Soil organic carbon dynamics in pastures established after deforestation in the humid tropics of Costa Rica

Currently, rates of deforestation in the tropics are probably higher than ever before in the past. As a consequence, changes in the earth's physical and chemical environments are proceeding at unprecedented rates. Increasing atmospheric concentrations of CO 2 , N 2 O and other trace gases, caused by enhanced emissions from soils after forest clearing, show that deforestation in tropical areas is of global importance. Recent estimates suggest a net release of carbon from the world's tropics, due to deforestation, of between 0.42 and 1.60 Pg C yr -1(1 Pg = 10 15g) of which 0.1 to 0.3 Pg C yr -1are attributed to decreases in soil organic matter content. This carbon release from tropical areas is second only to the global release from the burning of fossil fuels (which is about 5.3 Pg C yr -1).The main objective of this thesis was to quantify the changes in soil organic carbon storage and the resulting release of CO 2 after the conversion of tropical rain forest to pasture on two contrasting soil types in the humid tropics of Costa Rica. To study changes in soil organic carbon storage, sites of an Andisol and an Inceptisol, cleared at different times in the past (deforestation sequences) were compared. A deforestation map, based on aerial photographs from the period 1952 - 1984, was made for a part of the Atlantic Zone of Costa Rica, providing a well documented history of forest clearing. Using GIS techniques, this deforestation map was combined with an available soil map to select the study sites. Analysis of deforestation patterns on the map demonstrated a close relation of deforestation rate with accessibility and soil quality.Soil organic matter levels are the result of complex production and decomposition processes. The input of carbon from grass plant roots into the soil was quantified, using pulse labelling with 14C. The pulse labelling experiment revealed that root dry matter production of an improved pasture like Brachiaria (12 Mg ha -1yr -1) was about twice the root production of a low-productive species like Axonopus (6 Mg ha-1 yr-1). Root biomass of Brachiaria was about three times the root biomass of Axonopus due to higher residence time of carbon in the root biomass of Brachiaria as compared to Axonopus . Root exudates of grass plants were found to have a minor direct contribution to the longer term carbon dynamics, either because exudation rate was small or because decomposition was fast and complete.Decomposition of soil organic matter was measured using the δ 13C method, which uses differences in natural 13C isotope levels in vegetation (C3 and C4 vegetation) and soil organic matter to calculate changes in soil organic carbon. The method is applicable in soil organic matter studies where a change from C3 to C4 vegetation has occurred (or vice versa). It was demonstrated that for a correct application of the method, detailed information of changes in bulk densities accompanying changes in land use was vital. An uncertainty analysis of the δ 13C method demonstrated that the output of the δ 13C method in soil organic matter studies was highly variable due to variations in the input data. Spatial variability was the main source of the uncertainty in input data. However, variations due to sampling error and short scale variability were considerable and should not be ignored.Information on carbon input and decomposition was integrated, using a simple structured soil organic carbon (SOC) model which included carbon isotope fractionation during decomposition and depth dependent decomposition and humification rates. With this model, the observed changes in soil organic carbon and corresponding δ 13C levels during the conversion from a humid tropical forest to a cattle pasture were simulated successfully for the two soil types. With the calibrated model the cumulative net C02 release was calculated. The cumulative net release of CO 2 for pastures with low productive grass species (Axonopus compressus), varied from 31.5 (Humitropept) to 60.5 Mg C ha -1(Hapludand) in the first 20 years after forest clearing. These cumulative emissions could be reduced to 12.0 and 24.7 Mg C ha -1respectively, if higher productive grass species (e.g. Brachiaria dictyoneura ) would be introduced into the area.Decomposition rates were strongly influenced by depth. Inclusion of deeper layers in soil organic carbon simulation studies and considering carbon isotopes will probably improve the performance of SOC models in long-term studies.