, 'slow' C and 'passive' C, each with a 1-st order decomposition rate (ka, ks and kp). The model includes carbon isotope fractionation and depth-dependent decomposition rates. Transport of C between soil layers was described as a diffusion process, which accounts for physical and biotic mixing processes.Calibrated diffusion coefficients were 0.42 cm2 yr-1for the Humitropept and 3.97 cm2 yr-1 for the Hapludand chronosequence. Diffusional transport alone was insufficient for optimal simulation; it had to be augmented by depth-dependent decomposition rates to explain the dynamics of SOC, delta13C and delta14C. Decomposition rates decreased strongly with depth. Upon increased diffusion, differences between calibrated decomposition rates of SOC fractions between surface soils and subsoils diminished, but the concept of depth-dependent decomposition had to be retained, to obtain small residuals between observed and simulated data. At a reference depth of 15-20 cm ks was 90 yr-1 in the Humitropept and 146 yr-1 in the Hapludand. Slo [ABSTRACT FROM AUTHOR]