| Road heavy metal pollution is an important aspect of the road traffic pollution. On the onehand, it causes the surrounding air, soil and plant pollution, on the other hand, it affects humanhealth along the food chain or by the migration of atmospheric particulate matter. The foresttrees have strong heavy metal resistance ability, combined with the advantages of largebiomass and long life cycle, it plays a very important role in purifying the air and soil heavymetal pollution. This study takes the populus tomentosa forest belt besides Beijing CapitalAirport Expressway, the enrichment and distribution of Pb, Cd, Cr, Cu, Mn, Ni, Pb and Zn intrees were analyzed, and the influencing factors of heavy metal distributions in populustomentosa are discussed. The study results showed that:(1) On the vertical distance against the road, the horizontal distribution of the heavy metalconcentration in leaves is not a simple negative linear correlation, and the horizontaldistribution trends of different elements are not quite similar. In the seven elements, theconcentration of Zn decreased at first and then increased, while the peak value at100m andvalley value at50m. The remaining six elements increased firstly and decreased subsequently,with peak value at15to50m and valley value at50m or100m mostly.(2) The seasonal dynamics of the same heavy metal concentration in leaves is roughly thesame at different distances, of which the concentrations of Pb, Cd, Mn and Zn graduallyincreased with the growing seasons; the concentrations values of Cr and Ni are all highest inspring; the concentration of Cu changes relatively stable in all seasons.(3) In the horizontal direction of tree’s trunk xylem, thr tree-rings were divided into fiveyear periods from bark to the center, each of the heavy metal concentration levels in the everyring segment are different. The concentrations of Pb, Cd, Mn and Zn have a general downwardtrend, and which have a positive correlation trend with annual precipitation, minimumtemperature, sunshine hours, rainy days and windy days. (4) In the vertical direction of the trunk, with trunk height rises, the concentrations of Cd,Mn and Zn elements have an overall upward trend from the tree base to treetop. Theconcentration values of Cu and Pb are highest in the base of the tree, and are drasticallyreduced at diameter, then with height increases, fluctuation range of element concentrationvalues is not large. The concentration of Ni is overall decreased firstly and increasedsubsequently. From the accumulation of heavy metals in the each section of trunk, the amountof heavy metals increased firstly and decreased subsequently with trunk height increases, andthe peak value appeared at3.5to7.5m section.(5) The heavy metal concentration in tree organs are: Zn> Mn> Cu (Cr)> Pb> Ni> Cd.(6) The concentrations of the same heavy metal element are different among variousorgans, Cu, Mn, Ni and Pb are highest in the leaves of populus tomentosa, the concentration ofZn is highest in the bark, Cd and Cr are highest in the branch and root respectively.(7) The heavy metal accumulation in various organs depend not only on biomass, but onthe concentration, the accumulation of Cd, Cr and Ni elements are all highest in the trunk,while Cu, Mn, Pb and Zn elements are in the branch.(8) From the heavy metal accumulation of individual plant, each element in a descendingorder as: Zn>Mn>Cr>Cu>Ni>Pb>Cd.(9) The percentage of the amount of heavy metals in fallen leaves to that in the whole treebefore leaves fallen is: Mn>Ni>Pb>Zn>Cu>Cd>Cr, which illustrates that the populustomentosa has a relatively strong purification ability against Mn and Ni elements and arelatively weak purification ability against Cd and Cr elements.(10) In all organs, the cutting strunk measure led to the increase of Mn concentration. Inthe root, stem and bark, the concentrations of Zn and Cu in cutting-off plant are higher than notcutting-off plants. In the leave and branch, the concentrations of Cu and Zn in cutting-off plantare lower than not cutting-off plant. |
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