Effects Of Submerged Plant Decomposition On Nitrogen And Phosphorus Transfer And Transformation In Sediment-water Interface

Different submerged plant for the study of decomposition in the process of nitrogen and phosphorus in sediment and water interface migration and the effect of selected three kinds of submerged plant advantages of foxtail algae in Taihu lake（Myriophyllum verticillatum L）,micro gear pondweed（Potamogeton maackianus A.Bennett）and hornwort（Ceratophyllum demersum L.）,to establish"sediment-water-submerged plant"micro systems,indoor simulation experiment,the dissolved oxygen（DO）concentration in different condition,plants decomposition upper-middle overlying water and sediment in the form of nitrogen and phosphorus concentration change,and take the centrifugation and membrane diffusion velocity analyzer and gradient technology（DGT）for the interface between sediment and water distribution characteristics of nitrogen and phosphorus,calculation of sediment and water interface nitrogen and phosphorous diffusion flux,synchronous analysis the change of the interface between sediment and water microenvironment,explore the decomposition of plant transformation of nitrogen and phosphorus in sediment and water interface migration effects.The research conclusions are as follows:1)The decomposition of submerged plants exists in stages.According to the different decomposition rates,it can be divided into two stages:rapid decomposition stage（0-10d）and slow decomposition stage（10-55d）.In addition,the decomposition rates of plants in the two states are shown as follows:hornwort>foxtail algae>micro gear pondweed;and the increase of DO reduces the decomposition rates of plants.2)Under the two conditions,environmental factors such as DO,pH and pH of overlying water and sediments in the control group were relatively stable,while in each experimental group,decomposition of different submerged plants had little effect on pH of overlying water and sediments.During the experiment,the DO concentration of the overlying water in the aerated state was higher than that in the non-aerated state in all the experimental groups,and the decomposition of submerged plants reduced the DO concentration of the overlying water in all the experimental groups.At the same time,the pH of the overlying water in each experimental group was 6.5-8.5 and the neutral or weakly alkaline environment of the sediments 6.8-7.6,both of which were conducive to the migration and transformation of nitrogen and phosphorus in the sediment-water interface.3)Under the condition of short-term training,control upper overlying water total nitrogen（TN）, total dissolved nitrogen（DTN）,ammonium nitrogen（NH₄⁺-N）form of nitrogen and total phosphorus（TP）and dissolved total phosphorus（DTP）,phosphate(PO₄^3--P)various forms of phosphorus concentrations were low and stable,and the submerged plant different decomposition were made under the two states of the experimental group in overlying water nitrogen and phosphorus concentration increased significantly.Two kinds of state,foxtail,micro gear pondweed and hornwort all the experimental group in plant rapid decomposition stage 5 days in overlying water reach maximum TN concentration,aeration condition were5.32 mg/L,12.05 mg/L and 9.95 mg/L,which respectively are the inflatable air condition of 1.47,1.25,and1.94 times,while the experimental group in overlying water TP concentration in plant rapid decomposition period day 5 or 10 days at the upper end of the aeration condition were 1.14 mg/L,1.06 mg/L and 1.83mg/L,The results were 1.83 mg/L,1.49 mg/L and 1.26 mg/L respectively in the non-aerated state.After entering the slow decomposition stage of plants,the phosphorus concentration in the overlying water of each experimental group decreased and tended to the stable level of the control group,while the nitrogen concentration showed a similar change trend with the phosphorus concentration in the aerated state,and then increased again after falling in the non-aerated state.4)The experimental process,the control group in the condition of two kinds of sediment in various forms of nitrogen（TN,DTN and NH₄⁺-N）concentrations were slow to reduce the trend,and in addition to sodium hydroxide phosphorus in sediments（NaOH-P）concentration showed a trend of slow decline,TP,inorganic phosphorus（IP）,organophosphate（OP）and hydrochloric acid phosphorus（HCl-P）in various forms of phosphorus concentration after falling rapidly rising trend.In addition,during the rapid decomposition period of plants,except for the significant increase of NH₄⁺-N concentration in sediments of each experimental group,the decomposition of nitrogen and phosphorus receptor plants in other sediments was not obvious.At the end of the experiment,the concentrations of HCl-P in the sediments of the two groups were higher than the initial values,13.82%,6.83%and 6.33%respectively in the aeration state,11.55%,11.22%and 6.92%respectively in the non-aeration state,and the concentrations of nitrogen and phosphorus in other forms in the sediments were lower than the initial values.5)In the control group,the diffusion flux of nitrogen and phosphorus at the sediment-water interface was positive under both conditions,which showed that the sediment was always released from the overlying water as an internal source of nitrogen and phosphorus during the experiment,but it changed little with the passage of time.The decomposition of submerged plants resulted in the release of nitrogen from sediments to overlying water,and the aeration condition decreased the diffusion rate of nitrogen at sediment-water interface in each experimental group.The decomposition of submerged plants changed the diffusion rate of phosphorus in sediments to a certain extent.On the 5th and 10th day of the rapid decomposition period,each experimental group showed that phosphorus was released from overlying water under inflated conditions.However,the diffusion flux of phosphorus at the interface is negative in the non-inflatable state,which shows that the sediment adsorbs the overlying water phosphorus at this stage.Therefore,the adsorption and release of phosphorus at the sediment-water interface are affected by the concentration of phosphorus in overlying water,and the correlation between the synchronization of dissolved reaction phosphorus（DRP）and dissolved reactive iron(Fe²⁺)concentration and their diffusion flux.It also shows that it is also related to the concentration of Fe²⁺.