Spatial variations of sedimentary organic carbon associated with soil loss influenced by cascading dams in the middle Lancang River

•Lower cascade Reservoir preserved higher OCsed than upper Reservoir.•RUSLE and HSDR method can estimate the sediments delivered to the streams.•Sediments from lands decreased with the distance away the river channel.•Sediment loadings were correlated with OCsed under different delivery ratios.

Construction and operation of cascading dams can pose drastic effects on sediment transport and deposition, thus affecting sedimentary carbon concentrations in the riverine ecosystem. This paper examines the spatial variations of sedimentary organic carbon (OCsed) and its potential relationship with terrestrial soil loss in two cascade reservoirs (Manwan and Dachaoshan) along the mainstream of Lancang River. During both wet and dry seasons in 2013 and 2014, 25 sampling sections were chosen to evaluate the spatial variations of sedimentary organic carbon. Generally, the contents of OCsed in the upper 5 cm of the bottom sediment in wet seasons were higher than those in dry seasons in both reservoirs. Spatially, lower reservoir (Dachaoshan) preserved higher OCsed content than the upper reservoir (Manwan), at 36.1 kg m−2 and 27.8 kg m−2, respectively. Additionally, the contents of OCsed in the reservoir heads were higher than those in the reservoir centres and tails, this trend was also observed for terrestrial organic carbon (OCter) and liable organic carbon (LOC). Hillslope delivery ratio (HSDR) method are used to estimate the incoming sediment yields from the river basin. The results indicated that Dachaoshan catchment generated higher sediment stream deposits than the Manwan catchment with the high sediment loading (>40 t/ha/year) areas located within 3 km buffers encompassing a higher accumulation of farmlands. The cumulative effects of sediment transport on OCsed along the river channel were further investigated by setting sequential riverine sediment delivery ratios (0-0.55) to explore potential relationships between OCsed and total sediment delivered to stream. The results showed that in the Dachaoshan Reservoir the OCsed contents correlated with the total sediment delivered to streams when the ratios ranged from 0.10 to 0.55, in particular in delivery ratios ranging from 0.50 to 0.55. These findings emphasized the importance of direct and indirect connections between the terrestrial and riverine ecosystems, and also provided a preferable view to understand the cumulative effects of sediment transport on OCsed distributions in the reservoirs generated by cascading dams.