行道树毛白杨树干中重金属元素分布

采用电感耦合等离子体发射光谱法(ICP),测定分析首都机场高速公路旁毛白杨(Populus tomentosa Carr.)树干中Pb、Cd、Cr、Cu、Zn、Ni和Mn 7种重金属元素的含量及积累量,比较分析树干不同组织、不同方位、不同龄级年轮重金属含量差异及与交通量、关键气候因子的相关性。结果表明:树干木质部中各重金属平均含量由大到小依次为Zn、Cu、Cr、Mn、Ni、Pb、Cd,树皮中依次为Zn、Mn、Cr、Pb、Cu、Cd、Ni,树皮中各重金属元素的含量明显高于木质部;同一树干木质部中,各重金属元素在不同方位的分布有所差异,其中,靠近车道一侧的各重金属元素含量均高于背离车道一侧,南北向比较中,Ni、Zn为南侧含量高于北侧,其他5种重金属元素均为北侧高于南侧;以5a为一个龄级将年轮划分为5个龄组,各龄级年轮中重金属含量随时间的变化趋势各异,其中Pb、Cd、Cu、Zn总体呈递减趋势,且与年降水量、最低气温、日照时数、雨天日数和大风日数呈正相关趋势,与年平均气温和最高气温呈负相关趋势;各元素在毛白杨树干木质部中的积累量表现为ZnCrCuMnPbNiCd。

Heavy metals distribution in roadside tree trunks under a heavy urban transportation condition

Roadside trees play an important role in improving transportation safety and environment conditions such as soundproof, pollutant removal, and positive psychological influence. The distribution of heavy metal concentration in the roadside tree trunks are rarely investigated in wholesome way. To know the distribution of heavy metals in tree rings has important implications for researching on heavy metals enrichment and purification effect of trees. In this study, we measured the concentration of seven heavy metals including Pb, Cd, Cr, Cu, Zn, Ni and Mn in the tree trunks of Populus tomentosa Carr. growing along Beijing Capital Airport expressway, Beijing, China, by using the Inductively Coupled Plasma(ICP). The distribution was analyzed according to the tissues, directions, and growth of the tree trunks. The influence of major climate variables on the distribution was also studied by using correlation analysis. The result showed that the average concentration of each heavy metal was different in different stem parts. The concentration ranking from high to low was Zn > Cu > Cr > Mn > Ni > Pb > Cd in xylem and Zn > Mn > Cr > Pb > Cu > Cd > Ni in bark, respectively. Heavy metal concentrations in bark were signifcantly higher than that in xylem. Moreover, the heavy metal concentrations in the tree stem near the lane side were higher than the ones farther from the lane side. The concentration of Cu, Cr, Mn, Pb, and Cd in the northfacing tree stem was higher than that in the southfacing stem. We found there were temporal variations of the heavy metal concentration level in tree stem by measuring the concentration of heavy metals in each five year tree ring segment from the tree center to the bark. The concentrations of cadmium, lead, zinc and copper had a general downward trend, but manganese, nickel and chromium an upward trend. We concluded that the tree growth and the environment influenced the distribution of heavy metals in tree rings. When tree grew slowly, the stem xylem could absorpt and deposit more the heavy metal element and vice versa. There was a significant positive correlation between Zn, Pb, Cd and Cu concentration and climate factors including annual precipitation, annual strong wind days, annual rain days and annual minimum air temperature. The estimated content of heavy metals in stem were Pb (2822.69mg), Cd (1407.55mg), Cr (21482.56mg), Cu (17255.19mg), Zn (98123.60mg), Ni (1726.46mg) and Mn (15063.84mg), respectively, on the basis of trunk biomass. The current research provided an insight for assessing the capability of Populus tomentosa to curb the heavy mental pollution under a heavy urban transportation condition.