## 3 Results
### Spatial Distribution Characteristics of NEP  
The _NEP_ represents the net absorption or net storage of carbon in the vegetation ecosystem, which can quantitatively describe the carbon source/sink capacity of the vegetation ecosystem. When _NEP_ > 0, it means that the ecosystem plays the role of a carbon sink; otherwise, it plays the role of a carbon source [17]. Therefore, as a key parameter to characterize vegetation activities, the accurate estimation of _NEP_ can not only measure the health of vegetation ecosystem, but also can quantitatively analyze the carbon sequestration status and potential of a regional vegetation ecosystem.  
After removing the two land cover types of construction land and unused land in the Yangtze River Delta year by year, the annual mean _NEP_ of each pixel in the remote sensing image of the Yangtze River Delta region from 2000 to 2019 was calculated. The spatial distribution map of the 20-year _NEP_ mean value in the region was obtained (Figure 3), which showed that, from 2000 to 2019, the overall spatial distribution pattern of the mean vegetation _NEP_ in the Yangtze River Delta region presented the trends that were higher in the south and lower in the north, higher in the east and lower in the west, and increasing from the northwest to the southeast. In addition, it can be found that there is a clear boundary in Anhui Province, and the _NEP_ for the southern district of the Yangtze River is generally higher than that in the north. Although \(R_{H}\) for the southern district is higher than that in the north because of the higher temperature and precipitation in the south, it cannot change that the _NEP_ in the south is higher than that in the north in the Yangtze River Delta. Combined with the land cover data, it was found that there was more cultivated land in the north of the Yangtze River and more forest and grassland in the south of the Yangtze River. The mean _NEP_ of each pixel in the terrestrial ecosystem in this region is mostly higher than 0 in the past 20 years, therefore the carbon sink is dominant, and the proportion of pixels with _NEP_ > 0 is much larger than that of carbon sources (including construction land and unused land), accounting for more than 85% of the total area of the Yangtze River Delta region; however, the areas with a very high _NEP_ value (\(NEP\) > 600 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\)) accounts for less than 1.3%, and the high-value areas are mainly distributed in Wenzhou, Taizhou, Lishui, and Zhoushan (south of Zhejiang Province). These areas have high terrain, lush vegetation, and a forest coverage as high as more than 70% [37], and the carbon sequestration amount of vegetation is much higher than the carbon emissions from soil microbial respiration. In addition, the proportion of pixels with a mean _NEP_ < 0 is relatively small, and these pixels are mainly distributed in Shanghai, Suzhou, Hangzhou, and Nanjing of China, etc. These areas have a relatively low forest coverage rate,  
Figure 2: Comparison between MODIS-_NEP_ and vegetation net primary productivity model.  
their industrial structures mainly include the industries with high energy consumption and high carbon emissions, such as heavy industry and manufacturing industry, and the construction land expands rapidly [5], therefore the carbon sink capacity is weak, and the carbon source is dominant. After the statistics of the average annual _NEP_ of the Yangtze River Delta region in the last 20 years, the results show that the average annual _NEP_ of the Yangtze River Delta region is 253.2 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), of which the highest is 355.9 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in Zhejiang Province of China, then 212.4 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in Jiangsu Province, 208.0 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in Anhui Province of China, and the lowest is 188.5 g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\) in Shanghai of China. The first Zhejiang Province of China is much higher than the last three.