Study site
The study site was located at Haibei National Field Research Station of the Alpine Grassland Ecosystem (37°36′ N, 101°19′ E, 3215 m a.s.l.) in the northeastern part of the Tibetan Plateau, China. The mean annual air temperature and precipitation were -1.2 °C and 489.0 mm during 1980-2014, respectively (Liu et al. 2018). Approximately 80% of the precipitation was concentrated in the growing season (from May to September). This mesic alpine grassland is dominated by Stipa aliena , Elymus nutans and Helictotrichon tibeticum . The soil is classified as a Mollisol according to USDA Soil Taxonomy. The average soil bulk density, organic carbon concentration and pH were 0.8 g·cm-3, 63.1 g·kg-1 and 7.8 at the 0-10 cm soil depth, respectively (Lin et al. 2016).