Distribution And Pollution Assessment Of Nutrient Elements And Heavy Metals In Sediments Of Taxodium Ascendens Lake

Sediments is the accumulation pool of nutrients and heavy metals in lake wetlands,which has a profound impact on the growth and development of wetland plants,and may also be an endogenous source of carbon,nitrogen,phosphorus and heavy metal pollution in wetlands.In this study,the sediments of 15-year-old forest and 40-year-old near-mature forest in Taxodium ascendens Lake National Wetland Park at the junction of Jiangsu and Anhui were taken as the main research objects.A total of 144 columnar sediments samples（0～10 cm、10～20 cm and20～40 cm）were collected in four seasons.The distribution characteristics and variation rules of nutrient elements and heavy metals in the sediments of undergrowth and forest edge of Taxodium ascendens Lake in different seasons were systematically analyzed,and the content of nutrient,carbon,nitrogen,phosphorus and heavy metal pollution status of the bottom mud of Taxodium ascendens Lake were evaluated,so as to provide scientific basis for the growth of Taxodium ascendens Lake and its sediments pollution control.The main findings were as follows:（1）The bottom mud of Taxodium ascendens Lake was dominated by silt and sand particles,and the p H was weak acidity and neutrality predominating.The p H of sediments in near-mature forest was lower than that in young forest,and the p H of sediments under forest was lower than that in forest edge.The content of organic matter in sediments is 12.80～70.30g·kg^-1,mostly at high and excessive levels.The organic matter content of undergrowth sediments was higher than that of forest edge,and the organic matter content of near mature forest was higher than that of young forest.The average contents of total nitrogen,total phosphorus and total potassium in sediments were 0.89,0.78 and 18.25 g·kg^-1,respectively,mostly at moderate and high levels.The average contents of alkali-hydrolyzable nitrogen,available phosphorus and potassium were89.73,9.29 and 108.88 mg·kg^-1,respectively,with the content at low to moderate levels.In terms of seasonal variation,the organic matter and total nitrogen in sediments showed a pattern of high in summer and winter,and low in spring.The total phosphorus content in the bottom muds of Lake Taxodium ascendens was higher in summer,autumn and winter.Total potassium content in sediments was higher in summer and autumn.The content of available nitrogen,available phosphorus and available potassium was the lowest in spring.（2）The content of heavy metals in the bottom mud of Taxodium ascendens Lake showed Zn>Cr>Cu>Pb.Except for Pb,the content of Cu,Zn,Cr was significantly higher than the background value of this region,and the contents of Cu,Zn and Cr in undergrowth area sediment exceed 34.14%,18.31%and 12.89%of the national soil quality standard I.Cu,Zn,Pb,Cr consistently were greater than the forest edge.In vertical direction,the bottom mud of Taxodium ascendens Lake was high in surface layer and low in bottom layer.The seasonal variation of heavy metal elements Cu,Zn,Pb and Cr showed a high average content in winter.The comprehensive bioaccumulation coefficients of Cu,Zn,Pb and Cr in the sediments by T.mairei were 0.31,0.55,0.37 and 0.22,respectively,and the comprehensive enrichment ability followed the order of Zn>Pb>Cu>Cr.The accumulation of heavy metals in bark,branches and roots was higher,while that in leaves was generally lower.The heavy metal transfer coefficient of bark,branch and leaf to root was 1.19,1.18 and 0.75,respectively.The order of heavy metal transfer ability of each organ was bark>branch>leaf.Taxodium ascendens can play a certain role in purifying heavy metals in sediments.（3）Through single factor evaluation method,it was found that the average pollution indexes of organic carbon,total nitrogen and total phosphorus in sediments were 2.23,1.61 and 1.29,respectively,which exceeded the lowest ecological toxicity,indicating that organic carbon,total nitrogen and total phosphorus in sediments may pollute water bodies and organisms.The evaluation of organic nitrogen and organic pollution in sediments showed that the two forestlands were still clean in most periods,but organic nitrogen and organic pollution existed in the near-mature forest in summer.The potential ecological risk assessment method was used to evaluate the bottom mud of Taxodium ascendens Lake.The results showed that Cu,Zn,Pb and Cr in Taxodium ascendens Lake were in a slight ecological hazard state,and the risk index grade was mild.Through the geological accumulation index method,it is concluded that Cu,Zn and Cr have a certain accumulation in Taxodium ascendens Lake,especially in winter,Cu,Zn and Cr in the undergrowth sediment were slightly polluted,and pollution prevention and control should be done well.The organic matter in sediments had a good correlation with nutrients and heavy metal elements,such as Cu,Zn and Cr.The correlation coefficients between clay and Cu,Zn,Pb,Cr were 0.454,0.492,0.373,0.446,respectively,which were significantly positive.Heavy metals were mainly absorbed in organic matter and clay.As the main species of wetland,Taxodium ascendens has a certain purification capacity for carbon,nitrogen,phosphorus and heavy metal pollution of sediment,but the purification capacity is limited.It is necessary to strengthen the prevention and control measures of sediment pollution.