## 4 Discussion
### Dependence of CO\({}_{2}\) Flux on Soil Moisture  
The rate of each CO\({}_{2}\) flux component was expected to increase with soil moisture in semiarid regions [1; 41], but our results demonstrated decreased CO\({}_{2}\) flux with soil moisture (Table 3). This decreased CO\({}_{2}\) flux was mainly due to the asynchrony between soil moisture and plant growth or soil temperature during the growing season (Figure 3). Plants grew quickly in the early and middle stages of the season [29], and they had relatively high rates of assimilation and respiration, and more negative NEE (Figures 4 and 5). The air and soil temperatures were both higher, but soil moisture was lower at these stages, compared with the late stage of the growing season (Figure 3). In contrast, the plants stopped growing later in the season with lower (or less negative) GEP, NEE, and Re (Figures 4 and 5). The air and soil temperatures were lower, but soil moisture was higher at this stage (Figure 3). This asynchronous pattern between plant growth and soil moisture during the growing season results in a declining trend of CO\({}_{2}\) flux with soil moisture (Figure 7, Table 3). These results indicate that the ecosystem in the study region was more limited by soil moisture during the early and middle stages of the growing season but was more limited by temperature in the late stage of the season. This asynchrony was consistent with findings by [PERSON] et al. [66] in a grassland in Tennessee and implied that the dependence of CO\({}_{2}\) flux on soil moisture was modulated by this asynchrony. The precipitation treatments significantly changed the dependence of CO\({}_{2}\) flux on soil moisture. The DP resulted in 16% decrease in soil moisture, strengthening the limitation of soil moisture. In contrast, IP resulted in a 21% increase in soil moisture, which weakened the limitation of soil moisture and, thus, the dependence of GEP on soil moisture (Table 3). The dependence of respiration (Re and Rs) on soil moisture, however, decreased in DP (Table 3), which was consistent with results from a study in a pinon-juniper woodland [67]. The opposite influencing patterns of precipitation treatments on the dependence of GEP and Re to soil moisture may be due to differences in the mechanism between GEP and respiration in response to soil moisture conditions. Precipitation can directly affect GEP by changing stomatal performance, which is directly modulated by soil moisture [50; 51]. On the other hand, respiration response is mainly determined by the availability of substrates, which is indirectly affected by soil moisture. In comparison to wet conditions, drought affects the allocation of assimilates in the plant-soil system by transferring a larger proportion of assimilates to the roots [68] and increasing microbial biomass and enzymatic activities in the rhizosphere [69]. Therefore, it has a contrasting effect on soil respiration, in comparison to effects on GEP. Since long-term changes in precipitation might significantly change soil temperature and moisture (Figure 3), which determine ecosystem CO\({}_{2}\) flux, we hypothesized that increased precipitation would enhance the dependence of CO\({}_{2}\) flux components on temperature but weaken the dependence on soil moisture, while decreased precipitation would have opposite effects on such dependencies. The changes in the dependence of GEP, Re, and Rs on temperature (Table 2), and the changes in the