The Study On Removing Nitrogen From Eutrophic Water By Myriophyllum Aquaticum(Vell.)Verdc. And Its Mechanism

The fundamental reason of water eutrophication is the higher nitrogen and phosphorus concentration in water.Nitrogen is an essential nutrient element for plant growth,and aquatic plants need to absorb large amounts of nitrogen for their own growth.Myriophyllum aquaticum,which is an important plant for constructed wetlands and floating bed,has powerful purification ability for eutrophic water.However,the nitrogen removal ability and nitrogen absorption,transport and accumulation mechanisms of Myriophyllum aquaticum under different nitrogen levels and NH₄⁺/NO₃^-are still unclear.In this paper,we selected Myriophyllum aquaticum as experimental materials.Solution culture experiments were conducted with different nitrogen levels and NH₄⁺/NO₃^-to explore the nitrogen removal ability and nitrogen accumulation,distribution,transformation mechanisms of Myriophyllum aquaticum,in order to provide theoretical basis and technical support for ecological restoration of eutrophic water.The main results as follows:1.Solution culture experiments were conducted to investigate the effect of eutrophic water nitrogen levels and NH₄⁺/NO₃^-on growth characteristics of Myriophyllum aquaticum.The results showed that the growth characteristics of Myriophyllum aquaticum were influenced by nitrogen levels and NH₄⁺/NO₃^-.In the first 3 weeks,the biomass of plants treated with nitrogen 20 mg/L was the largest,and then that of plants with 200 mg/L nitrogen was the largest after 3 weeks.The plants with no more than 20mg/L nitrogen were higher than plants with nitrogen of 100-400 mg/L in the first 2 weeks,but after 2 weeks there were no significiant difference in height among plants with 5-200mg/L,and the height of plants with nitrogen of 2 mg/L or 400 mg/L was inhibited.The chlorophyll content of plants leaves was increased with water nitrogen levels.The biomass of the plants with NH₄⁺/NO₃^-1:0 was higher under the nitrogen levels of 20,100mg/L,while that with NH₄⁺/NO₃^-0.5:0.5 and nitrogen concentration of 200 mg/L was higher and reached the maximum from third to fifth weeks.The plant height increased with time,and it was the highest when the plant treated with NH₄⁺/NO₃^-1:0 under different nitrogen levels.2.Solution culture experiments were conducted to research the nitrogen removal ability of Myriophyllum aquaticum on eutrophic water.The results showed that the nitrogen removal ability of Myriophyllum aquaticum was influenced by nitrogen levels and NH₄⁺/NO₃^-.The removal rates of total and ammonium nitrogen were nearly 100%with nitrogen levels were no more than 20 mg/L after a week,however,the nitrate nitrogen levels were low and varied little;the removal rates of total nitrogen were 76.50%,71.49% and 48.08%,of ammonium nitrogen were 99.6%,99.3% and 60.2% with nitrogen levels were 100-400 mg/L respectively,while the removal rates of nitrate nitrogen were all about 50% after 5 weeks and there were no significant difference among different treatments.In the first week,the removal rates of total nitrogen,ammonium nitrogen and nitrate nitrogen were highest under different nitrogen levels and NH₄⁺/NO₃^-,and it was increased with nitrogen levels,the removal rates were 63.2%-96.7%,68.2%-96.4% and86.2%-97.8%;There were no significance between the removal rates of ammonium nitrogen and nitrate nitrogen with nitrogen level of 20 mg/L.While nitrogen levels were100,200 mg/L,the removal ability of nitrate nitrogen was higher than that of ammonium nitrogen.3.Solution culture experiments were conducted to investigate the effect of nitrogen levels and NH₄⁺/NO₃^-on nitrogen content,accumulation and distribution of Myriophyllum aquaticum and nitrogen deposition,release in sediments.The results showed that Myriophyllum aquaticum can absorbed nitrogen both of water and sediment with nitrogen levels were no more than 20 mg/L,and the nitrogen contents of Myriophyllum aquaticum had no significant difference,the nitrogen content of upper part were higher than underneath;When nitrogen concentrations were between 100-400 mg/L,nitrogen in water would deposition into sediment,the nitrogen contents of Myriophyllum aquaticum were increased with increasing nitrogen levels.but the difference between upper and underneath part were not remarkable,showing uniform distribution.Nitrogen accumulation and nitrogen accumulation rates of per unit dry matter increased with nitrogen levels,and there were significant difference among 20 mg/L,100 mg/L and 200mg/L.The nitrogen content and accumulation of Myriophyllum aquaticum increasedsignificantly with increasing nitrogen concentrations,and it was different because of the change of NH₄⁺/NO₃^-,nitrogen content and accumulation increased significantly in the first week,and then varied little.The nitrogen accumulation of Myriophyllum aquaticum and its contribution in the water and sediment nitrogen removal increased with increasing nitrogen levels,and the highest contribution rate of nitrogen accumulation was NH₄⁺/NO₃^-0:1 with nitrogen levels of 20 mg/L,while it was NH₄⁺/NO₃^-0.5:0.5 with nitrogen levels were 100,200 mg/L.Diffferent NH₄⁺/NO₃^-under nitrogen levels of 20 mg/L,nitrogen of sediment could release to water;while nitrogen levels were 100,200 mg/L,the contribution of sediment nitrogen deposition to water nitrogen removal was highest in the first week were 35.4%-58.6%,nitrogen deposition of sediment was the main way of nitrogen removal in the early stage of water.At 5weeks,the ratio of nitrogen content in upper part/underearth was the largest in NH₄⁺/NO₃^-1:0.4.Solution culture experiments were conducted to investigate the effect of eutrophic water nitrogen levels and NH₄⁺/NO₃^-on the content,mainly forms of nitrogen and activities of enzymes related to nitrogen metabolism in Myriophyllum aquaticum.The results showed that the content of protein,amino nitrogen and nitrate nitrogen increased with nitrogen levels,while it was different due to the change of NH₄⁺/NO₃^-,and protein content>amino nitrogen content>nitrate nitrogen content.The content of protein were higher with NH₄⁺/NO₃^-1:0 and NH₄⁺/NO₃^-0.5:0.5 under different nitrogen concentrations,the content of amino nitrogen was the highest with NH₄⁺/NO₃^-1:0,and the nitrate nitrogen content was the highest with NH₄⁺/NO₃^-0:1.The activities of enzymes related to nitrogen metabolism in Myriophyllum aquaticum were influenced by nitrogen levels and NH₄⁺/NO₃^-of eutrophic water.The activities of NR increaseed with nitrogen levels,and it was highest in NH₄⁺/NO₃^-0:1,so it could reduce the accumulation of nitrate nitrogen;Both the way of GS/GOGAT and GDH can work at the same time,and the circulation of GS/GOGAT played a major role,GDH played a supporting role in NH₄⁺assimilation.