## 1 Introduction  
The variability in soil hydrothermal conditions is closely linked to the stability of vegetation ecosystems and leads to variations in gross primary productivity (GPP) via regulating processes such as photosynthesis, respiration, and transpiration [1; 2; 3]. For example, a decrease in soil moisture reduces the water potential of vegetation leaves, hindering the diffusion of carbon dioxide and affecting the photosynthetic rate [4; 5]. On the other hand, an increase in soil temperature can stimulate vegetation metabolism and enhance carbon accumulation, but it also leads to rapid soil moisture depletion [6; 7]. With the influence of global warming, regional climate conditions become more unstable, and there is considerable uncertainty regarding the relationship between soil hydrothermal conditions and vegetation dynamics [8]. Thus, it is of great importance to identify the regulation of soil hydrothermal conditions on GPP values [9; 10; 11]. The regulations of soil hydrothermal conditions on vegetation dynamics vary depending on regional factors [12]. In arid and semi-arid regions, soil moisture plays a crucial role in limiting vegetation growth [13], while soil temperature exhibits a more significant regulation in humid regions [14]. Studies have demonstrated that when examining the influence of soil hydrothermal conditions on carbon sequestration capacity in the Tibetan Plateau, soil moisture and soil temperature are the primary drivers of carbon flux changes in the arid grassland of the western region and the humid meadow of the eastern region, respectively [15]. Additionally, there is a synergistic effect between soil moisture and soil temperature. Previous control experiments conducted on alpine meadows at different altitudes in Northern Tibet have found that an increase in soil moisture, induced by experimental warming of soil temperature, leads to a rise in GPP during the growing season [16]. Therefore, the regulation mechanisms of soil hydrothermal conditions on changes in gross primary productivity (GPP) vary in different regions. It is also possible that the soil hydrothermal conditions within different layers have differentiated impacts on vegetation dynamics. However, there is a lack of research specifically examining the regulations of soil hydrothermal conditions at different depths within the root zone on GPP. At the regional scale, previous studies have demonstrated a positive correlation between vegetation growth and soil moisture [17], while soil temperature has been found to promote vegetation carbon sequestration capacity [18]. [PERSON] et al. claimed that the imbalance of soil hydrothermal conditions caused by drought is the main stress that threatens GPP accumulation in more than 70% of regional ecosystems worldwide [19]. An experiment conducted by [PERSON] et al. in the Southeastern USA demonstrated a strong correlation between 0 and 100 cm soil hydrothermal conditions and vegetation greenness [20]. Additionally, studies have shown that soil profiles at different depths exhibit distinct physicochemical and hydrothermal properties. [PERSON] et al. proved that the hydrothermal conditions of the 0-20 cm soil layer are more susceptible to external environmental disturbances in Northwest China [21]. The study by [PERSON] et al. demonstrated that the soil hydrothermal conditions in the Qinghai Tibet Plateau region exhibit significant differences between active layers and permafrost during the freezing period [22]. However, most studies focus solely on a single element of soil moisture or soil temperature [23] or only conduct short-term research based on in situ sampling data [24]. There are obvious limitations in the existing research on the long time series regulations of soil hydrothermal conditions at different depths on vegetation growth. The Qilian Mountains area, as a representative geographical unit of the Tibetan Plateau, possesses a fragile ecosystem and complex climate conditions, leading to spatial variations in soil hydrothermal conditions [25].