长沙城市森林土壤7种重金属含量特征及其潜在生态风险

Abstract

采用调查分析方法,研究长沙城市森林土壤Zn、Cu、Ni、Pb、As、Cd、Hg7种重金属含量,并以长沙市土壤背景值和湖南省土壤背景值为参比值,采用Hakanson潜在生态危害指数法评价不同城市化梯度森林土壤重金属潜在生态风险。结果表明:7种重金属的平均含量均随着城市化程度提高而增加,Pb增幅最大,As增幅最小。同一城市化梯度森林土壤均以Zn平均含量最高,Cd最低,但均未超过土壤环境质量标准(GB15618-1995)Ⅱ级标准值。在城市中心区,桂花树林、樟树+桂花树混交林土壤Zn、Cu、Pb、As、Hg平均含量普遍较高,而樟树+马尾松混交林、桂花树+杜英混交林土壤Cu、Ni、As、Cd、Hg平均含量普遍较低,Pb、Zn空间分布差异明显,Cd、Ni、As空间分布比较均匀,Cu、Ni、Pb、As、Cd、Hg之间(除Cd与As、Ni之间外)均存在显著(<em>P</em>＜0.05)或极显著(<em>P</em>＜0.01)的相关性,与土壤有机质之间也呈显著(<em>P</em>＜0.05)或极显著(<em>P</em>＜0.01)的相关性,Zn、Cu、Ni、Pb、As、Cd、Hg主要是人为输入;中心区森林土壤重金属的潜在生态危害已达到中等生态危害程度,边缘区接近中等生态危害程度,郊区为轻微生态危害程度,Zn、Cu、Ni、Pb、As均为轻微生态危害程度,Cd、Hg已达到中等生态危害程度以上。;In order to understand the influence of urbanization on distribution of heavy metals in soils, concentrations characteristics of 7 heavy metals (Zn, Cu, Ni, Pb, As, Cd, and Hg) in various forest types were investigated along an urban gradient in Changsha, a capital city of Hunan Province, China. The concentrations and potential ecological risk of heavy metals in the urban soils was assessed when compared to the background values of heavy metals in soils in both Changsha city and Hunan Province. A total nineteen major forest types were selected at three locations (namely urban district location, urban fringe location, and suburban location) along the urban gradient degree in urban forests soil in the Changsha city. Two forest plots (each with the size of 20m × 20m) were built in each forest types. Three subplots were (each with an area of 1 m²) were set up in each of the plots. Five soil samples (from 0-20 cm depth) were randomly taken within each subplot and pooled to a soil sample for heavy metals chemical analysis, 0-20cm soil samples were collected from different urban gradient (urban district, urban fringe, suburban) forest to analyze the concentrations of Zn, Cu, Ni, Pb, As, Cd, and Hg, and the potential ecological hazard assessment indices by Hakanson was used to assess the soil heavy metals potential ecological risk degree in urban forest, based on the background values of soil in Changsha City and in Hunan Province. Results showed that the average concentration of the same heavy metal elements in urban forest soils tended to increase with degree of urbanization. Pb had the maximum increasing value and As the minimum. In the same urbanization location degree, Zn had the highest average concentration and Cd had the lowest one among the studied of all test heavy metals., On average, while the average the concentrations of all examined heavy metals them in study sites were lower than those in the Environmental Quality Standard for Soil (Ⅱgrade) in China.,. The heavy metals whose concentrations were higher than the background values in Changsha cite and the accumulation tendency of test heavy metals in urban forest were as in the order as of Cd＞Hg＞As＞Pb＞Cu (Ni)＞Zn in the study sites. The concentration of re existed some differences in the same heavy metal element varied concentration with among different types forest types in the city, The average concentrations of Zn, Cu, Pb, As, and Hg were as higher in <em>Osmanthus fragrans</em> forests and, <em>Cinnamomum camphora Presl.</em> and<em> Osmanthus fragrans </em>mixed forests, but that of Cu, Ni, As, Cd, and Hg were found to be was low concentrations in<em> Cinnamomum camphora Presl.+ Pinus massoniana </em>mixed forests and, <em>Osmanthus fragrans +Elaeocarpus sylvestris Poir </em>mixed forests, the spatial distribution patterns of Pb and Zn was significant different in the study area, while Cd, Ni and As were more evenly distributed. Significant correlations were found among Zn, Cu, Ni, Pb, As, Cd, and Hg concentrations in soils (<em>P</em>< 0.05), except the relationships between Cd and As, and Ni. The 7 heavy metals had positively significant correlations with soil organ matter. It was found that Zn, Cu, Ni, Pb, As, Cd and Hg mainly came form the anthropogenic inputs in the urban district forests. Based on potential ecological risk assessment, the situation of heavy metals was in the medium ecological risk level in urban district locations, was in closing to the medium level in urban fringe locations, and was in a slight level in suburban locations in Changsha cite. Among the studied heavy metals, Zn, Cu, Ni, Pb, and As were in a slight risk level, while Cd and Hg were in medium or strong ecological risk level. Our study provided scientific reference to estimate and evaluate the impact of urbanization on heavy metals in Southern China.