## 4 Discussion
### Analysis of Impact Mechanisms  
The change in the NEP in the MRYR is the result of the joint action of environmental and human activity factors. Anthropogenic impacts on land cover profoundly change the structure and function of natural ecosystems, which in turn changes the carbon fluxes of ecosystems [89]. From 2001 to 2022, 1407.19 km\({}^{2}\) of forest was converted to cropland, but the NEP increased. This shows that not only do human activities have a great influence on the NEP, but climate change also considerably contributes to the increase in carbon sinks.  
Figure 14: Spatial distribution of mean values of NPP (**a**) and R\({}_{\rm H}\) (**b**) in MRYR, 2001–2022.  
From 2001 to 2022, PRE and TEM in the MRYR increased by 68.74 mm and 0.29 \({}^{\circ}\)C, respectively, resulting in more favorable conditions for vegetation growth [90; 91]. In relatively arid regions, the effect of PRE on vegetation tends to be stronger relative to the TEM [92]. [PERSON] et al. [93] also found a lack of correlation between interannual changes in the temperature and carbon sinks in arid and semi-arid regions. In this study, the correlation of the NEP, WUE, and CUE with PRE was significantly higher than that with TEM. In the interannual anomalies, we found that an increase in precipitation not only affects a carbon sink in that year, but also has a strong effect on the accumulation of the carbon sink in the following year. This confirms that PRE is the main controlling factor for carbon sink accumulation in the MRYR.  
Precipitation and temperature affect photosynthesis and evapotranspiration, which in turn affect vegetation development [94]. An increase in PRE and TEM enhances ETa and carbon sinks, which lead to differences in the WUE and CUE across different vegetation types and regions. The correlation of WUE and CUE with PRE and TEM in the northern part of the MRYR is positive. One reason for this is that grassland is the dominant vegetation type, and increased PRE and higher temperatures enhance grassland NEP; the other is that the temperature is relatively low and there is low evapotranspiration and, in the face of drought stress, these reduce leaf stomatal conductance with increasing temperature to enhance its own WUE [95]. The correlation between WUE and PRE has a clear latitudinal zonality [96]. Excluding cropland, the value of WUE shifts from a negative to a positive correlation as the latitude increases. Similar results were found in the study of [PERSON] et al. [28] of the Loess Plateau. Changes in CUE are produced by a combination of factors, and the correlation between vegetation CUE and TEM may be confounded due to a combination of CO\({}_{2}\) fertilization, nitrogen deposition, saturated water vapor pressure, and other factors.  
There was a very high positive correlation (\(p<0.05\)) between the NEP and DSI in the MRYR. This indicates that droughs have a direct effect on carbon sinks. Carbon sinks have been reported to increase in many regions of the globe during drought years [14; 97; 98]. The reason for this may be that temperature affects vegetation respiration, which in turn promotes an increase in carbon sinks [40]. In particular, light droughs are coupled with high temperatures, and high temperatures may offset the reduction in carbon sinks caused by moisture deficits [97]. However, this phenomenon did not occur in this study area, and the moisture content in the MRYR was likely already at a very low level which supported an increase in carbon sinks. The correlation between the WUE, CUE, and DSI has a clear vegetation differentiation. There are very high correlations in the grasslands and croplands, lower correlations in the forests and shrubs, and even a large number of negative correlations in the eastern part of the MRYR. This phenomenon could be related to the resilience of vegetation to cope with drought; grassland and cropland do not have the function and structure of forests and shrubs, and drought affects them more rapidly and directly.