| As the city's lung, Urban forest vegetation bears the change of urban ecological environment and provides well conditions of work, life and living for urban residents. So, the development of urban forest has became important form and content of ecological city construction in the accelerating urbanization process in the new period. This article chiefly concerned with the study of Changsha urban forest soil, and discussed the spatial distribution characteristic and sources about heavy metal content in urban forest soil, compared and studied soil heavy metal content and pollution levels in different types of green. Try to regularly reveal the influence of urban soil heavy metal content in different forest types, urbanization degree and green types, to provide the scientific basis for findind out the main heavy metal pollution factors and Pollution ways in green land in changsha city, selecting green greening tree species for Changsha urban, preventing and restorating soil heavy metal pollution, improving the urban environmental quality and guaranteeing the health of residents. The main conclusions are as follows:(1) The results of Changsha Urban forest soil contents of heavy metals showed that the order of the average contents of the 5 kinds of heavy metals in green soil from high to low were Mn>Zn>Pb>Cu>Cd, the average content (mg/kg) were 166.34,50.22,31.12, 23.56,0.16. In xiaoyuan park,nanjiao park, Guihua park and Central South University of Forestry and Technology, five kinds of heavy metal content of four research areas were more than the far from the crowd areas which were shiyan lake, dashanchong wangling park in all the eleven study areas.5 kinds of heavy metals contents ranging distribution disparity in different urbanization gradient forest soil, general rule was city center> city limits> cities Surrounding.(2) Element absorption migration and accumulation ability of different kinds of heavy metal were different in various of forest types:The best is osmanthus and DuYing Lin for four types heavy metals of Cu, Zn, Pb and Cd; Osmanthus Lin Purify had the best effection ability. The reduction functions of heavy metal existed certain differences in greens which were composed by different plant species, woodland obviously was higher than the lawn. To the absorption and migration effect of Cu, Cd and Pb elements, the lawn was less than woodland, to the absorption and migration effect of Mn, the woodland was below the lawn. (3) To soil of the same forest types, the possibility of Cd and De elements from the same sources is larger, meanwhile, it was low about Pb and Mn. The sources of soil heavy metal were not completely the same in the same study area (Guihua park and botanical garden). Heavy metal element, Zn, Pb and Cd were likely to come from common sources of pollution in Lieshi park and forestry university campus soil. The possibility of Cu, Zn and Cd elements from the same sources of pollution was larger in yuelushan soil. Cu and Pb, Zn and Cd came from the identical sources in Xiaoyuan park soil. Pb and Zn, Pb and Cd were likely to come from common sources of pollution in Changsha wanglin park soil. Heavy metal sources might be not only one in Nanxiao park,Heinifeng, blackelk peak, ShiYan lake and Dashan blunt soil. In different research areas, Cu and Zn, Cd and Pb, Cd and Mn might have came from different pollution areas, but Pb and Cd, Zn and Mn, Pb and Zn, Cd and Zn might come from common sources of pollution.(4) The correlation of different metals and the same soil chemical index is different, even the correlation of the identical heavy metal element and the same soil chemical index is different in different types of urban forest. Because of human activities in different degree and natural environment, the correlation of identical soil chemical index and different metals, different soil chemical index and identical heavy metal element is different in different research area. The correlation of soil machine quality, TN P, TN N and identical heavy metal element in different natural environment is different.(5) Changsha forest soils with higher levels of some heavy metals may be caused by the pollution of traffic, and some may be derived from living pollution or industrial activities. Origin of urban forest soil heavy metals is very complex, meanwhile, the change of natural sources of metals and sediment particle size also may have some impact. Therefore, the source of heavy metals in Changsha forest soils are mainly restricted to qualitative analysis.(6) In urban forest soil of different research areas, from the perspective of single pollution index, the pollution index of Pb is the largest, the pollution degree is the highest, the pollution levels of Cd is second, the pollution index of Zn is minimum and the pollution level is the lowest. From the perspective of integrated pollution index, the heavy metal pollution levels of forest city soil in Changsha were:clean, still clean, lightly polluted.(7) Different urbanization gradient forest soil has different pollution of heavy metal, the degree at different levels ranking with high to low for:the city center> city limits> cities Surrounding. Looking from single pollution index, Zn pollution index is the lowest, Pb pollution index is the highest, according to the size of the order to pollution index ranking for:Pb> Cd> Cu>Zn in different soil of urban forest types. Looking from the composite pollution index,6 kinds of soil heavy metal pollution of forest type of ChangSha is level of clean (vigilance limit), from what soil heavy metal composite index of pinus forest up to a maximum of light pollution levels, belonging to the pollution is at its worst, Osmanthus fragrans and Elaeocarpus sylvestris Poir mixed comprehensive pollution index is minimum. Therefore, from the angle of purify the heavy metal of urban forest soil, pines should be reduced in the city, appropriately increasing mixed forest of Osmanthus fragrans and Elaeocarpus sylvestris Poir mixed forest.(8) The comprehensive pollution index of heavy metal in lawn is higher than in woodland, the pollution in lawn is at clean level, but is at clean alert level in woodland.The integrated purification ability of forest for heavy metal Cu, Zn, Cd, Pb is higher than the lawn.So, we should build as much as possible woodlands, rather than lawn in process of urban greening.(9) As much as possible to build more woodland and fewer lawn is sugested in unban greening planning. Based on the content of heavy metals in different areas, we plant different kinds of trees which can absorb the heavy metal well. All in all, from the perspective of decontaminating heavy mental of urban-forest soil, pines should be reduced and mixed forest like Osmanthus fragrans and Elaeocarpus sylvestris forest should be increased appropriately. |
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