## 4 Discussion  
### Causes of Changes in Urban Carbon Sequestration  
Urbanized areas, with their unstable terrestrial ecosystems, often see vegetation transition from carbon sinks to sources. Our study found that 29.86% of counties in the YKDUA experienced a decrease in carbon sequestration (CS) (Figures 4b and 6a), aligning with [72]'s findings on carbon sink stability across China's terrestrial ecosystems. Contrary to [73], who focused on climate as the dominant factor, our research additionally investigates the influential capabilities of five other factors. It was discovered that human activities (HUMAN) and vegetation photosynthesis (CSIF) are the predominant drivers in over 144 counties, accounting for 73.10% of CS changes (Figure 7i), surpassing the impact of climate factors. It should be explicitly noted that although photosynthesis was observed in this study as the largest driving factor of CS, we argue that urban CS is primarily driven by the interplay between the biophysical potential for photosynthesis (represented by CSIF) and the overarching constraints and modifications imposed by human activities (represented by GAR,  
Figure 8: SEM causal path modeling driven by changes in CS in the YKDUA from 2001 to 2020. The results are obtained by verifying the model assumed in Figure 2. (**a**) Outside the districts. (**b**) Municipal districts. (**c**) All counties. The black arrow indicates that the predictive factor has a significant impact on the response variable (\({}^{*}\)\(p<0.05\), \({}^{**}\)\(p<0.01\), \({}^{***}\)\(p<0.001\)), while the dashed line indicates no significant impact. The direction of the arrow represents the direction of causal action, and the number on the arrow is the standardized path coefficient \(\beta\). A larger \(\beta\) implies a stronger driving influence. WCR, and others), rather than being solely attributed to the dominance of photosynthesis. The relative influence of photosynthesis and anthropogenic factors (e.g., land cover change) can vary dynamically, especially under conditions of large-scale changes in GAR, which were not observed within the study area. Afforestation, the enhancement of green spatial structure, and the promotion of photosynthesis emerge as key drivers of CS changes, differing from studies focused solely on urban green infrastructure [73, 74]. Compared with other factors, the intrinsic driver of CS in vegetation photosynthesis is considered the most significant. This is reflected in the evaluation of the importance of driving factors in GWR-RF (Figure 6b) and SEM modeling, which indicates that the normalized path coefficient \(\beta\) of CSIF, as influenced by CS, is greater than that of other factors (Figure 7). This suggests that promoting the sustainable development of vegetation photosynthesis based on natural solutions is the most important way to increase vegetation CS in urbanized areas. Afforestation and the enhancement of urban green space structure are recognized as important strategies for urban CS capability enhancement. These initiatives not only directly increase CS but also indirectly contribute to this process by promoting photosynthesis. They can be effectively advanced through collaborative efforts. This study's findings indicate that afforestation has boosted vegetation CS but has also contributed to CS decline in certain areas (Figure S3c), potentially due to disruptive afforestation practices harming wetland CS capacities. This supports reference [75]'s observations of decreasing wetland areas due to increased afforestation at the county scale. Therefore, in regions like the East China Plain, with its rich wetlands and river networks, careful consideration is required to balance afforestation with ecosystem preservation. In municipal areas, urban green space optimization (GAR) significantly enhances CS compared to outside municipal areas, supported by additional light and heat resources from artificial lighting and urban heat islands (Figure 8b), more so than in non-municipal areas (Figure 8a).