Study On The Mechanism Of Remediating Cu And Pb By Poplar On Calcareous Soil

In recent years,It's a special research of using bioenergy plants such as poplar for remediating the heavy metal contaminated soil.However,the mechanism of how to remediate Cu and Pb is still not clear on calcareous soil which with high CaCO₃content,low organic matter content,and low nutrient availability of plants.In this paper,the research object is the calcareous soil-poplar plantation system which soil is contaminated by heavy metal.The effects of poplar afforestation on soil properties,the effects of Ca and Na redistribution in soil profiles on poplars absorption of heavy metals,and the absorption and accumulation characteristics of poplars on heavy metals under chelating agents are studied by using field location sampling and potted simulation experiment methods.The main results are as follows:1)Poplar afforestation can not change the pH and CaCO₃ content of calcareous soils significantly,but it can reduce the conductivity of rhizospheric soils significantly.In the early stage of poplar afforestation,the content of SOC in the rhizospheric soil is increased,but in the later period its content is reduced significantly,but the content of DOC has no significantly change in the whole growth period.In the late period of afforestation,planting poplars also reduce the total P content in the rhizospheric soil,but it can increase the total N content,while maintaining the bioavailable P level without significant decrease.After afforestation,the C/N ratio in soil is reduced,and the C/P and N/P ratios are increased.As the age increases,the activity of the hydrolytic enzymes?glucosidase?GLU?,urease?URE?and alkaline phosphatase?ALP??in the soil is gradually suppressed,and the activity of GLU and ALP was lower than that in non-rhizospheric soil in the later afforestation period.It shows that afforestation can inhibit the activity of soil hydrolose,and the time of afforestation is longer,the activity of soil hydrolose is suppressed.2)Poplar afforestation can not change the total Cu content in the rhizospheric soil,but it can increase the total Pb content as the tree age increases,and at the same time it can increase the proportion of bioavailable Cu and decrease the proportion of bioavailable Pb.Redundancy analysis?RDA?shows that the content of bioavailable Pb in soil is regulated by CaCO₃,C/P and N/P ratio,total Pb,and total P,while the content of bioavailable Cu is regulated by DOC,effective P,and total Cu.The results shows that there is an intrinsic link between the three hydrolytic enzymes associated with the C,N,and P cycles in the soil,and their activity is inhibited by the bioavailable Pb simultaneously.3)During the growth of poplar,Na and Ca are accumulated in the deep?20⁴0cm?and surface?0²0 cm?soils in the rhizospheric soil,and the content of bioavailable Na is increased and bioavailable Ca is reduced respectively.The organ,stand age,and the interactions between organs and ages of poplars have a significant effects on the absorption of Cu,Pb,Na,and Ca by poplars.Specifically,the distribution of Cu,Pb,and Ca in poplars follows the order of bark,leaves>branches>trunk xylem,while the distribution of Na is the xylem of the trunk>leaves>bark,branches.The changes of the bioavailable Na and Ca contents in soils are driven by afforestation age mainly affect the contents of Cu and Pb in bark and the content of Pb in leaves.The contents of Cu and Pb in leaf litters increase significantly with the increase of tree age,and they are also higher than those in leaves during the growth period significantly because of the bioconcentration of leaf litter.4)The application of chelating agents including EDTA,EGTA and CA increase the contents of bioavailable Cu and Pb in soil,and the activations of EDTA and EGTA are more persistent.The application of EDTA and EGTA increase the content of Cu in poplar roots and leaves,while EDTA could only increase the content of Pb in leaves.In summary,the properties of the calcareous soil and the redistribution of mineral elements in the soil profile are affected through the activity of the root system and the decomposition of litter,by which affect the bioavailability of the heavy metals in the soil during the growth of poplars.Poplars have a phytostabilization mechanism for Cu and Pb,and detoxification of heavy metals with bark storage and leaf litter.