Effects of N addition on photosynthetic characteristics and leaf senescence in Stipa bungeana of steppe grasslands in the Loess Plateau

• Bing-hong Han, 
• Zhen-yan Shang, 
• Xiao-bo Yuan, 
• Zhuo An, 
• Hai-yan Wen, 
• Jin-bo Li, 
• Hua Fu, 
• De-cao Niu