Author Contributions section
Yi Xian-Gui substantially contributed to the conception and design. Yi Xian-Gui, Li Meng, Zhu Hong, Chen Lin and Duan Yi-Fan conducted the field research and sample collection. Chen Jie and Li Yong-Fu did the laboratory work. Yi Xian-Gui, Zhu Hong, Li Yong-Fu, Li Xue-Xia and Li Meng analyzed data. Yi Xian-Gui and Chen Jie drafted the article. All authors participated in ameliorating the research and discussions of the results, as well as polishing up the manuscript. Wang Xian-Rong finally approved the version to be published.