Accumulation And Transportation Of Heavy Metals During The Growth And Decomposition Of Spartina Alterniflora And Phragmites Australis

Due to its unique hydrological conditions and physicochemical characteristics of sediments,the estuarine wetland has accumulated a lot of heavy metals?HMs?.As a typical wetland plants of estuarine,Spartina alterniflora and Phragmites australis play an important role in the biogeochemical cycle of HMs.The formation of the iron plaque?IP?of the root surface is the universal phenomenon of wetland plants,and plays an important role in the absorption and utilization of nutrient elements and the transformation of heavy metals.Periodic tidal flooding in estuarine wetland determines redoxi-orphic characteristics of wetland.Tidal flat elevation in the estuarine wetland determines the tidal flooding time and flooding frequency,which will inevitably affect the formation of iron plaque and accumulations of HMs in wetland plants.At the same time,the changing tidal flat environment also influences the decomposition of plant tissues and the exchange of heavy metals between plant tissues and sediments.The present study investigated the accumulation of copper?Cu?,zinc?Zn?,lead?Pb?,and chromium?Cr?in S.alterniflora and P.australis,the typical estuarine wetland species,at different tidal flat elevations?low,middle and high?in filed.The influence of the flooding time and concentration of HMs on IP formation and the accumulation and translocation of HMs in S.alterniflora were also investigated through controlled experiments.The decomposition of different parts of the plants and the dynamics of HMs content in sediments and plants were also followed at different tidal levels for about one year.Main conclusions of the present study were summarized as follows:1.The elevation change had significant effect on the accumulation of heavy metal in different parts of S.alterniflora and P.australis,the high tidal position of sediments accumulated more Cu and Zn,while Cr accumulated in low tidal position.The heavy metals were mainly concentrated in the roots,and the changes with elevation were consistent with those in the sediments;S.alterniflora had ability to accumulate and transport Zn,and the P.australis had ability to accumulate and transport Cu.The accumulations of Cu and Zn in the different parts of the plants generally increased with the tidal levels,the accumulations of Cr were the highest in the low tidal position and there was no significant difference in different tidal position for Pb.Acid extractable fraction of Pb and Cr in the sediments decreased with the increasing of tidal levels.There was no significant difference for the acid extractable fraction of Cu and Zn in rhizosediments of S.alterniflora,whereas the rhizosediments of P.australis had the highest accumulations of Cu and Zn in bareflat.Ratios of acid extractable fraction of Cu and Zn to total metal concentrations in sediments of S.alterniflora and P.australis all decreased with the tidal levels.All four HMs were mainly accumulated in roots of S.alterniflora and P.australis,followed by the stems and the least in leaves.At different tidal levels,the accumulations of Cu in roots and stems of S.alterniflora in the high tidal position was significantly higher than that in the middle and low tidal positions,while the accumulations of Cu in leaves was not significantly different at different tidal positions.The elevation changes had no significant effect on the accumulation of Zn in different parts of S.alterniflora;The concentration of Pb and Cr in roots of S.alterniflora were the highest in low tidal positions,but the the accumulation of these two metals in stems and leaves were not statistically different at different tidal positions.For P.australis,there was no significant difference for the accumulation of Cu and Pb in roots and stems,and in leaves,the accumulation of these two metals increased with the increasing of tidal levels.The highest accumulation of Zn in stems and leaves were observed at the middle tidal position.The concentration of Cr in roots of P.australis decreased with the increasing of the tidal elevation,which changed opposite in leaves.The bioconcentration factor?BCF?and translocation factors?TF?of Zn were highest in the two plants,but the changes of elevation were not obvious,the BCF of Cu decreased with the increasing of elevation.For S.alterniflora,the TF values for Cu,Zn,Pb and Cr were the lowest in low tidal position.The TF of the leaves in P.australis were increased with the elevation,and the maximum was in the middle and high tidal positions.2.Results of controlled waterlogging treatments in greenhouse conditions showed that the maximum value of Fe plaque concentrations was observed under the treatment of 3 hours per day of flooding,and the minimum value was observed under the treatment of 12 hours per day of flooding.Fe concentrations of P.australis peaked in the high tidal positions,while S.alterniflora in the middle tidal positions.Fe plaque concentrations of S.alterniflora firstly increased and then decreased with the increasing of HM levels.The adsorption of heavy metals by Fe plaque were mainly related to the Fe plaque content.DCB extraction method was used to study the Fe concentrations in the IP and its effect on the accumulation and translocation of HMs.Results showed that the Fe concentrations in the DCB extractant of roots of S.alterniflora peaked in the middle tidal positions,while the lowest level of Fe was observed in the roots of S.alterniflora that were collected from the high tidal positions in the field.In roots of P.australis,however,the Fe concentrations in the DCB extractant peaked at the high tidal positions.The concentrations of HMs,including Cu and Cr,in root DCB extractants all increased with the elevation of tidal levels,and the highest concentrations were all observed at the high tidal positions.However,Pb and Zn in the root DCB extractant showed no significant difference among the different tidal positions.Results of controlled waterlogging treatments in greenhouse conditions showed that the maximum value of Fe plaque concentrations was observed under the treatment of 3hours per day of flooding,and the minimum value was observed under the treatment of 12 hours per day of flooding.Cu in the DCB extractant showed a similar changing pattern with that of Fe.However,Zn,Pb and Cr in roots'DCB extractant increased with the increasing of waterlogging time.Under the treatment of different levels of combined HMs,Fe plaque concentrations firstly increased and then decreased with the increasing of HM levels,and the concentrations of HMs in the DCB extractant showed a consistent pattern.3.The mass loss rate in different parts of S.alterniflora and P.australis peaked in the high tidal positions,the mass loss rate of leaves of two wetland plants decreased with time;During the first 290 days of decomposition,the isotope ratio of ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb in roots of S.alterniflora both decreased significantly,and the ratios increased slightly in the last sampling time.During one year of decomposition,the mass loss rate,carbon to nitrogen ratio?C/N ratio?,lignin and cellulose concentration of S.alterniflora and P.australischanged consistently with the time.The root mass loss rate of S.alterniflora positively correlated with root C/N ratio and cellulose concentration.The leaf mass loss rates of S.alterniflora and P.australis also positively correlated with leaf C/N ratio.However,in stems,no significant correlation was observed between the mass loss rate and C/N,lignin and cellulose concentration.During the decomposition process,the concentration of all HMs?except Pb?in the sediments showed no significant variation compare to the initial values.With the increasing of decomposition time,the concentration of Cu in roots and leaves of S.alterniflora increased significantly.In roots and leaves of P.australis,however,Cu concentrations increased at the first 81 days and then decreased significantly at the last sampling time?350 days?.Compared with the initial value,the concentration of Zn in stems of S.alterniflora and P.australis both decreased gradually with time,whereas the changes of Zn concentration were different in leaves,which increased at the first 80 days and decreased with the time further prolonging.Pb concentration in leaves of P.australis and in S.alterniflora both increased with time.The changes of Cr concentration in roots of S.alterniflora and leaves of P.australis both showed a similar pattern with that of Zn.During the first 290 days of decomposition,the isotope ratio of ²⁰⁷Pb/²⁰⁶Pb and ²⁰⁸Pb/²⁰⁶Pb in roots of S.alterniflora both decreased significantly,and the ratios increased slightly in the last sampling time.The differences of the Pb isotops ratio between plant material?root?and sediments also decreased over time,and the lowest difference was observed on 26 October?Day 290of decomposition?.Overall,the elevation change had significant effect on the accumulation of heavy metals and mass loss rate in different parts of S.alterniflora and P.australis,all four HMs were mainly accumulated in roots,the HMs content of sediments and mass loss rate were higher in the high tidal positions.Fe plaque concentrations of S.alterniflora firstly increased and then decreased with the increasing of waterlogging time and HM levels.During one year of decomposition,the concentration of different heavy metals had different change trend in different parts of the two wetland plants with time.