Effects of Different Vegetation Types on Soil Hydrological Properties, Understory Diversity, and Water Conservation Capacity in the Horqin Sandy Land, China

The Northern Sand Prevention Belt is an important ecological security barrier in China. It is also one of the regions most severely affected by wind-eroded desertification. In this study, to investigate differences in soil properties among different vegetation types in sandy areas, a typical vegetation restoration area in the Horqin Sandy Land was selected. Widely distributed natural forests of Ulmus pumila, Populus plantations and Pinus sylvestris var. mongolica plantations were the study objects, with bare sandy land as the control. Understory herbaceous plant diversity and soil hydrological and physical properties were analysed, and Pearson correlation analysis, principal component analysis and the fuzzy membership function method were employed to comprehensively evaluate the soil water conservation function with respect to different vegetation types. The results showed that soil hydrological and physical properties differed significantly among the vegetation types. The U. pumila forest exhibited relatively low soil bulk density (1.53 g·cm−3) and high total porosity (32.42%) in the deep-soil layer (80–100 cm), indicating a strong capacity for deep-soil water storage. The Populus plantation showed the greatest soil improvement in the surface layer, with the highest capillary water-holding capacity and field water-holding capacity, reaching 17.82% and 16.05%, respectively. The P. sylvestris var. mongolica plantation had relatively high deep-soil bulk density (1.60 g·cm−3) and a pronounced decline in water-holding capacity. The unvegetated sandy land had the lowest capillary water-holding capacity (15.09%) and very poor water-retention capacity. Understory herbaceous plant diversity was highest in the P. sylvestris var. mongolica plantation and lowest in the U. pumila forest, and herbaceous plant diversity was significantly correlated with soil capillary porosity and capillary water-holding capacity. This comprehensive evaluation indicated that the poplar and P. sylvestris var. mongolica plantations had greater soil water conservation capacity than the natural U. pumila forest and unvegetated sandy land, with the tree–herbaceous vegetation structure exhibiting stronger synergistic characteristics and relatively favourable soil moisture conditions. These findings suggest that greater attention should be given to tree–herbaceous synergistic configurations in vegetation restoration in the Northern Sand Prevention Belt, while tree species selection should be optimised according to local conditions to enhance regional soil water conservation capacity.