Relationship of vegetation stand age to soil water dynamics and use in artificial shrublands and grasslands in a semiarid region

Vegetation restoration enhances ecosystem services but alters soil hydrological processes in arid and semiarid regions. Understanding vegetation-soil water interactions is crucial for effective water management and sustainable recovery. However, due to the limited long-term monitoring, soil water dynamics and plant water use characteristics at various stages of restored ecosystems remain poorly understood. In this study, we investigated the soil water conditions and water use strategies of typical revegetated shrublands (Caragana korshinskii) and grasslands (Medicago sativa) at 5-, 18-, and 50-yr ages across two growing seasons. Soil water dynamics within 300 cm profile were assessed using the oven-dried method, and plant water use strategies were analyzed using δ²H and δ¹ ⁸O in plant and soil water, and δ¹ ³C in leaves. The results showed a decline in the soil water content from 5 to 18 years in both shrub and grassland ecosystems, with replenishment observed in the 50-yr sites, exceeding that of the other stand ages. The 50-yr C. korshinskii and M. sativa exhibited higher deep soil water utilization with lower intrinsic water use efficiency (iWUE) compared to younger stands. However, both species exhibited consistent shifts in water use strategies across stand ages: during drought years, they relied more on shallow soil water (0–40 cm) with higher iWUE, while in wet years, they used more middle soil water (40–140 cm) with lower iWUE. These findings suggest that soil water conditions and water use characteristics are related to stand ages. After prolonged stand development, soil water deficits were alleviated, and water availability for vegetation increased, potentially improving ecosystem sustainability. Therefore, future management should assess the ecological effects of vegetation restoration over longer developmental stages and implement targeted strategies based on stand ages.