ACKNOWLEDGMENTS
This research was financially supported by National Key R & D Project of China (2017YFC0504802); Key Program of National Natural Science Foundation of China (41430749); and the funding from State Key Laboratory of Grassland Agro-Ecosystems and Program for New Century Excellent Talents in University (NCET-11-0210). We are grateful to Alpine Meadow Ecosystem Research Station of Lanzhou University for providing the experimental site.