Abstract: To explore the ecological stoichiometric characteristics of plants in different pond beds of constructed wetlands and their coupling relationships with water purification efficiency, so as to provide a theoretical basis for optimizing the allocation of aquatic plants in urban sewage treatment systems.Taking five plant pond beds in Chengdu Living Water Park as research objects, including Ⅰ: Lemna minor, Ⅱ: Acorus calamus, Ⅲ: Ceratophyllum demersum, Ⅳ: Cyperus involucratus and Ⅴ: Myriophyllum verticillatum. We determined the contents of carbon (C), nitrogen (N) and phosphorus (P) in plant shoots and roots as well as their stoichiometric ratios. Meanwhile, water quality indicators including total nitrogen (NTW), total phosphorus (PTW), chemical oxygen demand (CODCr), five-day biochemical oxygen demand (BOD5), dissolved oxygen (DO) and pH value were monitored synchronously. The results indicate that: (1) Along the water flow direction, The concentrations of total nitrogen NTW, total phosphorus PTW, chemical oxygen demand CODCr, five-day biochemical oxygen demand BOD5 and dissolved oxygen DO in water generally decreased. Among them, CODCr and BOD5 showed significant differences among different pond beds (P<0.05). (2) Among the five plant species, emergent plants (Acorus calamus and Cyperus involucratus) exhibited higher pollutant removal efficiency than floating plant (Lemna minor) and submerged plants (Ceratophyllum demersum and Myriophyllum verticillatum). The contents of C, N and P in shoots were higher than those in roots for all tested plants.(3) The C︰N, C︰P and N︰P ratios of shoots ranged from 9.47 to 18.77, 113.79 to 325.11, and 11.23 to 19.92, respectively; while those of roots were 13.01 to 34.94, 110.59 to 584.53, and 8.58 to 18.83. Correlation analysis indicated that the C︰N ratio of shoots was extremely significantly positively correlated with water CODCr and BOD5 (P<0.01), and the C︰P ratio of shoots was significantly positively correlated with DO (p<0.05). The roots N︰P ratio had an extremely significant positive correlation with DO and a significant positive correlation with CODCr and BOD5. The above results revealed that the C︰N ratio of plant shoots can indicate the removal efficiency of organic pollutants, and the roots N︰P ratio is closely related to water dissolved oxygen level and organic matter degradation. The constructed wetland system of Living Water Park presents remarkable purification performance for municipal wastewater, and the pond beds planted with emergent aquatic plants achieve the optimal comprehensive purification effect. Plant ecological stoichiometric characteristics, especially the C︰N ratio of shoots and the N︰P ratio of roots, can effectively reflect the removal efficiency of organic pollutants in water bodies. It is suggested that stoichiometric indicators be incorporated into the criteria for plant selection and routine operation & maintenance of constructed wetlands. Regular mowing of aboveground tissues and timely removal of dead plants are recommended to sustain the efficient elimination of nutrients and organic pollutants and prevent secondary pollution.

Key words: constructed wetland, aquatic plants, ecological chemometrics, purification capacity, plant configuration