Soil carbon dynamics in the humid tropical forest zone

Abstract

Conversion of natural forests to agriculture in the humid tropics leads to a reduction in ecosystem carbon storage due to the immediate removal of aboveground biomass and a gradual subsequent reduction in soil organic carbon. A considerable part of soil carbon is protected from microbial attack by a range of physical and chemical mechanisms and is not sensitive to landuse change. We analyzed the soils data base for Sumatra (Indonesia) developed by the Center for Soil and Agroclimate Research (CSAR) to estimate effects of landuse on soil C content. Sumatra has a considerable diversity of soils ranging from those of recent origin in the highlands, to older sedimentary and heavily leached soils in the pedimont peneplain and large areas of wetland soils along the coast. Peat soils (Histosols) and other wetland soils (Aquic and Fluvic suborders) contain the greatest soil C reserves, followed by young volcanic soils (Andisols). Agricultural use of these soils can have a disproportionately large effect on C release to the atmosphere. On the major part of the upland soils the difference in (top) soil C content between natural forest and agricultural land is in the range 0.5–1.0% C, equivalent to a change in total C stock of 10–20 Mg ha −1. These results agree with data collected in S. Sumatra in the 1930s. C org of forest soils is related to soil pH, and is lowest in the pH range 5.0–6.0. Wetland conditions, lower pH, higher altititude (lower temperature) and higher clay and silt content all contributed to higher soil C contents in a multiple regression analysis of the whole data set. Existing models and data sets are insufficient to predict changes in soil C contents under various landuse practices. Carbon isotope studies, and especially the δ 13C method may be used to study the effects of landuse change, especially when the vegetation was changed from one dominated by C3 plants (most forest species) to one dominated by grasses and crops with a C4 photosynthetic pathway. Results from Brazil documented a gradual decline of organic matter originating from the forest system and its partial replacement by organic matter derived from inputs of sugarcane during the first fifty years of cultivation. Forest conversion to well managed grasslands may lead to an increased soil C storage, after an initial decline. The consequences of erosion on losses of soil C depend on the scale at which these losses are considered, because of sedimentation processes. When net erosion losses are not expressed per unit area, but per length scale to the power l.6, erosion losses appear to be largely independent of scale. The `fractal dimension of erosion’ (on average around 1.6) probably is a landscape characteristic and estimates of its value are needed for extrapolation. Better understanding of soil C deposition sites is needed to evaluate overall erosion effects and test whether or not erosion can contribute to net C sequestration.