## 2 Materials and Methods
### Data Processing  
#### 2.3.1 Calculation of _Nep_  
The _NEP_ of a vegetation ecosystem is an important index to measure the carbon source and sink of vegetation in a region. Without considering the influence of other natural and human factors, _NEP_ is equal to the difference between vegetation _NPP_ and soil microbial respiration carbon consumption [25]. the lengthy sequenced variation of _NEP_ of terrestrial vegetation, and its response to climate needs to be rapidly, comprehensively, and precisely understood in the process of the rapid development of urban agglomeration with the strongest comprehensive strength in developing countries. If the influence of other natural and human conditions is not considered, it can be expressed as the difference between _NPP_ and the carbon emissions from soil microbial respiration (\(R_{H}\)), and the calculation formula is:  
\[NEP=NPP-R_{H} \tag{1}\]  
In Formula (1), _NEP_ is the net ecosystem productivity of vegetation, _NPP_ is the net primary productivity of vegetation, and \(R_{H}\) is soil microbial respiration, and the units of these three factors are all g C\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\). When _NEP_ > 0, it means that the carbon fixed by vegetation is more than the carbon emitted by soil respiration, and the carbon sequestration by vegetation is reflected as a carbon sink effect. On the contrary, when _NEP_ < 0, it is reflected as a carbon source effect. Therefore, it can be used for the quantitative analysis of the carbon sequestration status and potential of a regional vegetation ecosystem. The annual _NPP_ data of vegetation are based on MODIS satellite remote sensing parameters, and the data are calculated by using the BIOMe-BGC model and light energy utilization model [26]. At present, these data have been verified and widely used in carbon cycle studies in many regions [27; 28; 29; 30]. The annual total value is the total amount of _NEP_ of all pixels in a year, the mean value of the total value in a year is the sum of annual total values of _NEP_ of all pixels divided by the number of pixels, and the maximum and minimum values of the annual total value of _NEP_ are the maximum and minimum values of _NEP_ in all pixels in this region in the current year. \(R_{H}\) refers to the amount of soil microbial respiration, which is related to the number and types of microorganisms in the soil and the exudates of plant roots. Temperature and precipitation are the two most important factors among all the factors affecting soil microbial respiration [31]. Therefore, \(R_{H}\) was calculated by using the regression equation of temperature, precipitation, and carbon emission. Based on the field-measured data, the regression equation between soil carbon emission and conventional meteorological data (temperature \(T\) and precipitation \(R\)) is established, whichcan be readily applied to forestlands, grasslands, and croplands [13; 15]. The calculation formula is:  
\[R_{H}=0.22\times\left[\varepsilon^{0.0913T}+ln(0.3145R+1)\right]\times 30\times 46.5\% \tag{2}\]  
In Formula (2), conventional meteorological data are used. \(T\) (\({}^{\circ}\)C) is the monthly mean temperature, and \(R\) (mm) is the monthly precipitation.  
Since construction land and unused land are carbon sources, when calculating _NPP_, the pixel values of such land types are removed in advance.