Assessing soil organic carbon stability along a toposequence as a function of water and tillage erosion by in‐situ soil respiration measurements: A case study for a Belgian silt‐loam cropland

Abstract Soils represent a major terrestrial carbon (C) pool and play a key role in regulating atmospheric CO₂. Moreover, maintaining high soil organic carbon (SOC) levels in croplands enhances soil health, making SOC sequestration a crucial research focus. This study aims to improve our understanding of the impact of sediment transfers caused by water and tillage erosion on SOC stability in the Belgian loess belt. Within the same field managed under conventional practices, SOC quantity and quality were assessed at six sites distributed along a slope profile. SOC stock assessments in the top 50 cm revealed a 3D redistribution of C, with losses in eroded sites and accumulation in depositional sites. Mineralization rates, derived from in‐situ CO₂ efflux measurements, were significantly higher in eroded sites and lower in depositional sites compared to reference. Depositional sites also exhibited weak correlations between mineralization rates and soil temperature and moisture, suggesting limited SOC accessibility to microorganisms. These stability variations were mainly linked to differences in SOC depth distribution. While sediment deposition allows large carbon pools to be buried down, soil erosion brings C from deeper layers to the surface. Moreover, a comparison between Walkley and Black and dry combustion C‐measurements showed a higher proportion of recalcitrant organic matter in depositional sites. This study highlights the significant role of both water and tillage erosion and associated deposition in SOC dynamics and underscores the need for further research at the catchment scale, also integrating other erosion‐induced processes to further improve our understanding.