| Eutrophication is one of the most serious and prevalent environmental problems in the world. In recent years, the ecological engineering method, which has achieved significant treatment effects to purify eutrophic water, has been widely used. The floating macrophytes, with high capability to extract nitrogen (N) and phosphrous (P) from wastewater were frequently tested for treating eutrophic water on laboratory and pilot scale. So far, the mechanisms concerning how nitrogen in eutrophic water was mediated by the floating macrophyes remain unclear. Majority of previous studies focused on the assimiliation of N by the macrophytes. However, according to the series of experiments done by our research group, Eichhonia crassipes could enhance the denitrification process in eutrophic water. Accordingly, the thesis aims to study the effect of E. crassipes at different growth stages on the fate of nitrogen in eutrophic water, using simulative experiments under the controlled nitrogen concentrations of eutrophic water. The major studies include:1) the contribution of plant assimilation and the denitrification process mediated by E. crassipes at different growth stages to removal of nitrogen from eutrophic water; 2) characteristics of oxygen and organic carbon release from E. crassipes roots at different growth stages; 3) effects of releasing processes of oxygen and organic carbon by roots on transformation of NH4+ and NO3- in eutrophic water with increasing concentrations of nitrogen. Through the study, it is expected that the results would provide some valuable information during apply the ecological engineering method of using E. crassipes to purify wastewater. The detailed studies and results include:1) The study concerning the effect of E. crassipes at different growth stages on purification of water with different concentrations of nitrate (2 mg L-1、5 mg L-1、10 mg L-1). The results showed that E. crassipes can reduce nitrogen concentrations in water effectively and efficiently. Quantity of nitrogen assimilated by E.crassipes at three different growth stages increased with the increase of initial nitrogen concentrations from 2 mg L"1 to 10 mg L-1, whereas the proportion of nitrogen content accumulated in E. crassipes to the loss of nitrogen from overlying water declined with the increase of initial nitrogen concentrations. At the growth stage of Oct., the quantity of nitrogen accumulated in E. crassipes tissues, grown in water with 2 mg L-1、5 mg L-1、10 mg L-1 of nitrate were:725.1±41.41 mg,938.3 ±58.25 mg and 1391.9±201.26 mg; The proportion of nitrogen content accumulated in E. crassipes to the loss of nitrogen from overlying water were:86.5±2.61%,52.7±2.05% and 44.22±1.70%. At other two growth stages, the trends were similar. With the increase of growth stages, the proportion of nitrogen content accumulated in E. crassipes to the loss of nitrogen increased at first and then declined. In the water with nitrate concentration of 2 mg L-1, the proportion of nitrogen content accumulated in E. crassipes at different growth stages to the loss of nitrogen were 87.14±6.74%,145.59±6.28% and 86.50±2.61%. In water with nitrate concentration of 5 mg L-1 and 10 mg L-1, the trends were similar. Assimilation of nitrogen by E. crassipes accounted for 44.42±1.70%～87.14±6.74% of nitrogen removal from water.2) The study concerning the effect of E. crassipes at different growth stages on characteriscics of N2 and N2O emission via denitrification process from water with different concentrations of nitrate (2 mg L-1、5 mg L-1、10 mg L-1). The results showed that the cultivation of E. crassipes significantly enhanced the quatity of N2 and N2O production via denitrification. In the water without cultivation of E. crassipes, the amount of N2 and N2O released from eutrophic water was 11.89±2.65 mg～12.80±1.55 mg、234.0±16.45 mg～1004.4±69.29 mg, respectively. In the water with cultivation of E. crassipes at different growth stages, the amount of N2 and N2O released from eutrophic water was 18.32±0.91 mg～31.40±3.98 mg and 717.4±22.12 mg～3221.6±311.84 mg, respectively. The amount of N2 and N2O released from water with growth of E. crassipes was significantly higher than without, indicating E. crassipes enhaneced the dinitrification occurred in water. Quantity of N2 and N2O released from water with and without cultivation of E. crassipes increased with the increase of initial nitrogen concentrations from 2 mg L-1 to 10 mg L-1. At the growth stage of Jun. in the water with 2 mg L-1、5 mg L-1 10 mg L-1 of nitrate concentrations, the amount of N2O-N was 18.14±1.1mg、22.75± 1.00mg、24.98±2.17mg and amount of N2 was 1257.61±102.72 mg、2162.74±286.67 mg、2856.22±209.52 mg. At the growth stage of Oct., the trend was similar. The proportion of nitrogen loss via N2 and N2O emission to N loss from water increased with the increase of growth stages. In water with 5 mg L-1 concentration of nitrate, N loss via N2O production from water with growth of different seedling stages of E. crassipes (Jun., Oct.) accounted for 1.13±0.38% and 1.30±0.14% of N loss from water; N loss via N2 production from water with growth of different seedling stages of E. crassipes (Jun., Oct.) accounted for 1.13±108.25±5.33% and 119.15±3.53% of N loss from water. N2 production from water with cultivation of E. crassipes contributed to 85.65±2.81%～ 119.15±3.53% of N loss for overlying water, indicating the occurrence of denitrification in the sediment.3) Aquatic plants could modify the physical, chemical and biological environment of water body via releasing oxygen and organic carbon from roots, which would have a significant impact on transformation of nitrogen in water. In the present study, the characterization of releasing oxygen and organic carbon from roots of the floating aquatic plant, E. crassipes, at different seedling stage was studied by simulative experiments. The effect of this releasing process on the concentrations of dissolved oxygen (DO) and total organic carbon (TOC) and transformation of NH4+ and NO3- in eutrophic waterwas then discussed. According to the results, E. crassipes exhibited high capability to release oxygen and organic carbonfrom roots. Rates of releasing oxygen by the roots were as high as 56.19、93.15 and 106.32 umol O2h-1, and rates of releasing organic carbon were as high as 0.25、0.60 and 0.92 mg C L-1 h-1 at three seedling stages.The rates of releasing oxygen and organic carbon from roots increased significantly with the increase of E. crassipes seedling stage, while the order was reversed if the rates were divided by the fresh weight of roots. The removal rates of total dissolved nitrogen (TDN) from eutrophic water increased with the increase of E. crassipes seedling age. Besides the contribution of plant assimilation on the removal of nitrogen from water, the increased quantity of oxygen and organic carbon released from roots improved the nitrification and nitrification-denitrification process. These all made great contribution to the fast removal of the loaded nitrogen from eutrophic water by E. crassipes.To conclude, E. crassipes could efficiently purify eutrophic water through extract nitrogen from water and enhance nitrification and/or denitrification process by releasing oxygen and organic carbons. |
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