| As an important part of the constructed wetland ecosystem,aquatic plants are of great significance for the prevention and restoration of water environmental pollution.Although research on the use of aquatic plants to remediate polluted water bodies has been widely reported,the extraction efficiencies of different aquatic plants and their combinations on pollutants in real reservoirs and lakes is unclear.Therefore,in order to quantify the ecological purification function in constructed wetlands in subtropical areas,the aquatic plants such as Pontederia cordata,Eichhornia crassipes,Nuphar pumilum which are wildly planted in constructed wetlands at the inlet of Changtan Reservoir in the mountainous southeast coastal area of Zhejiang.We were planted singly or complexly in the tanks that mimicked real water enviroment.After a growing season,the biomass and the concentrations of total nitrogen(TN),total phosphorus(TP),potassium(K),calcium(Ca),Purification efficiency of sodium(Na),magnesium(Mg),iron(Fe)and other nutrients in different combined plant communities were measured,and the extraction efficiences of those combined modes were calculated.The main findings are as follows:1.Changes in the contrations of various elements in aquatic plants varied greatly with different treatment conditions.On the whole,the concentrations of total nitrogen,total phosphorus and potassium in P.cordata showed a trend of first decreasing and then rising,and the concentration of magnesium and calcium in the root system of P.cordata showed a downward trend due to the rapid growth of the biomass of emergent plants.In addition to total nitrogen and zinc,the concentration of other elements in E.crassipes showed an overall growth trend.Except for the decrease in potassium concentration,the concentration of other elements in N.pumilum was generally increasing.2.Plant biomass varied greatly with different treatment conditions.The net increase of plant stem and leaf biomass in spring was between 34.10～505.54 g/m2 and 73.89～734.04 g/m2 in summer.The net increase of biomass in the plant root was between 140.72～1912.38 g/m2 in spring and 59.50～1158.33 g/m2 in summer.Among them,the biomass growth of P.cordata and E.crassipes monoculture and E.crassipes+N.pumilum combination in spring was higher than that in summer.The biomass growth of N.pumilum monoculture and the combination modes of P.cordata+E.crassipes and P.cordata+E.crassipes+N.pumilum was higher than those in spring.No significant difference in biomass growth between in spring and summer was observed in P.cordata+N.pumilum combination mode.3.Element accumulation in aquatic plants also varied greatly with different treatment conditions.In the monoculture modes of aquatic plants the order of plant element accumulation was P.cordata>N.pumilum>E.crassipes.Among the various plant combination modes,the order of plant element accumulation was P.cordata+E.crassipes+N.pumilum>P.cordata+N.pumilum>P.cordata+E.crassipes>E.crassipes+N.pumilum.The results showed that P.cordata monoculture mode and its combination modes with other aquatic plants had the highest element accumulation.4.Removal contribution rate of plants to each element varied greatly with plant culture modes.The removal contribution rate of single plant planting mode was P.cordata>N.pumilum>E.crassipes.Among them,the removal contribution rate of total phosphorus,magnesium,iron and zinc of P.cordata in spring was greater than that in summer,while the removal contribution rate of total nitrogen,potassium,sodium and calcium was less than that in summer.The removal contribution rate of total phosphorus,sodium and calcium by E.crassipes was higher than that in summer,and the removal contribution rate of total nitrogen and potassium was higher than that in spring,but no significant difference was observed among magnesium,iron and zinc.Moreover,the removal rates of total nitrogen,potassium and iron in summer varied slightly,while those of total nitrogen,potassium and iron in spring changed greatly.Among the various plant combination modes,P.cordata+E.crassipes+N.pumilum combination mode had the highest removal contribution rate of elements,and the removal contribution rate increased with cultivation time.There was no significant difference in the removal contribution rate among the modes of two plant species combination in spring.But in summer,and the element removal rate of two plant species combination was P.cordata+N.pumilum>P.cordata+E.crassipes>E.crassipes+N.pumilum in summer.5.Removal rates of water pollutants by aquatic plants varied with planting modes.In the single plant planting mode,the removal rate of spring water body in descending order is:P.cordata>N.pumilum>E.crassipes and the order of summer removal rates is:P.cordata>E.crassipes>N.pumilum.Among the various plant combination modes,the spring removal rate was sorted from the highest to the lowest as P.cordata+E.crassipes+N.pumilum>P.cordata+N.pumilum>P.cordata+E.crassipes>E.crassipes+N.pumilum.There was no significant difference in the size of the removal rate in summer compared with that in spring.Meanwhile,the single species of P.cordata and its combined planting mode had the highest removal efficiency of water bodies,and the size of plant removal rate in different seasons was related to the planting mode and species type.6.The absorption of aquqtic plants for sediment pollutants varied with planting modes and season.Among the planting modes of single plants,P.cordata in spring has the highest absorption of other elements except sodium in sediment and in summer,E.crassipes has a higher absorption of total phosphorus,sodium and calcium,and N.pumilum has a higher absorption of total nitrogen,potassium and magnesium.Among the various plant combination modes,the P.cordata+N.pumilum combination in the spring had the largest absorption of each element,followed by the P.cordata+E.crassipes+N.pumilum combination;P.cordata+N.pumilum combination in the summer had higher absorption of total phosphorus,calcium and iron,but no significant difference in absorption between the other combinations was observed.So the absorption of aquatic plants for sediment pollutants in summer was slightly higher than that in spring.In summary,different aquatic plants and their combinations have certain purification effects on pollutants in ecological wetlands.P.cordata monoculture mode and P.cordata+E.crassipes+N.pumilum combination mode had the highest purification efficiencies,followed by the P.cordata+ N.pumilum combination mode in this study.The results suggested that,it is advisable to use P.cordata as main species in combination with N.pumilum and E.crassipes mixed culture method so as to restore the contaminated aquatic environment or prevent enviroment pollution.Meanwhile,it is necessary to standardize the management of aquatic plants in wetlands to prevent secondary pollution owing to nutrient inputs from plant debris decomposition. |
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