Geochemical characteristics and fluxes of organic carbon in a human-disturbed mountainous river (the Luodingjiang River) of the Zhujiang (Pearl River), China

This study aims to investigate the state of the riverine organic carbon in the Luodingjiang River under human impacts, such as reforestation, construction of reservoirs and in-stream damming. Seasonal and spatial characteristics of total suspended sediment (TSS), dissolved organic carbon (DOC) and particulate organic carbon (POC), as well as C/N ratios and the stable carbon isotopic signatures of POC (δ13CPOC) were examined based on a one-year study (2005) in the basin-wide scale. More frequent sampling was conducted in the outlet of the river basin at Guanliang hydrological station. DOC and POC concentrations showed flush effects with increasing water discharge and sediment load in the basin-wide scale. Atomic C/N ratio of POC had a positive relationship with TSS in the outlet of the basin, indicating the reduced aquatic sources and enhanced terrestrial sources during the high flood season. However, the similar relationship was not observed in the basin-wide scale mainly due to the spatial distributions of soil organic carbon and TSS. δ13CPOC showed obvious seasonal variations with enriched values in the period with high TSS concentration, reflecting the increased contribution from C4 plants with enhanced soil erosion. The specific flux of the total organic carbon (2.30 t km-2 year-1) was smaller than the global average level. The ratio of DOC to POC was 1.17, which is higher than most rivers under Asian monsoon climate regime. The organic carbon flux was estimated to decline with decreasing sediment load as a result of reforestation, reservoir construction and in-stream damming, which demonstrates the impacts of human disturbances on the global carbon cycle