Soil organic carbon dynamics: variability with depth in forested and deforested soils under pasture in Costa Rica.

Dynamics of soil organic carbon (SOC) inchronosequences of soils below forests that had beenreplaced by grazed pastures 3–25 years ago, wereinvestigated for two contrasting soil types (AndicHumitropept and Eutric Hapludand) in the Atlantic Zoneof Costa Rica. By forest clearing and subsequentestablishment of pastures, photosynthesis changes froma C-3 to a C-4 pathway. The accompanying changes inC-input and its δ13C and 14Csignals, were used to quantify SOC dynamics. C-input from rootturnover at a pasture site was measured by sequentialharvesting and 14C-pulse labelling. With aspatial resolution of 5 cm, data on total SOC,δ13C and δ14C of soil profileswere interpreted with a model that distinguishes threepools of SOC: ‘active’ C, ‘slow’ C and ‘passive’ C,each with a 1-st order decomposition rate(ka, ks and kp). The modelincludes carbon isotope fractionation and depth-dependentdecomposition rates. Transport of C between soillayers was described as a diffusion process, whichaccounts for physical and biotic mixing processes.