Freeze-thaw-driven soil moisture return significantly contributes to spring phenology on the warming Qinghai-Tibet Plateau

Climate warming advances the start of the growing season and enhances carbon uptake on the Qinghai-Tibet Plateau, primarily through the direct effects of air temperature and precipitation. However, the role of spring soil moisture dynamics driven by freeze-thaw processes remains underexplored. Here, using multi-source data from 32 Qinghai-Tibet Plateau sites (2003–2024), we develop a framework to decouple and quantify the moisture-limit relief from freeze-thaw-driven soil moisture return (ESMR) and concurrent precipitation effect (Eprec). Across study sites, soil moisture return contributes 20.7% ± 2.1% (mean ± SD) of the start of the growing season advance since 2003, surpassing the precipitation effect and air temperature, underscoring the role of pre-freezing moisture return in advancing start of the growing season in the following spring. At sites with active layer thickness exceeding 2.2 m, the ESMR exhibits ~31% higher sensitivity to surface soil moisture compared to sites with thinner active layer thickness. As active layer thickness deepens, the influence of soil moisture dynamics in the middle and lower active layers, associated with permafrost degradation, may gradually weaken in regulating start of the growing season. Our findings identify freeze-thaw soil moisture dynamics as an important, previously underappreciated control on start of the growing season and spring carbon uptake on the warming Qinghai-Tibet Plateau. Warming advances spring growth on the Qinghai-Tibet Plateau, yet the role of thawing-soil moisture is unclear. This study suggests that soil moisture return significantly contributes to spring phenology, regulated by active layer thickness and precipitation.