## 4 Discussion
### Characterization of Changes in NEP, WUE, and CUE  
In this study, the results show that the NEP, WUE, and CUE in the MRYR show an increasing trend in general. This is inextricably linked to China's ecological protection measures. The magnitude of the NEP is closely related to the biomass and, in general, the larger the biomass, the larger the NEP. The NEP of the forests and shrubs in this study show higher values, which is the same as the results of previous studies [81; 82]. However, the interannual growth rate of the NEP is the opposite, with the grassland being significantly higher than that of the other vegetation. This is related to the physiogonomy of the vegetation and the stability of the community, as the effect of climate on vegetation is inversely related to the stability of the vegetation community [83]. The trend of NEP growth in the southeastern and northwestern edges of the MRYR is significantly less than that of the other regions. This may be due to higher heterotrophic respiration (R\({}_{\rm H}\)) caused by high TEM and PRE in the southeastern part of the MRYR (Figure 14). In contrast, the arid climate in the northwestern part of the MRYR causes a slower growth of carbon sinks. The performance of WUE is forest \(>\) shrub \(>\) cropland \(>\) grassland. The forest and shrub ecosystems have a stronger ability to accumulate organic matter, and their biomass is significantly higher than that of the other vegetation types. Cropland has a higher carbon sink and sufficient water due to human intervention (irrigation and fertilization). The WUE shows a spatial distribution of being high in the southeast and low in the northwest, which is mainly related to the spatial distribution of the vegetation [28]. Moreover, the WUE of the natural vegetation around croplands show higher values, which is similar to the study  
Figure 13: (**a**): Time-varying characteristics of R\({}_{\rm H}\) and GPP, 2001–2022. (**b**) Characteristics of temporal changes in precipitation and temperature, 2001–2022. (**c**) Characteristics of temporal changes in R\({}_{\rm H}\) in region with winter temperatures \(>\)0 \({}^{\circ}\)C and \(\leq\)0 \({}^{\circ}\)C, 2001–2022. of [PERSON] et al. [84] of the Loess Plateau. In forests and shrubs, the increasing trend of ETa is higher than that of the NEP, resulting in a decreasing trend of its WUE. This may be due to the increase in tall trees, such as in forests under ecological conservation projects, which require water resources that are not only supplemented by precipitation but may also require the deep absorption of soil moisture and groundwater resources by the root system, thus enhancing plant transpiration [85]. This may be one of the reasons why the depth of the dry soil layer in the Loess Plateau has deepened [86]. Therefore, when implementing ecological conservation policies, they should be adapted to the local conditions and not involve the blind planting of trees and grasses. Although the soil moisture content is decreasing [54; 86], the vegetation still shows a greening trend, and the carbon sink is still increasing. This may be caused by atmospheric CO\({}_{2}\) fertilization [87]. Studies have shown that the WUE growth rate is higher in more arid areas, suggesting that the vegetation in areas of moderate water scarcity has a better ability to adapt to drought [28]. This study shows similar results. The CUE of the shrubs and forests is higher than that of the other two vegetation types, which is inconsistent with most studies [7]. This is because the CUE in this paper is calculated based on the NEP, and the other articles calculated it by the NPP. There is a process of heterotrophic respiration between the NPP and NEP. The multi-year mean value of the NPP in the MRYR is 351.31 gCm\({}^{-2}\) for grasslands, 439.47 gCm\({}^{-2}\) for croplands, and the mean value of the shrubs and forests are around 580 gCm\({}^{-2}\).