Heavy Metal Accumulation Of 5 Common Tree Species In Different Functional Zones In Beijing

With the continuous development of urbanization, population assembles to the urban and the size of the cities expands a lot. The high aggregation effect brings an positive impact on regional economic development, and also produces a series of environmental problems, one of which is heavy metal pollution. The arbor species, with long life cycle and large biomass, are an important part of urban green-land system and play an important role in absorbing and purifying heavy metal pollution.Taken five common tree species in four different functional zones(industrial,traffic,residential,park)of Beijing as an object, concentrations of 7 kinds of heavy metals in leaves, different organs, soil and TSP were measured by the Inductively Coupled Plasma(ICP). The distribution rule and relationship with environmental factors(soil and TSP) of heavy metal element in five tree species were studied, and the enrichment capability of each tree species were analyzed. The paper aimed at providing theoretical basis for the tree species selection under conditions of heavy metal pollution. The study showed that:(1) Interspecific variation characteristics of heavy metal concentration in leaves. ①In spring, summer and autumn, the maximum values were present in Populus tomentosa leaves in traffic zone; ②From the total average concentration in 5 tree species leaves, the concentration in Populus tomentosa leaves was the highest in tree seasons, the lowest concentration was in Ginkgo leaves; ③In terms of 7 kinds heavy metal, the concentrations of Cd, Cu, Zn in Populus tomentosa leaves were relatively higher than the rest of trees; the Cu element concentration in Sophora japonica leaves was also higher, which indicated that Sophora japonica had a certain absorption ability to Cu element; there were no obvious regularity about the absorption and change trend of Mn, Ni, Cr, Pb.(2) Seasonal variation characteristics of heavy metal concentration in leaves. Overall, with the growth of leaves, the change trend of heavy metal concentration in Populus tomentosa, Sophora japonica, Pinus tabulaeformis and Sabina chinensis leaves in different functional zones were different; which in Ginkgo leaves had the tendency of decreasing.(3) The heavy metal concentration in tree organs: Mn、Zn>Cu、Cr>Ni>Pb>Cd.The absorptive capacity to heavy metals by organ was inhomogeneity. The minimum total concentration of heavy metal in tree organ was in the trunk, the maximum concentration varied according to the tree species and environmental conditions, and distributes in different organs.The concentration of the same kind of heavy metal in tree’s various organs was different. The concentration value of Cd and Zn were highest in bark and lowest in trunk; the highest concentration of Cr varied from tree species to tree species; the Cu element concentration in Populus tomentosa trunk was the highest, on the contrary, in the remaining 4 tree species were the lowest; there was a dramatically difference for Ni element concentration in tree organs; the branch, bark and leaves had the highest Pb concentration, root second, and trunk the lowest.(4) According to the transfer rate of heavy metal in leaves before and after the growing season, Populus tomentosa and Pinus tabulaeformis had good purification on heavy metal pollution, Sabina chinensis was dominated by enrichment, Sophora japonica and Ginkgo depended on the type of elements.(5) The transfer coefficient to Pb and Zn elements by each tree species were larger, indicating their strong transport capability to Pb and Zn, while that to Cr was smaller.(6) The enrichment coefficient of 5 tree species to Cu, Zn were higher, and to Mn, Ni were smaller. The comprehensive enrichment ability of tree species from large to small was as follows: Populus tomentosa, Sophora japonica, Sabina chinensis, Ginkgo, Pinus tabulaeformis.(7) From the accumulation of heavy metal in 5 tree species, each element in a descending order as : Zn>Mn>Cr>Cu>Ni>Pb>Cd. The total heavy metal accumulation of tree species by an order of Populus tomentosa, Sophora japonica, Ginkgo, Sabina chinensis, Pinus tabulaeformis. The order of heavy metal total enrichment per unit area was Populus tomentosa, Sabina chinensis, Ginkgo, Sophora japonica, Pinus tabulaeformis; the order of heavy metal total enrichment per space was Populus tomentosa, Sabina chinensis, Pinus tabulaeformis, Sophora japonica, Ginkgo.(8) The overall concentration trend of 7 kinds of heavy metals in soil and TSP of Beijing was quite similar with that in tree. But, it could be found that there was statistically significant between a small number of heavy metal element in tree, soil and TSP, which showed that the absorbption of tree to heavy metal had a certain relationship with the heavy metal concentration in environment, but also affected by other factors.