1. a School of Economics & Management, Inner Mongolia University , Hohhot, Inner Mongolia, PR China
2. b Sino-US Center for Conservation, Energy and Sustainability Science in Inner Mongolia , Hohhot, Inner Mongolia, PR China
3. c Department of Rural Economy Research , Development Research Center of the State Council , Beijing, PR China
4. d School of Life Science & Global Institute of Sustainability, Arizona State University , Tempe, AZ, USA