Distinguishing the effects of climate change and vegetation greening on soil moisture variability along aridity gradient in the drylands of northern China

Drylands are particularly sensitive to climate change, and soil water availability is critical for dryland ecosystems. Climate change and large-scale ecological restoration have led to significant vegetation greening and soil moisture (SM) variations in the drylands. However, the influencing mechanisms of climate change and vegetation greening on SM variability were not well understood. In this study, the spatiotemporal variations of climate variables, vegetation cover, surface SM (SM_surf) and root-zone SM (SM_root) along aridity gradient in the drylands of northern China during 1981–2018 were investigated. The random forest model and partial least-squares structural equation modelling (PLS-SEM) analysis were employed to quantify the contributions of climate change and vegetation greening to SM variability and identify the influencing pathways. The climate change showed temporal trends of wetting to drying before 2000 and drying to more wetting in 2000–2018, and vegetation greening was significant in 2000–2018. The variations of SM displayed obvious east-negative and west-positive trends. The temporal trend of SM deficit decreased during 1981–2018 in most regions, and SM showed a significant increasing trend in all regions of different aridity gradient after 2000. The correlations of climate factors and vegetation cover with SM became stronger in the area with lower aridity. The vegetation greening-induced SM change was limited compared to climate change. The climate factors can directly impact SM availability, and also indirectly influence SM availability through vegetation (e.g., precipitation→LAI→SM_surf). The SM_surf had positive responses to vegetation greening, especially in semiarid subarea. In contrast, vegetation greening exacerbated SM_root consumption in semiarid and dry subhumid subareas, which became intensive after 2000. This study fills the knowledge gap in the underlying reasons for the variations of both surface and root zone SM in water-limited areas and reveals the potential risks of SM depletion, particularly in dry subhumid areas.