Understanding organic carbon dynamics in a river catchment through improved sediment fingerprinting

• OC sediment fingerprinting characterised seasonal dynamics in sediment sources. • Tracer conservativeness was assessed by comparing fingerprinting of suspended and bed sediments. • Arable soil dominant contributor to suspended sediment OC ranging from 37% to 61%. • Seasonal source variations linked to higher rainfall, livestock trampling and sparse vegetation. Agricultural practices accelerate the rates of soil erosion and organic carbon (OC) loss, increasing the input of nutrient rich sediment to surface waters. As climate change is increasing the frequency and intensity of hydrological disturbances that drive erosion, it is of vital importance to quantify the terrestrial to aquatic fluxes of OC to inform sustainable management strategies and mitigate the impacts of soil OC loss in river catchments. In this study, OC sediment fingerprinting was used to determine seasonal sources of sediment to a freshwater stream from different land uses in a river catchment. Multiple lines of evidence (soil and stream sediment sampling, local climate and agronomic data) were used to evaluate tracer properties and sources in order to improve the sediment fingerprinting technique. Within a mixed land-use catchment, four potential sources of sediment (arable, forest, pasture and moorland) were characterised between June 2018 and December 2019. Spatio-temporal differences in OC sources were observed at different times of year. Arable soil was the dominant contributor to suspended sediment OC, ranging from 37% to 61% at the catchment outlet. Increased rainfall, discharge, livestock poaching, and bare or sparsely vegetated areas were found to be the drivers of change in seasonal sources of sediment relative to land use. This study demonstrated a holistic approach to inform sustainable catchment management; using multiple lines of evidence to improve the characterisation of sediment sources and highlight remaining uncertainties in the sediment fingerprinting technique.