Decoupled spatial hotspots of carbon loss and sediment yield on typical black soil slopes

Soil erosion strongly regulates terrestrial carbon (C) cycling; however, the spatial variability of soil organic carbon (SOC) balance and associated loss pathways remains poorly quantified due to limited in situ hillslope-scale monitoring. We conducted an integrated assessment of SOC fluxes on a severely eroded black soil hillslope in Northeast China. The assessment, based on field monitoring during the 2023 and 2024 growing seasons, encompassed microbial mineralization, sediment and runoff transport, deep leaching, and root-derived C inputs. Results showed that the hillslope exhibited a net SOC loss and distinct spatial pattern. Despite weaker erosion, the upper position exhibited the most pronounced SOC imbalance and acted as the major C-loss hotspot (-1,730 kg C ha -1 yr -1 ), exceeding losses at the strongly eroded middle position (-1,054 kg C ha -1 yr -1 ), while the lower position functioned as a partial C sink (108 kg C ha -1 yr -1 ). The pronounced upper-slope loss mainly resulted from enhanced mineralization under favorable nutrient and aeration conditions, yielding 77.9% greater loss than at the middle position. In contrast, the middle position served as the primary erosion hotspot, with the share of sediment-associated C loss in total export rising from 11.9% (upper) to 24.5%. Notably, over 60% of the eroded SOC from the upper and middle positions was redeposited at the lower position, promoting long-term stabilization in depositional areas. These findings underscored the need for position-specific conservation to mitigate SOC loss and enhance sequestration in vulnerable agroecosystems. • Net soil carbon flux differed significantly among slope positions • Spatial decoupling between carbon loss and sediment yield hotspots • Position-specific conservation is critical for sloping farmland