Abstract: Chongming Island is a typical estuary alluvial island, seawater is upstream to the island, resulting that the soil salinity and moisture are different. It is unknown whether Phragmites australis population that are widely distributed in the Chongming Island is different across habitats, as affected by soil salinity. This research is to answer this question and to study the ecological Stoichiometry of P. australis and soil under the soil salinity gradient. We assume that the ecological stoichiometry of P. australis and soil will change under the soil salinity gradient. The object are P. australis and soil in the Chongming Island. The concentrations of carbon, nitrogen, phosphorus and the stoichiometry of them for P. australis and soil in three wetlands with different soil salinities were measured, to clarify the ecological adaptability of P. australis and its limiting factors in the Chongming Island. The results showed: 1) the average concentrations of soil carbon, nitrogen, phosphorus and ratio are 15.01, 0.69, 0.86 g/kg; 22.09, 21.87, 0.96. The average concentrations of P. australis carbon, nitrogen, phosphorus and ratios are 413.17, 10.75, 2.53 g/kg; 41.49, 293.58, 7.29. 2) With increasing soil salinity, the concentrations of soil carbon, nitrogen increased after the first reduced, they were maximum under the highest soil salinities, but the concentrations of soil phosphorus are minimum under the lowest soil salinities. The stoichiometry ratio of soil showed a steady increase. The concentrations of plant carbon and phosphorus were maximum under the lowest soil salinities, suggesting that plants nutrient absorption were effected by soil salinities. The C/P, N/P of plants were increasing with increasing soil salinity, which indicated P. australis would adapt the higher soil salinities and increase the nutrition use efficiency when it grow under higher soil salinities. The concentrations of plant nitrogen were continuously reduced with increasing soil salinity. 3) The concentrations of soil carbon, phosphorus had positive correlations with thoseof plants phosphorus, C/P, N/P ratios, while the concentration of soil nitrogen had positive correlations with C/P, negative correlations with N/P ratios of plants; the C/N in the soil only had negative correlations with phosphorus concentrations of plants. 4) The soil carbon, nitrogen elements was poor, but the phosphorus were abundant; the average N/P ratio were lower than 14, which indicate the P. australis growth were limited primarily by nitrogen.

Key words: wetland, the gradients of soil salinity, Phragmites australis, the carbon, nitrogen and phosphorus ecological stoichiometry