| Agricultural nitrogen and phosphorus emissions have become one of the most important sources in the eutrophication of Taihu Lake Basin. The application of aquatic macrophytes in wastewater decontamination can not only remove the nitrogen and phosphorus pollutant, but also harvest biomass. This technique combines wastewater treatments with source reuses and is a cost-effective and easy-spreading option for the purification of wastewater. Duckweed is a kind of floating plant,with broad tolerance, growing rapidly, being rich in protein, harvesting conveniently, high removal rate of N and P and also is a good landscape plant.This study was conducted in the Taihu Lake Basin and duckweeds were used for the research objects, on the basis of collecting and analyzing the duckweed species resources and their habitual nutrient conditions, simulated experiments were conducted to compare nitrogen and phosphorus removal efficiency among different duckweed species which grow in paddy field drainage water with low nutrient concentrations in the Tai Lake region. The main results were summarized as following:39 duckweed samples were collected from 4 cities, 13 counties in Taihu Lake Basin, which can be divided into 13 varieties according to different collection regions and species. The analysis of their habitual water quality showed that the concentration of TN, TP and COD in wastewater declined in the order : Domestic sewage> Pond Sludge Water> paddy field drainage.Each duckweed species can grow well in Steinberg culture solution which was formulated to closely resemble the nutrient profile of agricultural drainage water after 15 days. In the initial condition of 50% fonds coverage, nitrogen removal efficiency declined in the order: Spirodela polyrrhiza > Spirodela oligorrhiza, Lemna perpusilla > Lemna minor. Among the duckweed species in this study, HZ1 had the highest total nitrogen removal efficiency (85%) in 15 days, followed by YX1 (78%), while the nitrogen removal efficiency for WX3 was the lowest(31%), the TN removal rate was 34% in the control treatment after 15 days treatments. The nitrogen removal efficiency in the duckweed-covered treatments was significantly higher than the control treatment.The removal efficiency of TP differed with TN slightly, WX3 has the highest TP removal efficiency(96%), while YX2 has the lowest removal efficiency(55%), the TP removal rate was 43% in the control treatment after 15 days treatments. Discrepancy in TN and TP removal efficiency by duckweed treatments may be due to a difference between nitrogen and phosphorus removal mechanisms. COD removal efficiency by different duckweed treatments had no significant differences , which were almost between 78-96%, indicating that microbial carbon utilization had little differences in each duckweed culture system.The nitrogen removal mechanism study showed that, when adding soil microbial extract solution and glucose carbon source, HZ1 has the highest nitrogen removal rate, the residual nitrogen in the water was significantly lower than that of WX3, but its growth rate of biomass and plant nitrogen content is low, while WX3 is higher in both. When no microbial extract solution and carbon source added, although nitrogen removal by duckweed uptake and unknown pathways had small differences, but there were a large number of nitrogen residues in water after the experiment, and little difference can be found between the two different species.The three findings above can be concluded as, duckweed has a good effect for the removal of nitrogen in water eutrophication, but nitrogen removal from water is due to rhizosphere microbial effect or allelopathic interaction rather than bioaccumulation.
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