## Discussion
### Vegetation Carbon Source/Sink in Different Land Cover Types and Their Response to Land Cover Change  
For different land cover types, we found that the total and average carbon sinks for cropland in the ARNWC from 2001 to 2018 increased annually, which was caused by both the expansion of cropland area and anthropogenic management. In recent years, newly reclaimed land has mainly originated from the ARNWC due to the cultivated land requisition-compensation balance policy, which has contributed to the rapid expansion of the local cropland area [61; 62]. In addition, since the expansion of cropland mainly occupies the surrounding desert grassland or bare areas, the soil moisture and nutrient conditions of the newly reclaimed areas have improved compared to the original land cover type. Furthermore, it has been proposed that agricultural activities such as the application of  
Figure 10: Distribution of vegetation carbon sources/sinks along the elevation gradient in ARNWC. (**a**) the number of grids for carbon sinks or sources contained within different elevation ranges (**b**) the red dotted line is the dividing line where NEP is zero; the line representing the annual average NEP value above the red line denotes a carbon sink, and those below the red line denote a carbon source. Figure 9: Annual variation in average precipitation during the growing season (May–October) in ARNWC. organic fertilizers, anthropogenic irrigation, and conservative tillage practices contributed to 3.2-, 2.4-, and 4.2-fold increases in the yields of wheat, maize, and cotton, respectively, in Xinjiang during 1978-2015 [63], thus indicating that the implementation of effective agricultural activities and artificial inputs in the ARNWC is favorable for increasing the productivity of cropland [64]. From 2001-2018, the total carbon sink in forestland increased by a net of 0.55 Tg C. However, the average NEP decreased from 389.86 g C m\({}^{-2}\) a\({}^{-1}\) to 374.25 g C m\({}^{-2}\) a\({}^{-1}\), which was caused by the loss of the carbon sink in the original forest areas. Our study found a decrease of 0.37 Tg C (5.67%) in the carbon pools and a decrease in the average NEP from 392.09 g C m\({}^{-2}\) a\({}^{-1}\) to 369.64 g C m\({}^{-2}\) a\({}^{-1}\) in the regions with unchanged forestland, mainly located in the Altai Mountains. This observation indicates that the ecology of the regional forest is degrading. Previous findings have noted that the proportion of young, middle-aged, and nearly mature forests in the Altai Mountains is lower, the age composition is imbalanced, and the stands have been rapidly decreasing in recent years due to age and mortality. In addition, anthropogenic activities such as indiscriminate deforestation and the exploitation of mineral resources have also led to the destruction of forests [65]. However, the forest expansion and regeneration of young and middle-aged forests caused by the implementation of national afforestation and reforestation projects and the Natural Forest Protection Project in the Ili Valley and Qilian Mountains will contribute to the increase in the carbon sink [66; 67]. The total and average carbon sinks of grassland in the ARNWC have been increasing in the last 18 years. Although the effects of land cover type conversion led to a decrease of 0.44 Tg C, which offset a part of the increased carbon sink by enhanced vegetation growth, the combined effect of land cover change was still favorable to the accumulation of the carbon sink. It has been illustrated that the overall trend of grassland growth in the ARNWC has improved, and conservation measures such as fencing, prohibition, and rotational grazing have a positive impact on grassland growth [68; 69]. This is similar to our observations that carbon sinks increase in areas with unaltered grassland.