Variation of soil anti-scourability across land-use types and its dependence on soil depth in the loess hilly-gully region, China

Soil anti-scourability is a key indicator characterizing soil resistance to scouring by runoff, and its variation is closely related to differences in watershed land-use types. However, the mechanisms through which different land-use types regulate soil anti-scourability, as well as how such effects vary across soil depths, remain poorly understood. This scientific issue is particularly prominent under the background of large-scale vegetation restoration projects that have achieved remarkable ecological benefits. In this study, five typical land-use types and three soil layers were selected in the loess hilly-gully region, and through field sampling of undisturbed soil and laboratory-simulated scouring experiments, we systematically examined differences in soil anti-scourability among land-use types and quantified the contributions and potential pathways of root traits, soil physical structure (soil bulk density, disintegration rate, mean weight diameter of water-stable aggregates), and chemical cementing substances (soil organic matter, cation exchange capacity, pH) to soil anti-scourability across soil depths. The results showed that, in the 0–20 cm layer, ecologically restored land dominated by woodland, shrubland, and grassland exhibited 0.84–1.99 and 5.56–9.68 times greater soil anti-scourability than orchard and cropland, respectively, whereas such differences were minimal in the 20–40 and 40–60 cm layers. Soil anti-scourability was significantly positively correlated with root mass density, mean weight diameter of water-stable aggregates, soil organic matter content, and cation exchange capacity, but significantly negatively correlated with soil bulk density, disintegration rate, and pH. The mechanisms driving soil anti-scourability exhibited soil-depth dependence. In the 0–20 cm layer, soil anti-scourability was primarily regulated by soil physical structure ( SPC = 0.80, p < 0.001), whereas roots exerted mainly indirect effects. In the 20–40 cm layer, the relative contribution of roots increased, and soil anti-scourability was driven primarily by the direct effects of roots ( SPC = 0.53, p < 0.05). In the 40–60 cm layer, the relative contribution of roots increased further ( SPC = 0.66, p < 0.001), while the role of soil chemical cementing substances also became apparent. Overall, vegetation restoration significantly enhanced the resistance of topsoil to runoff scouring, but had limited effects on the anti-scourability of deeper soil layers, indicating a pronounced soil-depth dependence in the influence of land use on soil anti-scourability. This study provides a theoretical basis for soil erosion control in the loess hilly-gully region.