## 1 Introduction  
Semiarid grassland ecosystems are among the most vulnerable ecosystems and are highly susceptible to global climate change [1, 2]. Semiarid grassland ecosystems are also increasingly important drivers of the inter-annual variability of the global carbon (C) cycle [3]. Precipitation and the availability of soil water are the major limiting factors in semiarid ecosystems [4]. Precipitation directly affects soil moisture, which plays a prominent role in terrestrial ecosystems by affecting plant productivity and soil processes [5, 6]. In turn, soil moisture modulates the impacts of other drivers of global change, such as elevated atmospheric CO\({}_{2}\) levels, temperature, and nitrogen deposition [7, 8, 9]. Therefore, the responses of ecosystem processes to variations in soil moisture have become a focus of current ecological and environmental research. Photosynthesis and plant and soil respiration, which are the main components of the ecosystem CO\({}_{2}\) flux, are the major processes determining the productivity and C cycling in terrestrial ecosystems. Although the responses of these processes to precipitation changes (both increasing and decreasing precipitation) have been studied in various ecosystems, considerable uncertainty remains regarding the direction and magnitude of the responses [3; 4; 10; 11]. Studies in arid and semiarid regions, where precipitation is expected to decrease [12; 13], are particularly lacking. Such uncertainties and lack of knowledge make it difficult to accurately predict the responses to future scenarios of precipitation change [14; 15]. The CO\({}_{2}\) flux at various levels (from leaf to ecosystem) may acclimate to changes in temperature [16; 17; 18], which is characterized by a downward or upward regulation in the shape of long- or short-term response functions of CO\({}_{2}\) flux to temperature at different thermal environments [18]. Such shifts in the response function can modulate the feedback between C cycling in terrestrial and climate systems [18; 19]. For example, shifts in the response function of plant respiration due to warming are predicted to have the potential to reduce the annual CO\({}_{2}\) release by 35-80%, in comparison to a theoretical case without shifts in the response function [18; 19]. However, the shift in the components of CO\({}_{2}\) flux to soil moisture has rarely been examined. This information is urgently needed because such a shift in CO\({}_{2}\) flux would have important implications for ecosystem C cycling, which would be potentially similar to acclimation to temperature on ecosystem C cycling. This effect is particularly important for ecosystems limited by water, as changes in precipitation regimes can affect C dynamics and fluxes and are expected to have significant feedback on the terrestrial C cycle [20]. We investigated the components of ecosystem CO\({}_{2}\) flux (photosynthesis and plant and soil respiration) in plots with manipulated levels of precipitation in a _Stipu capilata_ L. grassland in a semiarid region of the northwestern China. The _S. capililata_ is a dominant native species adapted to the highly variable precipitation in this semiarid region [21; 22]. We hypothesized that CO\({}_{2}\) flux components in this _S. capilata_ grassland would not vary widely across precipitation regimes (H1). We also hypothesized that increased precipitation would enhance the dependence of CO\({}_{2}\) flux components (photosynthesis and respiration) on temperature, but decreased precipitation would weaken such dependence (H2) and that increased precipitation would weaken the dependence of CO\({}_{2}\) flux components on soil moisture, but decreased precipitation would enhance the dependence on soil moisture (H3). Hypotheses H2 and H3 were generated because long-term changes in precipitation might significantly change soil temperature and moisture, which in turn would lead to the shifts in the response curves of CO\({}_{2}\) flux components to the altered environment.