## Discussion
### Vegetation Carbon Source/Sink in Different Land Cover Types and Their Response to Land Cover Change
However, consistent with the negative effect of land cover type conversion, we found that the areas with changes in grassland had a decrease in carbon sink of 0.20 Tg C (16.39%), and the average NEP decreased from 71.37 g C m\({}^{-2}\) a\({}^{-1}\) to 47.98 g C m\({}^{-2}\) a\({}^{-1}\). The new grassland mainly originated from bare areas and was distributed in mountainous regions, for instance, the Tianshan Mountains, Kunlun Mountains, Qilian Mountains, and Helan Mountains. Since the 1980s, the warming climate in the ARNWC has led to partial snow melting [70]. These warmer conditions could promote the growth of alpine meadows and drive the bare alpine areas into higher elevations, with the former bare area boundaries being occupied by grassland [71]. However, the increase in new grassland carbon sinks still cannot compensate for the loss of grassland carbon sinks due to the expansion of cropland in the plains [72]. The total and average values of the vegetation carbon sink in urban areas increased, and this characteristic remained under the influence of land cover change. On the one hand, this was related to urban expansion. Our study found that the area of cities in 2018 was approximately four times greater than that in 2001 due to the influence of policies such as Western Development [73], which is in line with the results of prior studies [14]. On the other hand, although urbanization in its earlier stages occupied a large amount of the surrounding natural vegetation, fossil fuel combustion and consumption by industry also released large amounts of CO\({}_{2}\) into the atmosphere. However, the vegetation growth status and carbon sink capacity will gradually improve with the intensification of subsequent human management strategies [74; 75]. In addition, taking into account the natural environment in which the ARNWC is located, urbanization will sustainably improve the vegetation coverage to a certain extent [76]. The effects of land cover type conversion resulted in a decrease of 0.19 Tg C in the carbon sink of bare areas, corresponding to a decrease of 0.79 Tg C for the total vegetation carbon sink in the regions where bare areas changed. This mainly results from the reduction in bare areas. Our study found that only the bare land areas decreased from 2001-2018, and the decrease was substantial at 23,251 km\({}^{2}\). However, the total carbon sink has increased in regions with unchanged bare areas, which eliminates the loss of carbon sink caused by the reduction of bare land area. The effect of land cover change indicates that the growth of vegetation in bare areas is improving, which is consistent with the findings observed in other regions of the globe [54]. In general, our study reveals that the vegetation in the ARNWC mainly operated as a carbon sink and that the total amount increased. Among them, although the total net increase in the vegetation carbon sink in bare areas is relatively small, accounting for only 0.16% of the total carbon sink increase in the ARNWC, it still shows the same trend of increasing carbon sink as observed by previous studies [77; 78]. Under the influence of several national policies, land cover change has an obvious positive effect on the vegetation carbon sink. This further confirms that ecological restoration projects can be effective in achieving long-term improvements in terrestrial ecosystem carbon sink capacity [79]. However, it is still worth noting that although the carbon sink of cropland is increasing due to cropland expansion and anthropogenic management, in view of the constraints of natural conditions such as limited water resources and poor rainfall in arid regions, attention should still be paid to improving water use efficiency and avoiding the waste of water resources in the process of cropland reclamation [80]. Additionally, the windproof forest belt of cropland should be protected to prevent the occurrence of environmental problems such as land desertification [63].