## 4 Discussion
Finally, human interventions played a key role, such as large-scale afforestation projects contributing to over 70% of forest area expansion, natural forest protection, and ecological restoration programs, like the conversion of grazing land to grassland, which effectively promoted vegetation growth and enhanced ecosystemcarbon sequestration capacity [41, 42]. The combined effects of CO2 fertilization, climate warming, increased nitrogen deposition, and human interventions collectively drove the continuous growth of China's terrestrial carbon sink. Our analysis of the residual emissions after offsetting industrial carbon emissions with the carbon sink of China's terrestrial ecosystems revealed that, across all economic regions, the growth in ecosystem carbon sinks was smaller than the increase in carbon emissions from fossil fuel combustion during the same period (Figure 6). There were also regional differences in carbon neutrality levels. The economically developed eastern regions, including the Bohai Sea Rim and the Huang-Huai-Hai region, were far from achieving carbon neutrality. These areas were predominantly cropland with inherently weak carbon sink capacity, combined with high levels of industrial development and dense populations, leading to significantly higher apparent carbon emissions and residual emissions compared to other regions. In contrast, the northeastern region, with its relatively slower economic growth, showed a slower increase in residual emissions and exhibited better carbon neutrality levels than other regions. The western and central regions had relatively similar levels of carbon neutrality. Studies have suggested that as China's forests gradually mature and age, the carbon sink capacity might decline, further reducing the ability of terrestrial ecosystems to offset industrial carbon emissions [3, 43]. Therefore, achieving carbon neutrality requires the coordinated implementation of multiple strategies. The NEP simulation framework we developed also has some uncertainties. First, discrepancies may exist between the actual start and end dates of plant photosynthesis and the SOS and EOS derived from the phenology model. Studies have shown that photosynthesis can continue until chlorophyll depletion in the leaves [44]. Second, ecosystem process models, such as CASA, do not account for human factors, like irrigation, fertilization, and forest management, all of which can significantly influence plant growth [3]. This adds uncertainty to the NEP estimates, and caution is needed when applying these models to agricultural ecosystems. Furthermore, ecosystem process models rely on remote sensing data, and the accuracy of these estimates is highly dependent on the quality of the remote sensing data [11]. The relatively coarse resolution of remote sensing data may also obscure NEP variations in specific plant types [22]. In the future, when extending our NEP accounting framework to larger study areas, special attention should be given to data quality, and multi-source data should be used for cross-validation to improve the reliability of the estimates. Additionally, more factors influencing plant growth should be incorporated into carbon sink accounting models to improve their comprehensiveness and accuracy.