Differential response of soil moisture in surface and root zones to climate and land use changes

As a pivotal integrative variable in terrestrial ecosystems, soil moisture (SM) plays a critical role in regulating hydrological processes, vegetation growth, and climate feedbacks. However, the trends in global SM changes over recent decades and the mechanisms driving these changes remain uncertain. This study aims to reveal the dynamics and drivers of SM, especially the coupling effect of climate variability and land use or cover change (LUCC). A global SM product, derived from the Global Land Evaporation Amsterdam Model (GLEAM v3.6a), were utilized to investigate dynamic changes of SM in surface (SMsurf, 0–10 cm) and root-zone (SMroot, 10–250 cm) from 1982 to 2021. A series of mathematical methods, including the Theil–Sen trend estimation, Pearson correlation analysis, variance decomposition, contribution models and structural modeling, were applied to quantify the roles of climate and LUCC to SM changes. We found that there are significant regional differences in SM dynamics, SMsurf is higher than SMroot in humid and semi-humid regions, whereas the opposite is observed in extremely arid regions. Meanwhile, except for bare land, soil moisture is generally higher in the surface layer than in the root zone across different LUCC types. The greatest decline was observed in semi-arid and semi-humid regions, which is consistent with changes in precipitation (PCP), temperature (TMP), and solar radiation (RAD). The climate factors exerts stronger dominance on SM over LUCC, particularly for SMroot, which is largely regulated by long-term climate conditions. Meanwhile, the SMsurf is more sensitive to the coupling effect of land use–climate. Decreasing of PCP was the main cause of SM decline in humid and semi-humid regions, whereas increasing TMP and enhancing RAD played increasingly important roles toward arid and hyper-arid regions; notably, TMP became a more important regulator of SMroot in hyper-arid regions. Transitions associated with forests and wetlands generally maintained higher moisture levels, whereas those involving bare land showed substantially lower values. The findings advance the mechanistic understanding of terrestrial water cycles by quantifying the interplay between climatic and anthropogenic drivers on SM, which is essential for predicting future climate-ecosystem interactions.