Variation characteristics of nutrient contents in a 5-year Medicago sativa and Poa pratensis rotation

• A Yun, 
• SHI Shangli, 
• ZHANG Xiaoyan, 
• ZHANG Cuimei, 
• LI Wen, 
• WU Fang, 
• ZHOU Tong, 
• LAI Xingliang, 
• CHEN Yonggang, 
• SUN Shoujiang