Influence of roots on soil preferential paths for infiltration in typical vegetation of the Loess Plateau

Previous studies have indicated that preferential flow is likely to occur during infiltration on the Loess Plateau, with root systems being a key factor. Given the large-scale vegetation construction on the Loess Plateau, it is essential to investigate how root system changes influence preferential path formation. This study investigated the effects of roots of different diameter classes on preferential paths using in situ dye tracer experiments and profile imaging methods. Results showed that the cover of forest vegetation increased the density (root length density, root surface density and root volume density) of roots in soil. The total number of preferential paths in the studied 0–30 cm soil layer was 2524 in the natural secondary forest, which was 1.39 times higher than in the Pinus tabuliformis plantation and 1.48 times higher than in the mixed Robinia pseudoacacia-P. tabuliformis plantation. Preferential paths were predominantly distributed in the 0–15 cm soil layer. In forest soil, the maximum number of preferential paths consistently occurred at 5 cm depth, with measured values of 803 in P. tabuliformis plantation, 568 in mixed R. pseudoacacia-P. tabuliformis plantation, and 996 in natural secondary forest. By contrast, grassland soil exhibited the greatest number of preferential paths at 10 cm depth (1181 paths) and a lower number of preferential paths at 5 cm depth (414 paths). Most preferential paths exhibited an infiltration aperture radius below 5 mm. Roots of different diameter classes promoted the formation of preferential paths, and the effect of fine roots was more evident than that of coarse roots. Vegetation restoration on the Loess Plateau has altered infiltration patterns, necessitating attention to preferential flow, and root configuration and density critically influence preferential paths development, with implications for forest and hydrological management in this region.