## 3 Results
### Interannual Variation Characteristics of NEP  
The total _NEP_ value per unit area of the terrestrial vegetation ecosystem in the Yangtze River Delta region, in each year, from 2000 to 2019, was calculated, and the mean _NEP_ value of the Yangtze River Delta region in each year over the last 20 years, as well as the maximum and minimum _NEP_ values in the research area and the interannual variations of the proportion of pixels with _NEP_ > 0, were obtained (Figure 4a). It can be found that, from 2000 to 2019, the annual mean _NEP_ of vegetation in the Yangtze River Delta region fluctuated slightly around 253 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), the interannual variations were not large and presented a slightly increasing trend, and the slope of the trend line (k) was 2.25, indicating that the health of the vegetation ecosystem in this region was at a high level over the past 20 years; after eliminating the carbon consumption of soil respiration in the system, the vegetation in the entire Yangtze River Delta region played the role of a carbon sink, and the carbon sequestration capacity increased to some extent. The maximum values of _NEP_ under vegetation coverage in this region in these years appeared in southern Zhejiang, and all such _NEP_ values exceeded 750 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\); however, the interannual fluctuations were large, and the values generally presented a relatively significant upward trend, the slope of the trend line (k) was 3.50, indicating that the maximum carbon sequestration capacity  
Figure 3: (**a**) Spatial distribution of _NEP_ mean value in Yangtze River Delta from 2000 to 2019. _NEP_ mean distribution frequency; (**b**) Yangtze River Delta region; (**c**) Anhui; (**d**) Jiangsu (**e**) Zhejiang; (**f**) Shanghai (black curve is cumulative frequency). of the vegetation ecosystem in southern Zhejiang increased significantly and was greatly affected by the external environment. During the same period, the location where the minimum _NEP_ occurred in this region was not fixed, and there was no law. The minimum value fluctuated around \(-233\) g C\(\cdot\)m\({}^{-2}\)\(\cdot\)a\({}^{-1}\); moreover, although the interannual variation was small, the values generally presented a downward trend, and the slope of the trend line (k) was \(-3.13\), indicating that the carbon emissions from soil microbial respiration in the vegetation ecosystem in this region increased significantly, and the vegetations in these locations were weakened. Comparing these three _NEP_ values, it can be found that the change and increase in the mean value of _NEP_ in this region were mainly realized by the change of the maximum value of _NEP_ in this region. In addition, it can be found that the proportion of pixels with _NEP_ > 0 fluctuated around 90%, and the interannual change was relatively stable, and that the proportion increased slightly irrespective of the value, with the trend line slope being close to zero. Compared with the interannual variation of the average _NEP_ of three provinces and one city (Figure 4b), it can be found that the carbon sequestration capacity of Zhejiang Province of China is much higher than that of the other three places. Although there are large fluctuations in each year, there is no significant change trend, and the overall development is relatively stable. The average _NEP_ of Jiangsu Province of China is slightly higher than that of Anhui Province of China; however, the increasing trend of Anhui Province of China is higher than that of Jiangsu Province of China. The average value of _NEP_ in Shanghai of China is the smallest and is decreasing year by year. Through the identification of time series mutation points for the average _NEP_ of each year in the Yangtze River Delta from 2000 to 2019 (as shown in Figure 5), it is found that the UF curve in the Yangtze River Delta has multiple intersections with the critical value after 2011. Therefore, _NEP_ has an upward trend before 2013; however, the trend is not significant.