Wang, Qing, College of Grassland Science and Technology, Sichuan Agricultural University, Chengdu, Sichuan 611130, P. R. China; Institute of Ecological and Environmental Sciences, Sichuan Agricultural University, Chengdu, Sichuan 611130, P. R. China; Department of Chemical and Petroleum Engineering, Schulich School of Engineering, the University of Calgary, Calgary T2N 4H9, Canada, China