2.1 Study site
The investigation was conducted at Inner Mongolia Grassland Ecosystem Research Station (IMGERS, 116.8°E, 43.5°N, 1179 m a.s.l.), which is located in a temperate grassland in the Inner Mongolia, China (Fig. S1). The study site has a temperate continental climate. The long-term (1981-2011) mean annual temperature (MAT) is 0.78℃, the mean monthly minimum temperature is -21.3℃ in January, and the mean monthly maximum temperature is 19.3℃ in July. The annual precipitation (AP) is 330.1 mm, of which 85% falls in the growing season (from May to September) (Fig. S2a, 2b). Over the period 1981–2011, MAT showed an increasing trend with a rate of 0.06 ℃ year-1, while AP fluctuated between 166.1 and 507 mm (Fig. S2a). The meteorological data were collected from the IMGERS weather station, located approximately 9 km from the study area. The study site is dominated by two perennial rhizome grasses,Agropyron cristatum and Leymus chinensis , and two perennial bunchgrass species, Stipa grandis and Achnatherum sibiricum , which were the most widely distributed species and accounted for 62.9% of the above-ground biomass. According to Chinese classification, the soil type is chestnut soil, with an average bulk density of 1.29 g cm−3 and a pH of 7.68 (Yuan et al., 2005).