Effects of phosphorus application rate on switchgrass growth and soil nutrients in saline-alkali soil YAN Cheng-hong1,WU Na1,LIU Ji-li2,3,ZHAO Cong1,4,HE Hai-feng1 (1. College of Agriculture, Ningxia University, Yinchuan 750021, China; 2. College of Ecology and Environment, Ningxia University, Yinchuan 750021, China; 3. State Key Laboratory Breeding Base of Northwest Land Degradation and Ecological Restoration, Ningxia University, Yinchuan 750021, China; 4. Key Laboratory of Degraded Ecosystem