ORGANIC-CARBON TURNOVER IN 3 TROPICAL SOILS UNDER PASTURE AFTER DEFORESTATION

Deforestation is one of the main reasons for the global net release of CO2 from soil to atmosphere. Estimates of CO2 emission from soils are highly variable, mainly due to limited data of C dynamics in soils after forest clearing. The objective of this study was to calculate the changes in soil organic carbon (SOC) storage after deforestation in three soil types in the Atlantic Zone of Costa Rica with help of the deltaC-13 method. Changes in bulk density, which normally accompany land use changes, had a profound influence on the results of the calculations. Deforestation, followed by 25 yr of pasture, caused a net loss of 21.8 Mg ha-1 in SOC for an Eutric Hapludand and 1.5 Mg ha-1 for an Oxic Humitropept. The SOC changes in time were studied on a deforestation sequence on an Andic Humitropept. In the first years after forest clearing, decomposition of tree roots caused an extra input of SOC, which influenced the deltaC-13 signal. Decomposition of forest C and increase of pasture C were mathematically described for several depths. A considerable influence of depth on decomposition rates was found. The strong stabilization of organic C by Al-organic matter complexes probably caused the relatively small net C loss from SOC since forest clearing.