Acknowledgements
This work was supported by the National Natural Science Foundation of China (grant No. 31830015), the National Key Research and Development Program of China (grant No. 2016YFA0600801) and the Strategic Priority Research Program of the Chinese Academy of Sciences (grant No. XDB15010301). We are grateful to Wei Zhang, Shunzhong Wang, Edith Bai, Zongwei Xia, Jiabing Wu, Longchi Chen, Bing Fan and Qingui Wang for their helps during collecting soil samples.