Background The northern farming-pastoral ecotone is an ecologically fragile area and a region of severe soil erosion. Afforestation, which is mainly composed of man-made forests, struggles to effectively control soil erosion due to the varying influences of water, heat, wind, and sand in different habitats. Clarifying the differences in soil erodibility caused by different slope aspects and determining the impact of different vegetation restoration types on soil erodibility are crucial for the ecological restoration of this region.

Methods This paper focuses on the Yangcao Gully watershed in Chongli district, Zhangjiakou city, Hebei province as the research area. During the 2021 vegetation growing season, a comprehensive survey was conducted on the understory vegetation characteristics of various vegetation restoration types (wasteland, grassland, shrubland, and forest land) within the research area. Plots were set up as 10x10 m², and the survey indicators included vegetation height, richness, vegetation cover, aboveground biomass, belowground biomass, and litter biomass. A comprehensive soil erodibility index (CSEI) was established to characterize the soil's ability to resist erosion and to investigate the impact of slope aspect on soil erodibility. Data processing and statistical analysis were performed using Excel 2016 and SPSS Ver. 20. Principal component analysis, variance analysis, and Pearson correlation analysis were conducted using R language (R-3.5.1). Mapping was carried out using ArcGIS 10.4.1 and Origin 2021.

Results 1) Slope direction has a significant effect on most understory vegetation characteristics.Compared to other slopes, the growth condition of the understory vegetation on the west slope is relatively poor, with the characteristic values of understory vegetation being lower than those of the other three slope aspects. The vegetation cover on the north slope is 49.51% higher than that on the west slope. The aboveground and belowground biomass on the east slope is 24.54% and 40.09% higher than that on the west slope, respectively. The amount of litter on the east slope is 34.11% higher than that on the west slope. 2) Slope aspect has a significant impact on soil erodibility. The east slope has the highest (saturated hydraulic conductivity) SHC, which is 311.16% and 187.10% greater than that of the west slope and north slope, respectively. The west slope has the highest soil disintegration rate (SDR), which is 1.60 times, 1.97 times, and 3.89 times that of the north slope, south slope, and east slope, respectively. The east slope has the highest mean weight diameter (MWD) of 3.65mm, which is 55.66%, 36.60%, and 10.14% higher than that of the west slope, north slope, and south slope, respectively. The east slope also has the highest SSSI, which is 28.23%, 13.34%, and 10.71% greater than that of the west slope, north slope, and south slope, respectively. 3) Land use types, slope aspects, and their interactions significantly affect the CSEI, which can more effectively reflect the changes in soil erodibility among different slope aspects and land uses. The CSEI values for the south slope (0.26) and east slope (0.20) are low, indicating a strong capacity to reduce soil erodibility. The north slope (0.41) has a weaker capacity, and the west slope (0.65) has the weakest. Compared to the wasteland (0.81), the CSEI of the other three land use types is significantly reduced, with the best reduction effect observed in shrublands (0.3), followed by forest lands (0.42), and grasslands (0.61).

Conclusions The impact of slope aspect and different vegetation restoration types on soil erodibility during the vegetation recovery process is significant. Based on the differences in CSEI between various slope aspects and among different land use types within the same slope aspect, this study proposes the best vegetation restoration types for each slope aspect. In combination with the natural vegetation recovery conditions of the study area, Pinus sylvestris, Caragana korshinskii, Agalus melilotus and Lespedeza bicolor may be the best choices to improve the soil erodibility of semi-shade slope, shade slope, sunny slope and semi-sunny slope, respectively.