# Median to Strong Rainfall Intensity Favors Carbon Sink in a Temperate Grassland Ecosystem in China
## 1 Introduction  
Rainfall inputs alter ecosystem processes (e.g., primary production) and community structures over short (hourly and daily) and long (seasonal and yearly) timescales [1, 2]. Over the past decades, there has been an amplification of the hydrological cycle due to global warming [3]. Changes in amount, frequency, intensity, and extremes of regional rainfall by alteration of the hydrological cycle have been broadly observed [3, 4, 5]. Rainfall appears to shift from more frequent and smaller to fewer and larger precipitation events without change in total rainfall amount [6, 7]. Therefore, it is essential to understand the response of ecosystem processes to changes in rainfall regime, especially for water-limited ecosystems. Grassland ecosystems in arid and semiarid regions cover about 20% of land surface area and store about 10% of global soil organic carbon [8]. They dominate the trend and inter-annual variability of carbon sink, thus playing a critical role in carbon-climate feedback [9]. In grasslands, CO\({}_{2}\) exchangeis also controlled by variations in water availability [10; 11; 12]. Therefore, there is a strong relationship between carbon cycle and rainfall inputs [13; 14]. However, the response of CO\({}_{2}\) exchange to changes in rainfall regimes is unknown. Net ecosystem exchange (NEE), the balance between gross primary productivity (GPP) and ecosystem respiration (ER), is the shift signal from carbon source to sink, closely linking with the intensity of rainfall in grasslands [10; 15; 16; 17]. The different sensitivities of GPP and ER to variation in the intensity and frequency of rainfall control the magnitude and direction of NEE [1]. The sensitivity of GPP and ER varies along with change in rainfall patterns; therefore, it is difficult to predict the response of NEE to variations in rainfall. For example, small rainfall events may result in a larger degree of increase of ER than GPP; NEE is then found to be positive (carbon source) [18]. However, large rainfall events may result in a greater degree of augmentation of GPP than ER; NEE is then found to be negative (carbon sink) [10]. Therefore, there exists a rainfall threshold to meet the tipping point of NEE, along with changes in rainfall patterns when NEE moves from positive (source) to negative (sink). Unfortunately, the rainfall threshold has not yet been confirmed. Considering that changes in rainfall patterns in climate change experiments are key processes for understanding and predicting potential trends in CO\({}_{2}\) fluxes in semi-arid and arid regions in response to advancing climatic changes, it is an urgent need to seek for a rainfall threshold to explore the response of NEE to the variation in rainfall patterns. In grassland ecosystems, plant roots significantly impact ecosystem CO\({}_{2}\) fluxes to cope with the limit of water availability [19]. Rooting depth is sensitive to precipitation infiltration depth [20]. Large rainfall events infiltrate much deeper than smaller ones [1; 21]; therefore, rooting depth may be caused by large rainfall events. With variation in rainfall regimes in the future, rooting dynamics will be closely connection with rainfall events. However, we have little information on how root dynamics responds to changes in rainfall frequency and size of grasslands. In addition, GPP and ER have a significantly positively relationship with root production [22]. For example, an enhanced GPP will increase root production, and ER depends on root productivity [19]. Also, it is unknown whether the response of carbon fluxes has as consistent a trend as the response of root production to change in rainfall patterns. In order to examine the key gaps, we conducted three-year experiments in rainfall regimes with a gradient of five rainfall addition levels (0 mm, 2 mm, 5 mm, 10 mm, 20 mm, and 40 mm) during growing seasons in semi-arid grasslands in Inner Mongolia, China. The rainfall addition gradient offers the possibility to test how NEE responds to change in rainfall regimes.