Runoff-driven export of particulate organic carbon from soil in temperate forested uplands

We characterise the sources, pathways and export fluxes of particulate organic carbon (POC) in a headwater catchment in the Swiss Alps, where suspended sediment has a mean organic carbon concentration of 1.45% ± 0.06 . By chemically fingerprinting this carbon and its potential sources using carbon and nitrogen elemental and isotopic compositions, we show that it derives from binary mixing between bedrock and modern biomass with a soil-like composition. The hillslope and channel are strongly coupled, allowing runoff to deliver recent organic carbon directly to the stream beyond a moderate discharge threshold. At higher flows, more biomass is mobilized and the fraction of modern carbon in the suspended load reaches 0.70, increased from 0.30 during background conditions. Significant amounts of non-fossil organic carbon are thus transferred from the hillslope without the need for extreme events such as landsliding. Precipitation is key: as soon as the rain stops, biomass supply ceases and fossil carbon again dominates. We use rating curves modeled using samples from five storm events integrated over 29-year discharge records to calculate long-term export fluxes of total POC and non-fossil POC from the catchment of 23.3 ± 5.8 and 14.0 ± 4.4 tonnes km-2 yr-1 respectively. These yields are comparable to those from active mountain belts, yet the processes responsible are much more widely applicable. Such settings have the potential to play a significant role in the global drawdown of carbon dioxide via riverine biomass erosion, and their contribution to the global flux of POC to the ocean may be more important than previously thought.