SOIL ORGANIC-CARBON DYNAMICS - VARIABILITY WITH DEPTH IN FORESTED ANDDEFORESTED SOILS UNDER PASTURE IN COSTA-RICA

Dynamics of soil organic carbon (SOC) in chronosequences of soils belo w forests that had been replaced by grazed pastures 3-25 years ago, we re investigated for two contrasting soil types (Andic Humitropept and Eutric Hapludand) in the Atlantic Zone of Costa Rica. By forest cleari ng and subsequent establishment of pastures, photosynthesis changes fr om a C-3 to a C-4 pathway. The accompanying changes in C-input and its delta(13)C and C-14 signals, were used to quantify SOC dynamics. C-in put from root turnover at a pasture site was measured by sequential ha rvesting and C-14-pulse labelling. With a spatial resolution of 5 cm, data on total SOC, delta(13)C and delta(14)C of soil profiles were int erpreted with a model that distinguishes three pools of SOC: 'active' C, 'slow' C and 'passive' C, each with a 1(-st) Order decomposition ra te (k(a), k(s) and k(p)). The model includes carbon isotope fractionat ion and depth-dependent decomposition rates. Transport of C between so il layers was described as a diffusion process, which accounts for phy sical and biotic mixing processes. Calibrated diffusion coefficients w ere 0.42 cm(2) yr(-1) for the Humitropept and 3.97 cm(2) yr(-1) for th e Hapludand chronosequence. Diffusional transport alone was insufficie nt for optimal simulation; it had to be augmented by depth-dependent d ecomposition rates to explain the dynamics of SOC, delta(13)C and delt a(14)C. Decomposition rates decreased strongly with depth. Upon increa sed diffusion, differences between calibrated decomposition rates of S OC fractions between surface soils and subsoils diminished, but the co ncept of depth-dependent decomposition had to be retained, to obtain s mall residuals between observed and simulated data. At a reference dep th of 15-20 cm k, was 90 yr(-1) in the Humitropept and 146 yr(-1) in t he Hapludand. Slow C contributed most to total organic C in surface so ils, whereas passive C contributed most below 40 cm depth. After 18-25 years of pasture, net loss of C was 2180 g C m(-2) for the Hapludand and 150 g m(-2) for the Humitropept soil.