| The Min River estuary is one of the important estuaries entering the sea in the southeast coast of China.The marsh of Min River estuary plays an important role in maintaining the ecological security of Min River basin.In this paper,the native species of Cyperus malaccensis,the invasive species of Spartina alterniflora and the ecotone of C.malaccensis and S.alterniflora in the Shanyutan marsh of the Min River estuary were selected as study objects based on hot topics of the invasion of S.alterniflora and the limited studies on the bioaccumulation of heavy metals in the marsh plant-soil(water)system of Min River estuary.The characteristics of heavy metal content and chemical fractions distribution in marsh soil-water system,the accumulation and distribution of heavy metal in different marsh plants,the accumulation and release of heavy metal during decomposition of different plant litters were discussed.Furthermore,the effects of the invasion of S.alterniflora on the biological circulation characteristics and ecological risks of heavy metals in marsh were analyzed.The main results were drawn as follows:i)The contents of Cr,Cu and Pb in the marsh soils increased after S.alterniflora invasion.However,Ni and Zn contents in the C.malaccensis marsh were higher than the ecotone marsh and S.alterniflora marsh.The contents of Cr,Ni,Cu,Zn and Pb in different marshes increased as the soil goes deeper,and then the concentrations of heavy metal decreased with the increase of soil depth,and most of them showed high values in the 20-40 cm soil layer.ii)Heavy metal fractions in marsh soil were significantly influenced by the invasion of Spartina alterniflora.Generally,the residue fraction contents of Cr,Ni,Cu,Zn and Pb in different marsh soils ranked the highest.In contrast,lowest contractions of Cr,Ni,Cu and Pb were generally observed in the acid-soluble fraction,and lowest contractions of Zn were generally observed in oxidizable fraction.The proportions of weak-acid-soluble fraction(except for Cu)in the total amount of heavy metals were the highest in the ecotone marsh.Except for Pb,the proportions of reducible fraction and oxidizable fraction in the total amount of heavy metals decreased in the order of ecotone marsh > S.alterniflora marsh > C.malaccensis marsh.The proportions of residual fraction(except for Pb)in the total amount of heavy metals were in the order of C.malaccensis marsh > S.alterniflora marsh > ecotone marsh.The fractions and transformations of different heavy metal in marsh soils might be changed by altering the physical and chemical properties of marsh soil during the invasion of S.alterniflora.iii)The contents of different heavy metals in water of marsh soils also changed under the influence of the S.alterniflora invasion.The concentrations of Cr,Ni,Cu and Zn in the pore water decreased followed the sequence of C.malaccensis marsh >S.alterniflora marsh > ecotone marsh.Pb concentration in the pore water decreased in the order of ecotone marsh > S.alterniflora marsh > C.malaccensis marsh.The concentration of heavy metals in pore water of different marshes in the Min River estuary showed obvious seasonal variations,which was generally higher in winter and spring while lower in summer and autumn.iv)The content and enrichment ability of heavy metals of plants were significantly influenced by the invasion of Spartina alterniflora.Heavy metal contents in the root,stem,leaf,litter and ear of C.malaccensis in the ecotone were generally higher than that of C.malaccensis in the pure community.The contents of heavy metal in the root,stem,ear and sheath of S.alterniflora in the S.alterniflora marsh were higher than that of S.alterniflora in the ecotone marsh.In contrast,higher contents of heavy metal in the leaf and litter were generally observed in S.alterniflora of the ecotone marsh.Compared with the C.malaccensis of the ecotone,lower annual enrichment coefficients of heavy metal were observed in the root,stem,leaf and litter of C.malaccensis in the pure community.Higher annual enrichment coefficients of Cr,Zn and Pb occurred in the root and stem of S.alterniflora of the pure community,but these coefficients showed lower differences.Compared with the S.alterniflora in the pure community,the litter of S.alterniflora in the pure community showed lower annual enrichment coefficients of heavy metal.Furthermore,the annual enrichment coefficients of Cr,Zn and Pb in leave and sheath of S.alterniflora were not significantly different between the pure community and the ecotone.The difference of heavy metals enrichment ability in different plants was colsely related to the tolerance or avoidance mechanism of heavy metals by the invasion of Spartina alterniflora.v)Decomposition of plant litters and heavy metal pools in the marsh were significantly influenced by the invasion of S.alterniflora.The litter remaining rates of S.alterniflora in the ecotone and the pure community were higher than C.malaccensi,respectively.Compared with the C.malaccensis and S.alterniflora of the ecotone,lower litter decomposition rates were observed in the C.malaccensis and S.alterniflora of the pure community.Compared with the original heavy metal content of different plant litters,heavy metal contents(except for Cr content of CM'-SA' and SA)presented an increasing tendency during decomposition.During decomposition,Cr,Ni and Zn in different plant litters showed a releasing tendency although some fluctuations occurred.However,Pb accumulation occurred in the different plant litters during decomposition.Cu in the litters of S.alterniflora of the ecotone presented a releasing tendency.In contrast,Cu accumulation occurred in the litters of S.alterniflora of the pure community.Compared with the plants of pure community,the releasing abilities of Cr,Ni,Cu and Zn in the ecotone were enhanced by the invasion of S.alterniflora.By contrast,accumulations of Pb in the litters of ecotone were strengthened by the invasion of S.alterniflora.vi)The pollution level and ecological toxicity risk of heavy metals in marsh soils and water were relatively low under the invasion of S.alterniflora.Nevethless,after S.alterniflora invasion,Cr,Cu and Pb contents in the marsh soils increased significantly,and meanwhile the contents of Ni and Zn decreased.The C.malaccensis of the ecotone improved the absorption capacity of heavy metals in the process of competition between invasive species S.alterniflora and native species of C.malaccensis,which increasing the ecological risk of heavy metals(particularly for Cr and Ni)in the C.malaccensis.However,the potential toxicity of heavy metals(especially for Cr and Ni)to the S.alterniflora of the ecotone decreased due to the lower absorption capacity of heavy metals.This might be an important reason for the successful invasion of the S.alterniflora due to its comparative competitive advantage.The relatively low cycling capacity of Cr in C.malaccensis might lead to increased ecological risk of Cr on microorganisms that feed on litters in the marshes of Min River estuary.In contrary,the relatively high annual restitution capacity of Cr in S.alterniflora might increase the ecological risk of Cr in the marshes soil of the Min River estuary.Since root turnover was also an important way for Cr to be returned to the soil,the higher Cr turnover at the root of S.alterniflora could also increase the ecological risk in the marshes soil.Based on the above conclusion,effective control of the S.alterniflora can decrease the toxicity risk of heavy metals in the native marsh(C.malaccensis marsh)and the ecological risk of heavy metals in marsh soil,which helps maintain the stability and health of native marsh system. |
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