# Median to Strong Rainfall Intensity Favors Carbon Sink in a Temperate Grassland Ecosystem in China
## 2 Materials and Methods
### Measurement of CO\({}_{2}\), and Abiotic and Biotic Variables  
Ecosystem CO\({}_{2}\) fluxes, ecosystem respiration (ER), and net ecosystem exchange (NEE) were directly measured in middle growing season (July to August) by a multichannel automated measurement system developed by the National Institute for Environmental Studies of Japan. The system consists of an infrared gas analyzer (IRGA, Li-840, Li-Cor Inc., Lincoln, USA) and a data logger (CR3000, Campbell Scientific, Inc., Logan, UT, US). More details on this multichannel automated measurement system can be found in our previous studies [5; 6]. In each plot, the system measured NEE (with sunlight) and ER (with lightproof cloth covering). Each measurement lasted 150 s. The data logger monitored the CO\({}_{2}\) concentration output signals from the IRGA at a rate of 1 Hz and recorded their averages at 10-s intervals. Only mid-values of 130 s (deleting the first and last 10 s) were used [5; 6] to calculate the flux, as shown in Equation (1):  
\[F=\left(\left(V\times P(1-W)/\left(R\times S\times T\right)\right)\times\left(U _{c}/U_{t}\right)\right. \tag{1}\]  
where \(F\) is the CO\({}_{2}\) flux (umol m\({}^{-2}\) s\({}^{-1}\)) (NEE or ER), \(V\) is the volume of the chamber (m\({}^{3}\)), \(P\) is the air pressure (Pa), \(W\) is the water vapor mole fraction, \(R\) is the universal gas constant (8.314 J mol\({}^{-1}\) K\({}^{-1}\)), \(S\) is the base area of the chamber (m\({}^{2}\)), \(T\) is the air temperature in the chamber (K), and \(U_{c}/U_{t}\) is the rate of increase in the CO\({}_{2}\) mole fraction (mmol mol\({}^{-1}\) s\({}^{-1}\)) in the chamber calculated by least squares. Negative NEE indicates that the ecosystem absorbs CO\({}_{2}\) from the atmosphere, while positive values indicate CO\({}_{2}\) emission.  
We used Equation (2) to assess daytime ecosystem respiration (\(ER_{d}\)) due to nighttime fluxes representing night ecosystem respiration:  
\[ER_{d}=ER_{ref}\times\left(b_{1}+b_{2}SWC\right)\times\left(\frac{T_{s}-T_{ref }}{10}\right) \tag{2}\]  
where the \(ER_{ref}\) is the ecosystem respiration at the reference temperature (10 \({}^{\circ}\)C) and optimal soil moisture, \(T_{s}\) is the soil temperature, \(b_{1}\) and \(b_{2}\) are constant coefficients evaluated from the linear relationship between nighttime ER, temperature, and SWC. Finally, GPP was indirectly estimated as Equation (3):  
\[\text{GPP}=ER_{d}-NEE \tag{3}\]  
There were two thermocouples in each chamber to measure air temperature and soil temperature (at a depth of 5 cm) at the same time as CO\({}_{2}\) flux. Soil water content at depths of 10 cm, 20 cm, 30 cm, and 40 cm were measured every three days by Diviner 2000 (Sentek Pty. Ltd., Australia).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Rainfall Treatment** & **2 mm** & **5 mm** & **10 mm** & **20 mm** & **40 mm** \\ \hline Number & 40 & 12 & 6 & 3 & 2 \\ Frequency & 2 days & 3 days & 7 days & 15 days & 1 month \\ Start date & June 1 & June 1 & June 1 & June 1 & Mid-July \\ \hline \hline \end{tabular}
\end{table}
Table 1: The number, frequency, and start date of the rainfall treatment.  
Aboveground biomass (AGB) and belowground biomass (BGB) were clipped in a 1 \(\times\) 1 m quadrat of each plot on 15 August. All living plants (AGB) above the soil surface were clipped by the harvest method [17]. For each year, the locations of quadrats were randomly changed to prevent resampling of the same quadrat. Plant root (BGB) were estimated at three depths (0-10 cm, 10-20 cm, and 20-40 cm) with an iron tube (10 cm diameter and 1 m length), based on soil coring methods.