Effect of long term application of super absorbent polymer on soil structure, soil enzyme activity, photosynthetic characteristics, water and nitrogen use of winter wheat oleh Yonghui Yang, Yonghui Yang, Yonghui Yang, Sensen Zhang, Jicheng Wu, Jicheng Wu, Jicheng Wu, Cuimin Gao, Cuimin Gao, Cuimin Gao, Defu Lu, Darrell W. S. Tang