When Sediment Moves but Carbon Lags: Topography and Bioturbation Control Soil Carbon Burial jointly

Carbon redistribution along with accelerated erosion in agricultural landscapes is an important component of the global carbon cycle. However, it remains largely untested whether soil particles and soil organic carbon (SOC) are always preserved synchronously across hillslopes, and what mechanisms govern potential decoupling. In hilly Northeastern China, carbon-rich black soils have experienced only recently severe agriculture-induced erosion over the past ~130 years, providing an ideal setting to examine sediment and carbon behaviour over the Holocene and to assess how accelerated anthropogenic erosion influences long-term carbon dynamics. In this study, we investigated single-grain post-infrared infrared stimulated luminescence (pIRIR) signals of feldspar along two black soil catenae with contrasting slopes. 31 luminescence samples collected from five soil profiles along a catena with a slope gradient of 0.37°, 19 luminescence samples from three soil profiles along a catena with a slope gradient of 1.91°.  Geochronological constrains from luminescence and ¹³⁷Cs were combined with soil properties to trace soil redistribution and reconstruct Holocene erosion phases along the catenae. We identified a clear topographic control on whether sediment and carbon are redistributed synchronously. On gentle slopes, colluvial deposits formed primarily at backslope positions, yet no buried carbon horizons were preserved. This decoupling results from prolonged soil residence times for sufficient bioturbation , as indicated by abundant krotovina and distinct equivalent dose (De) distributions between krotovina and non-krotovina samples from same depth (~80 cm) in the summit profile. In contrast, steeper slopes favour rapid deposition and carbon burial at toeslope position due to higher erosion relative to soil mixing. Overall, our findings demonstrate that topography and post-depositional bioturbation jointly determine whether sediment flux translates into long-term carbon preservation. Gentle slopes promote redistribution without carbon burial, while steeper slopes facilitate synchronous sediment and carbon accumulation. This contrast is further amplified by anthropogenic erosion, leading to situations in which sediment transported is enhanced while carbon burial lags behind. These results highlight the critical role of landscape configuration and biological processes in mediating carbon fate in cultivated soils and emphasize the need to account for such controls when quantify carbon dynamic under human-induced erosion.