Effects Of Nitrogen Forms On Uptake And Translocation Of Cadmium By Dominant Submerged Plants In Baiyangdian Lake

With the increase of sewage,the aggravation of agricultural non-point source pollution,and the rapid development of aquaculture and tourism,Baiyangdian Lake presents the characteristics of eutrophication and heavy metal pollutions.As a nutrient junction of water and sediment,submerged macrophytes which have shown tremendous ability to repair the pollution of water,playing an important role in the water ecosystem.Therefore,the screening experiment of submerged plants was carried out and Potamogpus crispus L.has been selected due to its high fixed capacity to Cd in sediment.Furthermore,the kinetic test and indoor simulation experiments were conducted to investigate the effects of N on Cd uptake,translocation and accumulation by P.crispus under different nutrient conditions.These findings contribute to the application of submerged aquatic plants for remediation of contaminated water in Baiyangdian Lake.The major conclusions are as follows:（1）Indoor simulation experiments were conducted to investigate the tolerance of Cd,and accumulation and transportation of Cd from contaminated sediment by four kinds of submerged macrophytes which were Hydrilla verticillata（L.f.）Royle,Myriophyllum verticillatum L.,Ceratophyllum demersum L.and P.crispus.The results of the toxicity test showed that the tolerance for Cd from high to low is M.verticillatum,H.verticillata,P.crispus and C.demersum.The results of remediation Cd-contaminated sediment by H.verticillata,M.verticillatum and P.crispus indicated that the concentrations of Cd in shoots of submerged plants decreased in order of H.verticillata>P.crispus>M.verticillatum,while contents of Cd in roots decreased in order of P.crispus>H.verticillata>M.verticillatum.Furthermore,the transportation capabilities of Cd from Cd-contaminated sediment to plants in the pattern of H.verticillata>M.verticillatum>P.crispus.To sum up,P.crispus,which is the dominant submerged plant in Baiyangdian Lake and has large biomass,is suitable for remediation of low concentration of Cd polluted water in Baiyangdian Lake by"phytofixation"because it has the strongest enrichment ability and the weakest migration ability of Cd from the sediment.（2）The effect of nitrogen on Cd uptake by P.crispus showed that the accumulation of Cd by stems and leaves can be promoted by NH₄⁺-N,NO₃^--N and NH₄NO₃.For the treatment of lower Cd concentration（0.1μmol/L）,no significant differences were observed among different N forms.While for the treatment of higher Cd concentration（10μmol/L）,the uptake of Cd decreased in order of NH₄⁺-N>NO₃^--N>NH₄NO₃.The study on subcellular fraction and the staining of chemical and organization results showed that regardless of treatment,Cd mainly distributed in cell wall fraction（>70%）,followed by organelles（chloroplast）and cell sap fractions.（3）Laboratory simulating experiments with separated containers were conducted to research the effect of nitrogen on transport of cadmium in sediment-P.crispus-water.The results showed that N in water can promote the accumulation of Cd by stems and leaves of plant,with the accumulation of Cd decreased in order of NH₄NO₃>NH₄⁺-N and NO₃^--N.Only the treatment of NH₄NO₃ significantly improved the translocation of Cd from the stems and leaves to root and TF is 3 times as high as that of no nitrogen treatment;In addition,application of N in sediment also promoted the accumulation of Cd in the roots of P.crispus,but there was no significant difference in Cd accumulation in the roots between different concentrations of N treatments（P>0.05）.Cd migration from the roots to stems and leaves was inhibited by the addition of N in sediment.The upward transpotation rates decreased 63.64%compared with no nitrogen treatment.P.crispus could release 70.84%of the Cd accumulation in stems and leaves to the water,whearas the release of Cd can’t be affected by the addition of N in sediment.In summary,P.crispus is suitable for the remediation of low concentration of Cd contaminated water in Baiyangdian Lake by"phytofixation".The accumulation of Cd by stems and leaves can be promoted by NH₄⁺-N,NO₃^--N and NH₄NO₃.The accumulation of Cd by stems and leaves of plant can be promoted by addition of N in water,with the accumulation of Cd decreased in order of NH₄NO₃>NH₄⁺-N,NO₃^--N.Only the NH₄NO₃ treatment significantly improved the translocation of Cd downward to root.The addition of N in sediment promoted the accumulation of Cd in roots of P.crispus and inhibited the Cd migration from sediment to stems and leaves.