## 3 Results
### Spatial Distribution and Temporal Variation of Carbon Source/Sink
#### 3.3.2 Temporal Variation in Vegetation Carbon Sources/Sinks  
From 2001-2018, the total NEP in the ARNWC increased from 73.68 Tg C in 2001 to 85.06 Tg C in 2018, with an average rate of increase of approximately 0.67 Tg C/a, and the average NEP increased from 35.84 g C m\({}^{-2}\) a\({}^{-1}\) to 41.37 g C m\({}^{-2}\) a\({}^{-1}\) (Table 7). The total NEP of all land cover types also showed a net increase. Among them, the urban areas showed the most significant increase in total NEP, which more than tripled compared to 2001, although the total increased by only 0.08 Tg C. The contribution by cropland was the largest, with a net increase of 8.36 Tg C in recent years, accounting for approximately 73.46% of the total regional NEP increase, and the average rate of increase was approximately 0.49 Tg C/a. Following cropland were grassland, forestland, and bare areas, whose net increases in NEP were 2.36 Tg C, 0.55 Tg C, and 0.03 Tg C, respectively. Comparing the average NEP of each land cover type, we found that all land cover types except forestland had an increasing trend. This further indicated that the vegetation growth of each land cover type except forestland was improving. \begin{table}
\begin{tabular}{c c c c c} \hline Cropland to Forestland (12) & Cropland to Grassland (13) & Cropland to Urban Areas (14) & Cropland to Bare Areas (15) \\ Forestland to Cropland (21) & Forestland to Grassland (23) & Forestland to Urban areas (24) & Forestland to Bare areas (25) \\ Grassland to Cropland (31) & Grassland to Forestland (32) & Grassland to Urban areas (34) & Grassland to Bare areas (35) \\ Bare areas to Cropland (51) & Bare areas to Forestland (52) & Bare areas to Grassland (53) & Bare areas to Urban areas (54) \\ \hline \end{tabular}
\end{table} 
Table 6: Codes for interannual land cover type conversions. \begin{table}
\begin{tabular}{c c c c c c} \hline \multirow{2}{*}{**From**} & \multicolumn{2}{c}{**To**} & & & \\ \cline{2-6}  & **Cropland** & **Forestland** & **Grassland** & **Urban Areas** & **Bare Areas** \\ \hline \multirow{2}{*}{Cropland} & 133,151.75 & 339.50 & 2433.00 & 1083.75 & 756.50 \\  & 96.65\% & 0.25\% & 1.77\% & 0.79\% & 0.55\% \\ \hline \multirow{2}{*}{Forestland} & 150.00 & 16,635.00 & 318.50 & 1.00 & 43.75 \\  & 0.87\% & 97.01\% & 1.86\% & 0.01\% & 0.26\% \\ \hline \multirow{2}{*}{Grassland} & 6332.50 & 2328.50 & 434,464.00 & 556.75 & 7914.75 \\  & 1.40\% & 0.52\% & 96.21\% & 0.12\% & 1.75\% \\ \hline \multirow{2}{*}{Urban areas} & 0.00 & 0.00 & 0.00 & 730.25 & 0.00 \\  & 0.00\% & 0.00\% & 0.00\% & 100.00\% & 0.00\% \\ \hline \multirow{2}{*}{Bare areas} & 13,228.00 & 29.25 & 18,465.25 & 243.50 & 1,416,609.00 \\  & 0.91\% & 0.00\% & 1.27\% & 0.02\% & 97.79\% \\ \hline \end{tabular}
\end{table} 
Table 5: The land use transfer matrix for the ARNWC during 2001–2018 (unit: km\({}^{2}\)). In Figure 5, the annual carbon sink of cropland in the northern part of Tacheng Prefecture, the economic belt on the northern slope of the Tianshan Mountains, the II Region, the edge of the Tarim Basin, and the Qilian Mountains has been increasing over time. In contrast, the high carbon sink areas of forest located in the Altai Mountains and Tianshan Mountains, as well as the grassland in the II Valley, have decreased annually, indicating that the vegetation carbon sequestration capacity in these areas is decreasing. The regions that behave as carbon sources, such as the northern and southern slopes of the Tianshan Mountains, have continued to increase carbon emissions at a slow rate in recent years. The remaining sparsely vegetated areas, mainly within the hinterlands of the Tarim Basin, Junggar Basin, Tuha Basin, and Alxa Plateau, showed minor changes in NEP. As indicated by land cover type conversions from 2001 to 2018, the results in Figure 6 show that in areas where the land cover type remains unchanged, the total NEP of cropland, grassland, urban areas, and bare areas has increased by 4.91 Tg C, 2.57 Tg C, 0.025 Tg C, and 0.82 Tg C, respectively, which indicates that their vegetation growth has improved.