Evaluating soil water dynamics and vegetation growth characteristics under different soil depths in semiarid loess areas

Weathered bedrock alleviating vegetation death has been gradually appreciated, yet the hydrological effects of shallow soil layers and underlying bedrock impervious layers on the upper soil moisture and vegetation growth are less excavated in semiarid loess areas. To demystify these variations, this study evaluated soil water storage (SWS) changes and vegetation growth differences of alfalfa and caragana by characterizing different soil depths. The results showed that soil reservoirs changed under different soil depths and varied with vegetation types. From the beginning of Jun. to the end of Oct. 2021, SWS in alfalfa increased by 3.5 ‰ and 2.8 ‰ under soil depths of 0.6 m and 1.0 m but decreased by 3.6 ‰ under soil depth of 1.4 m; For caragana, SWS increased by 9.9 ‰ and 8.0 % under soil depths of 0.6 m and 1.4 m, while it decreased by 1.5 % under soil depth of 1.0 m. Moreover, different soil depths and vegetation types affected precipitation redistribution. Soil depth inhibited water consumption and accumulated water storage of alfalfa within 0.6 m (increasing 3.9 %) and caragana within 1.0 m (increasing 5.6 % on 0.6 m soil depth and 3.0 % on 1.0 m soil depth). These changes might be caused by root distribution and water absorption zone differences between alfalfa and caragana. Furthermore, limitation soil depth was one strategy of vegetation dwarfing, which had diverse effects on root systems and aboveground for different plants. By setting soil depth at 0.6 m, the underground parts of alfalfa and caragana were restricted, while the aboveground parts were promoted in alfalfa but reduced in caragana. However, the aboveground and underground parts were extended in alfalfa and caragana on 1.4 m soil depth. These vegetation growth differences might be due to the physiological response to restrictions on soil and seasonal precipitation. This study would provide a new understanding of the water distribution distinction and growth difference between herbaceous and shrub in shallow semiarid loess areas.