## 4 Discussion
### Interannual Fluctuation Trends  
To facilitate a more comprehensive comparison of the dynamic changes in soil hydrothermal conditions and GPP, the annual data was normalized [51]. Figure 8 illustrates the interannual fluctuations of soil hydrothermal conditions across different layers. Notably, the soil moisture values for all three layers reached their lowest point in 2013 during the study period. From 2007 to 2013, the soil moisture values across three layers exhibited a consistent decreasing trend, followed by a fluctuating increasing trend from 2013 to 2018. Previous studies have indicated that the Qilian Mountains experienced a mild drought event lasting for eight months in 2013, which likely contributed to the observed valley value in soil moisture [52; 53]. Furthermore, in the deep layer, it is evident that the decreasing trend of soil moisture is less pronounced, while the rising trend is more prominent. This finding supports the previous conclusion that the deep soil in the Qilian Mountains exhibits strong water retention characteristics.  
Figure 7: Feature importance of the regulations of soil hydrothermal conditions on GPP, (**a**) soil moisture, (**b**) soil temperature.  
Similar to the interannual trend analyzed in Section 3.1, the interannual fluctuations of soil temperature values across three layers exhibit noticeable similarities. The years 2004, 2012, and 2019 correspond to valley values, while peak values occur in 2009, 2014, and 2020. The Lenglong Mountain and Wushao Mountain in the eastern part of the study area serve as transition regions from cold to warm soil temperatures, and the interannual variation trends in these areas also indicate an extremely significant increase. This region exhibits a strong response to regional climate warming, with the persistent phenomenon of permafrost degradation contributing to a notable interannual rise in soil temperature [54]. The regulation of soil heat in the Qilian Mountains is significantly influenced by the freezing and thawing processes [55]. The presence of snow cover on the surface soil affects the exchange of heat with the soil, with thicker snow cover resulting in higher soil temperatures [56; 57]. However, the process of snow melting absorbs heat from the soil, leading to a fluctuating decrease in soil temperature to some extent [58]. Consequently, the annual variation characteristics of soil temperature are closely linked to the intensity of freezing and thawing processes.  
Figure 9 illustrates the interannual fluctuation of GPP in the Qilian Mountain vegetation zone. The data reveal a significant increasing trend throughout the study period, reaching its peak in 2018. Additionally, GPP values in the second decade are higher than those in the first decade. The relatively warm and humid climate conditions in the south-east region contribute to luxurian vegetation growth, resulting in a stronger capacity  
Figure 8: Interannual fluctuations of normalized values of the soil hydrothermal conditions across three layers: (**a–c**) is soil moisture at different depths, (**d–f**) is soil temperature at different depths.  
for carbon sequestration [59]. The continuous improvement in the vegetation carbon sequestration capacity of the Qilian Mountains is likely attributed to robust afforestation efforts [60; 61]. Moreover, the interannual fluctuation trends of GPP align more closely with SM-L3, providing further evidence of their strong correlation.