Impact of landscape positions on soil erodibility indices in typical vegetation-restored slope-gully systems on the Loess Plateau of China

Landscape position of the slope-gully system has considerable effects on the near soil surface characteristics, which in turn affect soil erodibility. However, little attention has been paid to the spatial variation of soil emdibility in vegetation-restored slope-gully systems in arid and semiarid regions. This study aimed to detect spatial variations in the soil emdibility indices of soil cohesion (Coh), saturated conductivity (K-s), aggregate stability (the number of drop impact (NDI) and the mean weight diameter (MWD)), penetration resistance (PR), emdibility K factor, and a comprehensive soil erodibility index (CSEI) under a north-facing slope-gully system (NSG) and a south-facing slope-gully system (SSG) on the Loess Plateau. The results showed that Coh, K-s, NDI, and MWD increased generally from the high to low landscape positions for both the NSG and SSG, whereas PR and K had the opposite trend. Coh, K-s, NDI, and MWD of the NSG were significantly greater than those of the SSG, but PR and K of the NSG were notably lower than those of the SSG. For both the NSG and SSG, the CSEI increased first then declined sharply until reaching the gully floor. For the NSG, the ratios of the CSEI of the summit, upper ridge slope, middle ridge slope, lower ridge slope and gully slope to the CSEI of gully floor were 23.5, 33.7, 28.1, 25.2, and 13.6. For the SSG, these ratios were 2.8, 4.1, 3.8, 3.7, and 2.4. The CSEI of the NSG was significantly less than that of the SSG at all positions, which indicates that the soil erosion resistance of the NSG was significantly greater than that of the SSG. The variations in soil erodibility was attributed to the differences in biological crust thickness, plant litter and root densities, soil bulk density, texture and organic matter content with landscape position and aspect.