ACKNOWLEDGEMENTS
This work was supported by National Key Research and Development Program of China (No. 2018YFC0507005, No. 2016YFA0602301) and National Natural Science Foundation of China (No. 41401304; No. 41571526). We thank staff of Jiusan soil and water conservation experimental station of Beijing Normal University, Prof. Hongyan Zhang and Dr. Yanfang Hao for data collection, Dr. Zhongqiang Wang for comments. The authors indicate no potential conflicts of interest.