1 | INTRODUCTION
Plant leaf carbon isotope composition (δ13C) is the integration of photosynthetic activities during plant leaf tissue synthesis, which can reflect the relative relationship between plant water loss and carbon harvest (Shen et al., 2019). It is often used to indirectly indicate the long-term water-use efficiency (WUE) of plants (Liu et al., 2016; Stewart et al., 1995; Farquhar et al., 1982). A large number of studies have shown that the δ13C value of plant leaves has a robust positive correlation with water use efficiency. Larger of δ13C indicated higher plants WUE (Su et al., 2005; Qu et al., 2001; Farquhar et al., Shen et al., 2017; Pan et al., 2011). Grazing is the main utilization mode of grassland ecosystems. The species richness and coverage of grassland will decrease significantly during grassland degradation processes (Su et al., 2005).
More than 60% of alpine meadows were degraded seriously than moderate degradation, and grazing is the leading factor in the degradation of alpine meadows (Cao et al., 2018). Previous studies focused on impacts of grazing intensities on grassland multi-functions (Du et al., 2019; Shen et al., 2019; Zhang et al., 2019). The grazing activities played a role in balancing and stabilizing the plant community (Wang, 2018). Grazing activities decreased the biomass and diversity of grassland ecosystems (Lin et al., 2017; Xu et al., 2015; Gang et al., 2019), soil water content (Zhang et al., 2019; Lin et al., 2011), and evapotranspiration process (Zhang et al., 2019; He et al., 2014; Shu et al., 2019). The stable carbon isotope technology is fast and accurate due to its labeling, integration and indication functions (Jiang et al., 1996). Some studies based on the δ13C stable isotope technology have found that the long-term WUE of plants is studied through the monthly and annual changes of the WUE in desert plants (Su et al., 2005). Grazing has a significant impact on plants stable isotope indicators (Chen, 2013). Grazing pastures in Guinea, Brazil, Tanzania, plant WUE shows an increasing trend under moderate grazing (Cavalcante et al., 2016; Wang, 2016). The δ13C of Stipa breviflora plants showed a downward trend with the grazing activities increasing (Wang, 2016). The WUE of typical plants showed a downward trend under grazing conditions, but the soil shows an upward trend in the desert steppe of Inner Mongolia (Zhu et al., 2015).
The δ13C value is affected by environmental factors such as evapotranspiration, soil water content, soil organic carbon, and soil bulk density, which in turn affects plant WUE. Zhang (2003) studied the changes in the δ13C values of terrestrial C3 plants in 72 regions of China’s Xinjiang, Gansu, Inner Mongolia, Qaidam Basin and the Qinghai-Tibet Plateau and found that precipitation and δ13C values are negatively correlated. With an increase of 100 mm, the δ13C value of the plant is negative 0.33‰ (Stewart et al., 1995). Soil water content has a weak negative correlation with WUE (Wu et al., 2019). The WUE of Haibei alpine meadow in Qinghai is positively correlated with the change of evapotranspiration (Shi et al., 2020). Zhu (2020) et al studied the correlation between the δ13C value and soil organic carbon under different grazing treatments in the Xilamuren desert steppe in central and western Inner Mongolia, and found that they are negatively correlated. The content of soil organic carbon in Panyang Lake is different from δ13C. The relationship between the two was also found to be negative (Wang et al., 2016).
Although, the responses of WUE to grazing at the species level and community level were vastly researched. However, few studies focused on the response of WUE of alpine meadows to grazing intensity and adaptation mechanisms. This study aimed to reveal the long-term WUE variation characteristics under different grazing intensities and its driven factors. Thus, we study different species, function groups and community level WUE under long-term grazing. Furthermore, main driven factors were discussed including soil available nitrogen, organic carbon, evapotranspiration, soil water content, and bulk density. The following scientific questions were answered: (1) How did the WUE ​​of the dominant species and coexisting species respond to different grazing intensities? (2) How did plant functional group species WUE ​​respond to different grazing intensities? (3) How did the coupling factors affect WUE in different grazing intensities?