Anthropogenic and environmental controls on particulateorganic carbon fluxes and sources in the Kasai Basin, DRCongo

Abstract

Erosion of upland soils represents a potentially large but relatively unconstrained flux of sediment and organic carbon (OC) to river systems. This flux is particularly important in the African tropics, where population growth, accelerating agricultural land use, and climate change lead to substantial erosion rates. However, few studies to date have investigated how climatic or anthropogenic factors impact sediment yields and OC signatures in tropical Africa. The Kasai Basin, DR Congo, is the largest source of sediment to the Congo River and exhibits a clear gradient in climatic (biomes, and rainfall) and anthropogenic (agricultural land-use change) parameters; it is therefore an ideal study area to explore how these factors influence sediment yields, POC yields and sources of POC. To provide new insight, we quantified total suspended sediment (TSS) yields, particulate OC (POC) yields, and stable and radiocarbon isotopic signatures of POC from 32 rivers within the Kasai Basin that span a range of both climate and land-use gradients. These sampled during the wet and dry season. From initial results, we observe that rivers draining predominantly forested ecosystems yielded 3.0±0.2 kg TSS km -2 y -1 while the savannah regions yielded an average of 26.7 ± 41.2 kg TSS km -2 y -1 . Preliminary POC data showed that rivers draining the woodland mosaics ecosystems yielded an average of 0.7±0.5 kg POC km -2 y -1 with an average fraction modern of 0.9274±0.04. In contrast, the average fraction modern of POC in rivers draining savannah ecosystems was 0.889±0.09. From this preliminary data, we show that the yield of TSS and the age of POC is lower in tropical rainforest-draining rivers) compared to savannah draining rivers, indicating the slower erosion of topsoil compared to faster erosion of deeper mineral soils. Older POC was found in rivers draining areas with high anthropogenic activity carbon likely due to soil disturbance and erosion of subsoil. We conclude that the type of biome and the extent of anthropogenic impact influences the quantity and composition of POC. With this preliminary data, this study provides better understanding of how climate and land-use influences C export to tropical rivers.