# Quantifying Soil Moisture Impacts on Water Use Efficiency in Terrestrial Ecosystems of China
## 4 Discussion
### The Coupling Relationship between Soil Moisture and WUE  
Studies have shown that \(WUE\) is sensitive to changing environments [59]. Generally, many factors, such as the CO\({}_{2}\) concentration, nitrogen deposition, climatic factors, vegetation, human drivers, and \(SM\), significantly influence \(WUE\)[6; 60; 61; 62]. \(SM\) is one of the critical factors affecting \(WUE\)[21; 63]. However, most studies have not thoroughly explored the mechanisms and accurate quantification of the effects of \(SM\) on \(WUE\). Our results showed that \(SM\) had a completely negative influence on \(WUE\) via the \(ET\) pathway. However, \(SM\) also directly influenced \(WUE\) via the \(NPP\) pathway, which included negative (44.29% of the total areas) and positive effects (55.71% of the total areas); the overall positive effect was dominant. The areas in which \(SM\) had a negative impact on \(WUE\) via \(NPP\) were mainly distributed in the extremely arid desert areas of Northwest China, the cold temperate areas in Northeast China, and humid areas in Southeast China, which might have been caused by the negative impact of \(SM\) on \(NPP\)[64].  
Under the dual effects of \(SM\), there was a clear negative feedback relationship between \(WUE\) and \(SM\), which was generally consistent with existing research [7; 31]. The increase in \(SM\) often led to a decrease in \(WUE\) in most areas. The strength of the feedback relation between \(SM\) and \(WUE\) may have partly depended on the hydrothermal conditions of the local environment. However, the most significant feature of the spatial variability of the \(WUE\) in this study was that \(WUE\) showed a downward trend in the arid area of northwestern China. In contrast, there was an upward trend in the humid eastern region of China. Therefore, China's terrestrial ecosystem \(WUE\) may be more dominated by \(ET\) than by \(NPP\)[21]. Because the \(NPP\) of tropical and subtropical vegetational zones is higher, it is generally approximately 8-10 times that of an arid desert and alpine vegetation areas in northwest China [65]. However, the \(WUE\) in the tropical and subtropical vegetation zones was smaller than that of the desert and alpine vegetation areas, which may have been due to excessive \(ET\) reducing the \(WUE\). Furthermore, studies have shown that whether the effect of \(SM\) on \(WUE\) is positive or negative depends primarily on whether the \(ET\) or \(NPP\) process dominates the \(WUE\) process [21]. This study used the absolute value of \(\frac{\partial WUE}{\partial ET}\frac{\partial ET}{\partial W}\) and \(\frac{\partial WUE}{\partial NPP}\frac{\partial NPP}{\partial W}\) to determine which pathway of \(ET\) or \(NPP\) dominated the feedback relationship between \(SM\) and \(WUE\). The results showed that the \(ET\) pathway dominated the feedback relationship between \(SM\) and \(WUE\) in 72% of the total area. The region in which the \(NPP\) pathway determined the feedback relationship only accountedfor approximately 28% of the total area, especially in the Qinghai-Tibet Plateau. Therefore, terrestrial ecosystems can partially offset the influences of soil dryness by improving _WIE_. In addition, this adjustment should be more remarkable in areas where _ET_ controls the _WIE_ process, such as in the arid regions of the northwest.