## 3 Results
### Seasonal and Diurnal Changes in Ecosystem CO\({}_{2}\) Flux  
The components of ecosystem CO\({}_{2}\) flux varied significantly with the stage of the growing season. There were relatively higher GEP and respiration rates (Re and Rs) during the mid-season (July and August) but more negative NEE and, thus, a high rate of C sequestration in the early season (May and June) (Figures 4 and 5, Table 1). Figure 4: Seasonally dynamics of ecosystem CO\({}_{2}\) flux through 2013 to 2015 of each precipitation treatment in a semiarid grassland of northwest China. GEP, Re, Rs, and NEE are gross ecosystem productivity, ecosystem respiration, soil respiration, and net ecosystem CO\({}_{2}\) exchange, respectively. Values are mean \(\pm\) standard error. The unit of CO\({}_{2}\) flux is \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). Vertical dashed lines separate different years. The sampling size for each treatment is 4. Figure 3: Daily mean soil temperature and moisture at 0–10 cm depth for each treatment during the experimental period (July 2013 to September 2015) in a semiarid grassland of northwest China. Values are means of the replicates. Vertical dashed lines separate different years. The precipitation treatment did not significantly affect the rate of most CO\({}_{2}\) flux component across the three growing seasons, with less than 4% changes in these components compared with ambient treatment. When tested within each stage of the growing season, DP showed a trend to decrease GEP and Re in the early stage of the season. Furthermore, both DP and IP increased GEP in the late stage of the season (Figure 5). NEE decreased (less negative) in the early stage of the season (+15 and +19%) but increased (more negative) in the late stage of the season (\(-15\) and \(-19\%\)) in both precipitation treatments. The Rs was not affected at any stage of the season (Figure 5). \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & **GEP** & **Re** & **Rs** & **NEE** \\ \hline \multirow{4}{*}{SS} & Stage of season & 1321.0 & 741.2 & 286.9 & 332.7 \\  & Treatment & 6.2 & 12.5 & 0.1 & 7.6 \\  & Interaction & 120.5 & 19.8 & 6.0 & 114.3 \\  & Date & 346.9 & 5.6 & 26.9 & 440.4 \\ \hline \multirow{4}{*}{F} & Stage of season & 51.3 & 153.3 & 175.4 & 21.9 \\  & Treatment & 0.2 & 2.6 & 0.0 & 0.5 \\  & Interaction & 2.3 & 2.1 & 1.8 & 3.8 \\  & Date & 27.0 & 2.3 & 32.9 & 58.1 \\ \hline \multirow{4}{*}{P} & Stage of season & \textless{}0.0001 & \textless{}0.0001 & \textless{}0.0001 & \textless{}0.0001 \\  & Treatment & 0.785 & 0.0755 & 0.9674 & 0.6047 \\ \cline{1-1}  & Interaction & 0.0538 & 0.0857 & 0.1187 & 0.0048 \\ \cline{1-1}  & Date & \textless{}0.0001 & 0.1292 & \textless{}0.0001 & \textless{}0.0001 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: ANOVA results of precipitation treatment and stage of season on gross ecosystem productivity (GEP), ecosystem respiration (Re), soil respiration (Rs), and net ecosystem CO\({}_{2}\) exchange (NEE) in a semiarid grassland of northwest China. Figure 5: Averaged ecosystem CO\({}_{2}\) flux at early, middle, and late part of the growing season through 2013 to 2015 of each precipitation treatment in a semiarid grassland of the northwest China. GEP, Re, Rs, and NEE are gross ecosystem productivity, ecosystem respiration, soil respiration, and net ecosystem CO\({}_{2}\) exchange, respectively. Values are mean \(\pm\) standard error. The unit of CO\({}_{2}\) flux is \(\upmu\)mol m\({}^{-2}\) s\({}^{-1}\). For each stage of the growing season, bars with different lowercase are significantly different at \(p<0.05\). Degree of freedoms of the model, date, season, treatment, and interaction were 8, 1, 2, 2, and 4, respectively. Degree of error was 679. Most diurnal patterns of CO\({}_{2}\) flux components were unaffected by precipitation treatments (Figure 6). Among the seven measurements of diurnal patterns, NEE was unaffected, and only one measurement of GEP was affected (decreased by DP in August 2014).