1. College of Life Sciences, Northwest A&F University, Yangling 712100, China
2. Biomass Energy Center for Arid and Semi-arid Lands, Northwest A&F University, Yangling 712100, China
3. Department of Plant and Environmental Sciences, University of Copenhagen, DK-1871 Frederiksberg, Denmark
4. Department of Physics, Technical University of Denmark, 2800 Kongens Lyngby, Denmark