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?