The Characteristics Of Heavy Metal In The Sediment And The Enrichment And Transfer Process Of Heavy Metals By Triarrhena Lutarioriparia In East Dongting Lake Wetland

In recent years,due to the rapid development of urbanization,industrialization and the increase of agricultural activities,a large number of pollutants containing heavy metals have been carried into lakes wetlands.Most of the heavy metals that enter the lakes wetlands cannot be degraded naturally or biologically,but are deposited in the sediment,and then enter the animals and human body through the food chain,thus posing a great threat to the wetland environment and human health.Wetland plants can not only remove metals from sediment by absorption and accumulation of plant organs,but also remove heavy metal pollutants from sediment by harvesting plant aboveground organs.Therefore,using wetland plants to remove heavy metals from sediment is an ecological and environmental protection treatment method.In this study,a large-scale field investigation was conducted on the sediment of the East Dongting Lake Wetland to determine the content of heavy metals in the sediment,analyze its spatial distribution characteristics and ecological risks,and explore the relationship between the content of heavy metals in the sediment and environmental factors;At the same time,through field sampling and fixed-point monitoring,the accumulation characteristics of heavy metals,the key ecological factors affecting the accumulation of heavy metals and the dynamic characteristics of the enrichment and transfer of heavy metals were analyzed;In addition,in order to study the cycling process of heavy metals in the wetland,the litter decomposition bag method was used to reveal the decomposition changes of the litter and the accumulation and return characteristics of heavy metals in the decomposition process;The results show that:（1）The average contents of Cd,Pb,Mn and Zn in the sediments of the East Dongting Lake wetland are 10.7,1.38,1.36 and 1.27 times higher than the background values of the sediments of the Dongting Lake,while the average contents of Cd,Cu,Pb and Zn are 12.3,1.21,1.08 and 1.26 times higher than the national first-class environmental quality standard for sediments.However,the content of Cd in sediment exceeds the third grade of national environmental quality standard for sediment.From the perspective of spatial distribution characteristics,the spatial distribution of Cd,Pb and Zn gradually decreases from east to west,while Cu and Mn show patch distribution characteristics.According to the correlation analysis,redundancy analysis（RDA）and linear regression analysis between the content of heavy metals in sediment and soil physical and chemical properties,the content of Cu in sediment is significantly positively correlated with total potassium,and the content of Pb and Zn is significantly positively correlated with available phosphorus and total phosphorus.The pollution load index（PLI）was used to assess the risk of heavy metals in sediment.It was found that the spatial distribution of pollution risk decreased gradually from east to west.（2）In this study,24 typical sample plots were selected for investigation.It was found that there were significant differences in the accumulation and distribution of heavy metals in various organs of Triarrhena lutarioriparia.Among them,Cd content was the highest in ears and leaves,and the lowest in stems,roots and buds;Cu content was highest in roots and lowest in leaves and stems;Mn content was the highest in roots and the lowest in rhizomes and buds;Pb content was the highest in panicles and the lowest in stems;Zn content was the highest in ears and buds,and the lowest in leaves,stems and rhizomes;Among them,the stem and root are the main organs to absorb heavy metals.In addition,the linear regression analysis between the content of heavy metals and environmental factors shows that the content of Cu is positively correlated with elevation and sediment organic matter,and negatively correlated with sediment moisture and conductivity.The Mn content is positively correlated with the organic matter of sediment and negatively correlated with the water content of sediment.The content of Pb and Zn is positively correlated with total phosphorus in sediment.The mean value of the total amount of Cd,Cu,Mn,Pb and Zn in each square meter of the T.lutarioriparia is 2.2 mg m^-2,86.7 mg m^-2,290.3 mg m^-2,15.9 mg m^-2and307.8 mg m^-2respectively.Combined with the distribution range of the T.lutarioriparia in the Dongting Lake wetland,it is calculated that the annual aboveground organ accumulation of the T.lutarioriparia is 0.7t Cd,22.9t Cu,77.5t Mn,3.1t Pb and 95.9t Zn.（3）Though three typical sample plots of Louxiwan,Junshan Park and Liumenzha to explore the dynamic process of the enrichment and transfer of heavy metals by T.lutarioriparia.It was found that Cd content was higher in November and December in the leaves,stems,roots and rhizomes of Southern silver grass.The contents of Cu and Zn were higher in February,and the contents of Pb were higher in April.The contents of Cu,Mn and Pb were higher in the roots and rhizomes of T.lutarioriparia,while the contents of Zn were higher in the roots and rhizomes of T.lutarioriparia in February.The content of Cd content in leaves,stems,roots and rhizomes of the three sample plots had higher enrichment coefficient in December,and Cu concentration coefficient in leaves of the T.lutarioriparia was higher in February.In addition,Mn and Zn in the leaves and stems of T.lutarioriparia have higher enrichment coefficients in the sample plots of Louxiwan and Junshan Park in February.The transfer coefficients of Cu and Zn in the three samples of each part of the T.lutarioriparia were higher in February,and the transfer coefficients of Mn and Zn were greater than 1.（4）The characteristics of litter decomposition and heavy metal accumulation and release were studied in Junshan,Beizhouzi and Luhu.With the increase of decomposition time,the mass residue rate of the litter stems and leaves of the three sample plots decreased significantly,and the mass residue rate of the leaves was significantly lower than that of the stems.According to the Olson exponent decay model,the decomposition time of the litter of T.lutarioriparia was less than that of the litter stems,and the decomposition time of the litter stems and leaves of Junshan sample plot was faster than that of the other two sample plots.The Cu content of the litter leaves in Junshan,Beizhouzi and Luhu was higher than the initial Cu content of the leaves in the decomposition process,and the Cu accumulation index of the fallen leaves was higher than 100%during the decomposition process,indicating that the Cu accumulation characteristics were present in the decomposition process.The content of Zn in the litter stems of Beizhouzi plot and Luhu sample plot was higher than the initial Zn content of the litter stems during the decomposition process,and the Zn accumulation index of the litter stems was lower than 100%during the decomposition process,indicating that the change characteristics of release were presented as a whole.