Effects Of Spatial Expansion Between Phragmites Australis And Cyperus Malaccensis On Migration And Transformation Of Heavy Metal And Its Ecological Risk In The Min River Estuary

Estuarine marshes are transitional ecosystems in which the material-energy exchanges between continental river water and marine water occur.Due to its unique hydrological,soil and biological characteristics,the formation and development of vegetation are significantly different from other ecosystems,and the interaction between plants is more complicated.Spatial expansion,as an important type of interaction between plants,mainly referred to the competitions of two species for environmental resources such as light,water,and nutrient.It is one of the main driving forces to shape plant morphology and regulate plant life history,community structure and dynamic characteristics.Spatial expansion between two plants in marshes could not only impact vegetation ecological characteristics and adaption strategies but also alter the community structure,which might affect biogeochemical cycles of macro/micro in the ecosystems.Currently,studies on the biological cycle process and ecological risk of heavy metals in plant-soil system under the influences of spatial expansion between plants remains scarce.In this paper,the Phragmites australis marsh（PA）,the ecotonal marsh formed by the spatial expansion of P.australis and Cyperus malaccensis（P.australis-C.malaccensis marsh,PA’-CM’）and the C.malaccensis marsh（CM）were studied.The temporal and spatial distribution of heavy metals values and the adsoption-desortion characteristics of heavy metals in marshes soils during the spatial expansion between P.australis and C.malaccensis were discussed.Meanwhile,the bioaccumulation and allocation of heavy metals by vegetation and litter decomposition and variations of heavy metals stocks in litters were investigated.On this basis,the ecological risk of heavy metals in plant-soil system under the influence of spatial expansion between the two plants was comprehensively evaluated.i)The concentrations of heavy metals in soils of ecotonal marsh and C.malaccensis marsh were higher than those of P.australis marsh.The alteration of environmental variables,litter decomposition,changes of root turnover rates,and the differences in root distribution under the influence of spatial expansion were the main reasons for the variations of heavy metals levels in soils of PA’-CM’.Higher values of heavy metals were observed in surface soils of PA’-CM’compare to the surface soils of PA and CM,which was probably dependent on the alteration of soil grain composition affected by plant ecological characteristics changed in ecotonal community.Moreover,soil grain composition was the critical factor affecting the vertical variation of heavy metals levels in different marshes.ii)The adsorption capacities for Cr⁶⁺,Ni²⁺and Cd²⁺and the maximum buffer capacity（MBC）for Cr⁶⁺and Ni²⁺in soils of CM were lower than those in soils of PA’-CM’and PA.Comparatively,the adsorption capacities and the MBC for Cu²⁺and Zn²⁺were higher in soils of PA at 0-30 cm depth,while those were higher in soils of CM at 30-60 cm depth.The differences in adsorption capacities for heavy metals among the three marsh soils were mainly dependent on their differences in grain composition and p H.The desorption amounts of Cr⁶⁺,Ni²⁺,Cu²⁺,Zn²⁺and Cd²⁺in the three marsh soils generally increased with increasing adsorption amounts,and there was hysteresis in desorption.With increasing p H,the adsorption amounts of Cr⁶⁺generally decreased while those of Ni²⁺,Cu²⁺,Zn²⁺and Cd²⁺increased,but the responses of desorption amounts for Cr⁶⁺,Cu²⁺,Zn²⁺and Cd²⁺to p H variations were just in opposite.iii)The concentration of heavy metals and their bioaccumulation capacities in different tissues of different plants changed greatly during the spatial expansion between PA and CM.Compared to pure communities,Cr levels in leaf and Ni and Pb values in sheath of PA’increased significantly,and levels of the six metals and their accumulation factors（AF）in litters of PA’increased evidently,but Ni and Zn levels and their AF in litters of CM’decreased greatly.Higher concentrations of Cr,Ni,Cu,Zn,Cd and Pb and their AF and the responding allocation ratios were observed in roots of different plants,indicating that roots were the key tissues for heavy metals accumulation.Compared to pure community,allocations of the six metals in litters of PA’increased clearly,while allocations of which in litters of CM’decreased obviously,suggesting that P.australis might maintain competitive advantage primarily by distributing more metals in senescent tissues for detoxification purposes.iv)The decomposition of C.malaccensis（CM and CM’）were faster than those of P.australis（PA and PA’）whether at the stage of before their spatial expansion（BE）or at the stage of during their spatial expansion（DE）.Compared with litters at BE stage,the decomposition rate of PA was increased by 24.40%whereas the decomposition rate of CM was decreased by 15.67%.Substrate quality（C/N ratio）and salinity were crucial variables affecting the decomposition of the four litters.The concentrations of Ni,Cu,Zn,Cd and Pb in decomposing litters of P.australis（PA and PA’）and C.malaccensis（CM and CM’）generally showed increasing tendency and the values in the former were significantly lower than those in the latter.The physicochemical sorption onto recalcitrant organic fractions and the substrate quality（C/N and M/C ratios）of decomposing litters were two important factors affecting the differences in metals variations between species.Whether at BE stage or at DE stage,stocks of metals in decomposing litters of P.australis（PA and PA’）were generally higher than those of C.malaccensis（CM and CM’）.The lower stocks of metals in CM and CM’might be more dependent on its lower mass remaining.Compared with PA at BE stage,the accumulation of As/metals in decomposing litters of PA’at DE stage decreased greatly,which might be ascribed to the lower precipitation of metal sulfides in PA’.Stocks of Cr in PA’and stocks of Cr,Ni,Zn and Cd generally evidenced the export of metals from decomposing litter to environment,indicating that the potential exposure risk of Cr,Ni,Zn and Cd might be increased as the spatial expansion between PA and CM.v)Compared with PA or CM,stocks of Cr,Ni,Cu,Zn and Pb in soils of PA’-CM’increased,stocks of Cd in soils of PA’-CM’increased compare to PA while the stocks of Cd in soils of PA’-CM’decreased compared to CM,indicating that the spatial expansion between PA and CM increase the pollution level and the potential ecological risk of Cr,Ni,Cu,Zn and Pb in soils of PA’-CM’to some extent.Whether at BE stage or at DE stage,high pollution level and potential ecological risk of Cd were observed in soils of different marshes.Moreover,the mobility of Cr⁶⁺in soils of PA’-CM’and CM,Cu²⁺in soils of PA’-CM’,Zn²⁺and Cd²⁺in soils of PA and PA’-CM’were strong,thus its ecotoxicity risk to plants and soil organisms might increase.With decreasing p H,the adsorption amounts of Ni²⁺,Cu²⁺,Zn²⁺and Cd²⁺in soils of PA and PA’-CM’would decrease significantly compared to CM while the desorption amounts would increase greatly,implying that Ni²⁺,Cu²⁺,Zn²⁺and Cd²⁺could be more easily desorbed from soils into pore-water in PA and PA’-CM’,which might increase their ecotoxicity risk to plants and soil organisms.vi)The toxic effects to marsh plants were distinctly altered due to the spatial expansion between PA and CM.When competition caused by the spatial between PA and CM occurs,on the one hand,allocations of the six metals in litters of PA’increased clearly,while allocations of which in litters of CM’decreased obviously,indicating that P.australis might maintain competitive advantage by distributing more metals in senescent tissues for detoxification purposes.On the other hand,the adsorption coefficients of Cr,Ni and Pb（especially Cr and Ni）in PA’increased while those of the six metals in CM’decreased,indicating that the two plants might maintain their competitiveness by enhancing their tolerance and avoidance of heavy metals respectively.Compare with litters at BE stage,the stocks of heavy metals in litters of PA’and CM’at DE stage decreased evidently.Cr,Ni,Zn and Cd in litters of PA’or CM’generally evidenced the export of metals from decomposing litter to environment,indicating that the exposure risk of Cr,Ni,Zn and Cd to soils in ecotonal marsh might be increased.In addition,the circulation rates of metals in PA’or CM’increased obviously compare with PA or CM,suggesting that the return rates and amounts of heavy metals in litters of ecotonal community increased distinctly.Due to root turnover is an important pathway to return heavy metals to marsh soils,especially Cr and Ni accumulated in roots of P.australis（PA and PA’）and C.malaccensis（CM and CM’）might lead toxic effect,and the circulation rates of Cr and Ni in PA’or CM’increased,thus the return amounts of Cr and Ni in plants of ecotonal community caused by root turnover also increased,and this could increase the potential ecological risk of Cr and Ni in soils of ecotonal marsh.This paper found that,as the spatial expansion between P.australis and C.malaccensis,the potential ecological risk of Cr,Ni,Zn and Cd in soils and the toxic effect of Cr and Ni in plants of ecotonal community should be emphasized.In future,as the frequency of sulfuric acid precipitation in the Min River estuary increased,the mobility of Ni,Cu,Zn and Cd in soils of PA and PA’-CM’might enhance,which could increase the ecotoxicity risk to plants and soil organisms.