## 3 Results  
### Interannual Variation Trends  
Overall, Table 1 shows nine different interannual variation trends obtained by Sen's slope estimator and the Mann-Kendell trend test. Meanwhile, the interannual variation trends of soil moisture in the Qilian Mountains vegetation zone are investigated, as shown in Figure 2. The multi-year average values of soil moisture across three soil layers exhibit a spatial pattern characterized by lower values in the northwest and higher values in the southeast. Within the middle zone of the Qilian Mountains, there are scattered patches on the map where soil moisture exceeds 50%. Geographically, these areas with high soil moisture are primarily concentrated in the Shule River basin, which suggests a potential association with surface and underground runoff transport processes [42]. Additionally, it is evident that soil moisture increases with depth, with values of 0.2974, 0.3128, and 0.3139 from top to bottom, respectively. This indicates that the deep soil is provided with stronger water retention characteristics compared to the shallow soil. The interannual variation trends depicted in Figure 2d-f exhibit notable spatial heterogeneity, with a drying trend in high-altitude areas (above 4000 m) and a wetting trend in the northeast and surrounding areas of Qinghai Lake. Notably, the pixel percentage of extremely significant increase trend in deep soil reaches 23.06%, which means that the area showing a positive trend in deep soil has increased by about 20% compared with other depths, emphasizing the deep soil has undergone a humidifying stage [43]. Figure 2: The background investigation of soil moisture across three layers, including (**a–c**) spatial pattern of multi-year average values and (**d–f**) spatial pattern of interannual variation trends. The background and variation in soil temperature are shown in Figure 3. The soil temperature values across three layers exhibit a spatial pattern characterized by a gradual increase from the northwest to the southeast. Also, the spatial distribution of soil temperature aligns closely with the elevation altitude, suggesting that variations in air temperature resulting from vertical zonality may influence soil heat absorption [30]. The lowest soil temperature sections are concentrated in the mountainous areas of Haixi Prefecture in Qinghai Province. However, there is no distinct differentiation of soil temperature observed across soil layers. The interannual variation in soil temperature generally demonstrates an increasing trend [44], with more than 50% of pixels exhibiting no significant increase trend. However, in the eastern areas, the interannual variation indicates an extremely significant increase. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Trend** & **SM-L1** & **SM-L2** & **SM-L3** & **ST-L1** & **ST-L2** & **ST-L3** & **GPP** \\ \hline Unchanged & 1.17\% & 1.17\% & 1.17\% & 1.17\% & 1.17\% & 1.17\% & 1.17\% & 23.45\% \\ No significant decrease & 41.13\% & 35.10\% & 27.72\% & 5.38\% & 4.69\% & 3.48\% & 0.75\% \\ Slight significant decrease & 4.53\% & 3.37\% & 3.04\% & 0.00\% & 0.00\% & 0.00\% & 0.06\% \\ Significant decrease & 4.27\% & 2.66\% & 3.30\% & 0.00\% & 0.00\% & 0.00\% & 0.07\% \\ Extremely significant decrease & 0.84\% & 0.71\% & 0.91\% & 0.00\% & 0.00\% & 0.00\% & 0.11\% \\ No significant increase & 37.89\% & 35.36\% & 28.04\% & 60.62\% & 57.62\% & 55.19\% & 9.02\% \\ Slight significant increase & 4.73\% & 9.97\% & 4.99\% & 12.50\% & 12.87\% & 14.66\% & 5.32\% \\ Significant increase & 3.95\% & 9.07\% & 7.77\% & 13.54\% & 16.51\% & 17.59\% & 15.57\% \\ Extremely significant increase & 1.49\% & 2.59\% & 23.06\% & 6.80\% & 7.14\% & 7.90\% & 45.65\% \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Statistical analysis of interannual variation trend in soil moisture, soil temperature, and GPP. Figure 3: The background investigation of soil temperature across three layers, including (**a–c**) spatial pattern of multi-year average values and (**d–f**) spatial pattern of interannual variation trend.