An investigation of the safety threshold of a floodplain wetland:a case study of the Er-Ka Nature Reserve, China Soil improvement effect of different management models on low hills in the lower and middle reaches of Yangtze River Effects of human disturbance on species diversity and niche of herbaceous plants along Qiyijiang riparian zone in Ya‘an City, Sichuan Province Ammonia volatilization from marsh soils of typical floodplains with different flooding frequencies