## 1 Introduction  
As one of the three largest carbon pools on Earth, terrestrial ecosystems absorb approximately 28-31% of anthropogenic CO\({}_{2}\) emissions from the atmosphere [1, 2], so enhancing the carbon sink function of terrestrial ecosystems can be effective in mitigating rising atmospheric CO\({}_{2}\) concentrations and limiting global warming [3]. The terrestrial ecosystem carbon cycle has great spatial and temporal heterogeneity, and monitoring its spatial and temporal variability is the key to accurately predicting future climate change and formulating climate negotiation strategies [4]. Therefore, the terrestrial ecosystem carbon cycle has become one of the core contents of research on global change and has received widespread attention from the international community [5]. Above all, the carbon sink effects provided by terrestrial ecosystems are essential for preserving nature and the survival of human society. However, such things as economic development and the intensification of human production activities (fossil fuel use, irrational land use, etc.) have led to increasing greenhouse gas emissions, which have disrupted the original source-sink balance of terrestrial ecosystems [6]. Among them, land cover change has altered the original land cover pattern, with a wide range and intensity of impact. Therefore, land cover change can directly disturb the terrestrial ecosystem carbon cycle [7] and have a great influence on terrestrial ecosystem functions [8]. Arid terrestrial ecosystems are generally fragile and vulnerable to external disturbances. However, the research regarding how land cover change affects vegetation carbon sources/sinks in arid regions is limited, and most studies have concentrated on the carbon fluxes of individual terrestrial ecosystems (e.g., grasslands, forests, etc.) without comparing the interactions between different terrestrial ecosystems [9]. The arid region of Northwest China (ARNWC) is far inland, experiences an arid climate with little rainfall, and its fragile ecosystems have a low self-regulatory capacity against external disturbances. Since the 1940s, the dramatic increase in population, socioeconomic development, and large-scale soil and water resource exploitation in the ARNWC have caused excessive depletion of resources, giving rise to many ecological issues, including land degradation and desertification, the degradation of riparian forest ecosystems, and the reduced carrying capacity of grasslands [10]. To solve these problems, a number of ecological restoration projects were launched in China between the late 1990s and the beginning of the 21 st century, for instance, the grain for green program and the natural forest protection project, which were aimed at strengthening environmental restoration by closing hills for afforestation, planting trees on wastelands, and prohibiting grazing [11; 12; 13]. In this context, the land cover pattern of the ARNWC has experienced substantial changes [14]. However, the effects of land cover change on the ecosystem vegetation carbon sources/sinks in the ARNWC are still unclear, and whether the ARNWC, especially the bare areas that contain desert ecosystems, is a carbon source or a carbon sink remains uncertain [15; 16; 17; 18]. Given these findings, it is crucial to analyze the spatial distribution and temporal variation characteristics of vegetation carbon sources/sinks and assess their impacts on land cover change in the ARNWC. This work can provide a scientific basis for the conservation and management of the ecological environment in arid regions, provide a reference for the rational use and planning of land resources, and regulate human activities under the premise of reducing carbon emissions. The methods for estimating carbon sinks mainly include four approaches: field surveys [19; 20; 21; 22], flux measurements [23; 24; 25], atmospheric inversions [26; 27; 28], and model simulations [29; 30].