2.1 Study area
The experiment was conducted on a karst alpine grassland in the Liangshuigou sheep breeding farm in Guizhou, which is located on the Yunnan-Guizhou Plateau, China (103º36′–104º45′E, 26º36′–27º26′N; 2200 m a.s.l.). It has a subtropical monsoonal humid climate with an annual average temperature of 10–12 °C and precipitation of 962 mm. The grassland is a perennial Lolium perenne +Trifolium repens grassland. The main plant species areLolium perenne , Trifolium repens , Festucaovina ,Poa annua , Carex liparocarpos , and Potentilla chinensis . The soil type in the study site is alpine yellow-brown loam (H. Sun, 2014).