Interactive amplification of forest soil erodibility by slope aspect and topographic relief in a karst trough valley

Intensified climate extremes and land-use shifts under global change have increased soil erosion uncertainties, demanding precise assessments of the erodibility K value for sustainable management. However, how slope aspect (SA) and topographic relief (TR) interact to influence K value (K EPIC ) related to the Environmental Policy Integrated Climate model (EPIC) remains poorly understood, especially in complex karst terrains. This study addresses this gap by investigating the interacting effects of SA and TR on forest K EPIC in a typical karst trough valley. A total of 88 forest sites were sampled using a 1000-m grid across varying SAs and TRs grades was combined with laboratory analysis, Geo-detector modeling and path analysis to quantify their individual and combined impacts on K EPIC . The results showed that: (1) K EPIC was highest on south-facing slope (0.0366 ± 0.0035 t·hm 2 ·h/(MJ·mm −1 ·hm 2 )) and in high-TR areas (0.0347 ± 0.0060 t·hm 2 ·h/(MJ·mm −1 ·hm 2 )), significantly exceeding east−/west-facing and medium-TR areas. (2) SA and TR synergistically enhanced K EPIC , with interaction detection confirming a bi-factor enhancement pattern. (3) Soil texture components (silt and sand) showed the highest sensitivity in the calculated K EPI C , reflecting the structure of the EPIC model, accounting for 89.11% and 61.39% of its variability, respectively. This does not imply independent causal control but highlights parametric sensitivity. (4) Path analysis revealed that only the indirect effect of TR via Ca 2+ on K EPIC was statistically significant (β = 0.0244, 95% CI: 0.0001–0.0008), suggesting that TR influences erodibility partly through its control over Ca 2+ retention. These findings reveal how topographic factors shape erodibility, guiding targeted conservation and restoration in fragile karst ecosystems.