Characteristics of leaf photosynthesis in Leymus chinensis grown in the saline-sodic soil area of the Songnen Plain, at different groundwater depths Ecological Responses of Different Maturity Leymus chinensis Seeds to Soil Burial Depths in Songnen Grassland Effect of different SH6 buried depths on hormone and 15N-urea utilization of dwarf apple trees STUDY ON THE ECOLOGICAL CHARACTERISTICS OF WATERLOGGING FIELD IN JIANGHAN PLAIN