Distinct mechanisms shaping global surface and root-zone soil moisture

Soil moisture (SM) plays a vital role in the global water and carbon cycles, with long-term impacts on ecosystem functioning and vegetation growth. However, under the background of climate change, a decoupling phenomenon have occurred between surface soil moisture (SMsurf) and rootzone soil moisture (SMroot). The variations and primary driving factors of SM across different layers have not been studied comprehensively. Therefore, this study explored the spatiotemporal dynamics of global SMsurf and SMroot from 2001 to 2021. The Random Forest coupled with numerical simulation experiments were applied to measure the influences of climate and vegetation dynamics to SM changes. The Partial Least Squares Structural Equation Modeling was employed to demonstrate the direct and indirect pathways of them to SM variability. Additionally, the copula functions were applied to examine the probability of SM loss under different stress scenarios caused by climate or vegetation changes. The results indicated that SM variation exhibited a significant spatial heterogeneity. Global greening significantly contributed to the increase in SMsurf at a rate of 0.000087 m3/m3/a, while precipitation (Pre) had the most significant impact on replenishing SMroot, with a contribution rate of 0.000117 m3/m3/a. Atmospheric water demand (Ep) was identified as the primary cause of global SM drought, with rates of -0.000089 m3/m3/a and -0.000075 m3/m3/a for SMsurf and SMroot respectively. Although vapor pressure deficit (VPD) had a significant dominant effect in regions with high VPD values, rather than globally, as global positive and negative VPD effects offset each other. Vegetation typically acted as an intermediary variable transmitting the indirect effects of climate factors on SM. Under the extreme scenarios, Precipitation, Standardized precipitation evapotranspiration index and vegetation resulted in the highest probability of SM loss. This research will furnish a theoretical underpinning for global water resource management and hold significant implications for the sustainable development of ecosystems.