## 4 Discussion
### Response of EVI and GPP to Soil Moisture  
Studying the impact of soil moisture at different depths on EVI and GPP is essential for understanding and predicting ecosystem functions, water resource management, and climate change adaptation strategies [35]. In this research, significant correlations were observed between soil moisture at specific depths and both EVI and GPP in arid and semi-humid regions, whereas such correlations were not significant in humid regions. These findings are consistent with those of [PERSON]'s team, who demonstrated that ecosystem photosynthesis adapts to soil moisture levels, exhibiting a hump-shaped response curve of GPP to increasing soil moisture, with an optimal soil moisture level at which GPP peaks [36]. This suggests that the optimal soil moisture for arid ecosystems is typically higher than that for humid ecosystems, reflecting the water adaptability of ecosystems. Additionally, [PERSON]'s (2021) [36] research indicates that in arid and semi-arid regions, water use efficiency (WUE) is positively correlated with precipitation, suggesting that increased rainfall enhances vegetation's carbon assimilation rate more than the rate of ecosystem water loss. Collectively, these findings elucidate the impact of soil moisture on ecosystem productivity and its variability across different climatic zones, providing new insights into understanding and predicting carbon-climate feedbacks. In particular, the influence of soil moisture on vegetation growth and photosynthesis is more pronounced in arid and semi-humid regions, likely due to the scarcity of water in these areas, which heightens vegetation's sensitivity to water use efficiency and response. In contrast, in humid regions, the impact of soil moisture on EVI and GPP may be overshadowed by other environmental factors, resulting in non-significant correlations. The EVI and GPP of croplands and grasslands exhibited significant negative correlations with SM4. In contrast, the GPP of wetlands significantly increased with rising soil moisture. This finding aligns with previous research suggesting a unimodal relationship between GPP and soil moisture, indicating an optimal soil moisture level at which GPP reaches its maximum value [3]. For croplands and grasslands, exceeding optimal soil moisture levels can trigger root oxygen stress. Under hypoxic conditions, aerobic respiration in roots is significantly weakened, resulting in insufficient ATP synthesis and severely restricting the active absorption of key nutrients like nitrogen and phosphorus. Meanwhile, the accumulation of harmful substances like ethanol produced by anaerobic respiration within cells can damage cell membrane structure and function. Once root physiological functions are impaired, the synthesis of photosynthetic pigments in the aboveground parts is affected, stomatal conductance is reduced, and carbon dioxide uptake is decreased, thereby inhibiting photosynthetic efficiency. Ultimately, this results in a significant negative correlation between EVI, GPP, and deep soil moisture [37; 38]. Conversely, wetland ecosystems, characterized by their unique hydrological conditions and plant adaptations, may more effectively utilize increased soil moisture, thereby promoting increases in EVI and GPP. Additionally, the correlation between soil moisture and other vegetation types did not reach a significant level, possibly due to differences in water use strategies and ecological adaptations among vegetation types. For instance, some vegetation types may access water through deep root systems, demonstrating greater resilience under water stress [39; 40], which may account for the less pronounced relationship between EVI and GPP and soil moisture compared to croplands and grasslands.