Selective removal of fine particles by erosion decouples mean weight diameter from aggregate stability in Mollisols

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Soil aggregate stability and soil organic carbon (SOC) are fundamental components of cropland agroecosystem functioning, yet both are highly vulnerable to erosion-induced physical disturbances. Here, we investigated variations in aggregate-size distribution, aggregate stability indicators, and SOC fractions, including coarse particulate organic carbon (cPOC), fine particulate organic carbon (fPOC), mineral-associated organic carbon (MAOC), microbial biomass carbon (MBC), and dissolved organic carbon (DOC), across four depth intervals (0–10, 10–20, 20–30, and 30–40 cm) along an erosion gradient in a Mollisol cropland. Results showed that soil erosion markedly decreased the proportions of <0.053 mm and 0.25–0.053 mm aggregate fractions throughout the 0–40 cm soil profile, while increasing the relative abundance of larger aggregate-size classes. Although mean weight diameter (MWD) increased with increasing erosion severity, this pattern likely reflected a compositional shift resulting from the selective loss of fine aggregates rather than enhanced intrinsic soil structural stability. Consistent with this structural degradation, total SOC and most carbon fractions, particularly cPOC, MAOC, and MBC, declined throughout the 0–40 cm profile. In contrast, fPOC (0–20 cm) and DOC (0–40 cm) peaked at moderately eroded sites, indicating nonlinear responses of mobile and aggregate-associated carbon pools to erosion-driven redistribution. To improve the assessment of soil stability in eroded landscapes, we suggest combining active carbon fractions (e.g., cPOC and MBC) with the proportion of fine aggregates (<0.25 mm), because the latter is particularly sensitive to the selective removal of carbon-rich fine particles. Furthermore, pathway analysis revealed depth-dependent controls on soil stability. In the topsoil (0–20 cm), erosion influenced stability through both direct effects and indirect pathways mediated by microbial and physically associated carbon. Conversely, in the subsoil (20–40 cm), erosion effects were primarily driven by indirect pathways involving physical carbon fractions. Overall, these results highlight that stability mechanisms and their carbon controls differ between topsoil and subsoil in eroded Mollisols, providing process-oriented guidance for managing erosion-degraded croplands.