## 4 Discussion  
### Response of CO\({}_{2}\) Flux to the Precipitation Treatments  
Our results showed a seasonal pattern of CO\({}_{2}\) flux in this semiarid grassland (Figures 4 and 5), which was mainly due to variations in plant growth throughout the growing season. A previous study at the same site showed that _S. capillata_ is characterized by a higher growth rate early in the season and higher aboveground biomass accumulation at mid-season [29]. High growth rates often produce more negative NEEs and, thus, high rates of ecosystem C sequestration [30; 31; 32]. High above-ground biomass accumulation corresponds to a high GEP and, thus, high Re and Rs because respiration is driven by photosynthesis-provided substrate in most ecosystems [33; 34]. In our study, Re and Rs were positively correlated with GEP (r = 0.683 and 0.419, respectively, \(p<\) 0.01). The growth and accumulation of biomass in the ecosystem was lowest in the late stage of the growing season, resulting in a lower rate of each CO\({}_{2}\) flux component (Figure 4). Our results are consistent with the dependence of the seasonal pattern of CO\({}_{2}\) flux on plant growth observed in various grasslands [35; 36; 37; 38; 39]. We demonstrated that the seasonal and diurnal patterns of most CO\({}_{2}\) flux component were not affected by increased or decreased precipitation in this study (Figures 4 and 5, Table 1). These results indicated that most components of CO\({}_{2}\) flux were relatively stable in this semiarid grassland across the various precipitation scenarios, supporting our first hypothesis. The response of ecosystem CO\({}_{2}\) flux to precipitation treatments was mainly manifested by changes in ecosystem productivity [40; 41; 42]. Ecosystem productivity generally increases with increasing precipitation [43; 44; 45] and decreases with decreasing precipitation [46; 47]. In our study, however, above-ground biomass averaged across 2013 to 2015 was not affected by changes in precipitation, with mean aboveground biomass (post-experiment biomass) of 87.2, 88.0, and 88.9 g m\({}^{-2}\) in AP, DP, and IP plots, respectively. The most direct effect of the precipitation treatments on plant growth was the changes in the availability of soil water to plants [44]. However, soil water availability to plants remained constant at levels of 40-80% field capacity in the same region as this study [48], corresponding to 10.4-20.8% volumetric soil moisture at the same site of this study [49]. Soil moisture in our study was within this range in the three precipitation treatments during the early and middle stages of the growing season (Figure 3), indicating that the treatments did not affect the availability of soil water to plants. Soil moisture in DP was sometimes less than this range and, thus, may have influenced the stomatal performance of the leaves [50; 51]. Furthermore, the little response of available soil moisture further indicated that soil water recharge from non-growing season precipitation might be important for determining the main processes of this ecosystem. Soil temperature in DP, however, was relatively higher than in AP, which may have partly enhanced the photosynthetic rate in this temperature-limited ecosystem [52; 52], thus, may have offset the decreases in CO\({}_{2}\) flux due to stomatal limitation. A lack of an effect of precipitation treatments on GEP was expected because the adaptation of plants would decrease the response to environments [18; 19; 53]. This indicated that productivity in the semiarid grassland was relatively stable to variation in precipitation. The Re and Rs were not affected by precipitation treatments, primarily due to the lack of response of GEP (\(-1\%\) across three seasons), which drives ecosystem and soil respiration, as proposed by previous studies [33; 34; 54] and suggested by the linear relationship between the variables in our study (Re = 1.662 + 0.334 \(\times\) GEP, R\({}^{2}\) = 0.467, RMSE = 1.381, \(p\) < 0.0001; Rs = 1.381 + 0.121 \(\times\) GEP, R\({}^{2}\) = 0.175, RMSE = 1.016, \(p\) < 0.0001).