| With the development of urbanization and the increase of population density, a large quantity of heavy metals were discharged into urban soils thus threatened human life and contaminated ecological environment. These heavy metals came from different sources including food chain, flying dust, surface runoff and human activities such as industrial wastes, automobile emissions, mining and agricultural practice. The contents of heavy metals in urban soils can be a reflection of the impact on people's health to some extent. However, the influence of heavy metal forms is much more direct since they determine the remobilization ability in soils and then contribute to the microbial characteristics of heavy metals. Moreover, the urban soil is greatly different from natural or agricultural soil in physicochemical properties as well as the structure and function of microorganism due to urbanization. This paper conducted in the urban soils of Changchun and the studies are as follows: The change of physicochemical properties, the content and heterogeneity of heavy metals; Evaluation on the contamination of heavy metals with one-factor contamination index and composite contamination index, chemical fractionation of heavy metals by sequential extraction; The main factors of influencing heavy metals enrichment and distribution of fractionation of heavy metals by stepwise regression analysis, Adsorption-desorption characteristics of heavy metals under the mono-and complex-factors and influencing factors, Microbiology characteristics as well as the relationships between microbiological properties and distribution of fractionation of heavy metals by partial correlation methods. All the studies above would help to recognize the mechanism of heavy metal contamination in Changchun, supply theoretical bases for the evaluation of heavy metal contamination through microbiological parameters and chemical fractions of heavy metals activities in soils and for the determination of critical concentration of heavy metal toxicity. Also, they are significant to make planning on urban ecological condition and to prevent and control the pollution of heavy metals in Changchun city. Results from the studies were showed as follows: The physicochemical properties of urban soils, especially the pH, organic content and texture of soils, changed greatly with the development of urbanization. The soil pH increased with the development of urbanization because of the increasing content of carbonate in urban soils. Comparing with urban agriculture land, sand grains content in urban soils is higher and tends to increase with more and more human activities while the organic content in urban soils decreases gradually as the development of urbanization. The heavy metal content in urban soils changed greatly and there are obvious differences in distribution between different heavy metals in each domain. There are different degrees and classes of heavy metal contamination in urban soils, with Pb and Ni at four-level and moderate contamination, Cd at five-level and severe contamination, Cu and Zn at three-level and minimal contamination. Soils in different domains or even from different units in one domain, polluted by the same kind of heavy metal take on obviously different extent. Most urban soils are severely polluted by heavy metals at five-level and the degree of contamination increases with the development of urbanization. The main influencing factors of heavy metal enrichment in soils are different to each metal element. Pb determined by contents of organics,calcium carbonate and Eh, Cd by CEC and water content, Cu by contents of organics and Fe, Zn by contents of organics and calcium carbonate and sand grains, Ni by contents of Fe and Mn. There is great heterogeneity for the distribution of fractionation of heavy metals in urban soils. The residual phases of Pb, Cd, Cu, Zn and Ni are first dominating forms. Though they are severely contaminated by heavy metals, urban soils would not do great potential harm to human beings because the high content of inert forms. The chemical forms and content of Pb, Cd, Cu and Zn increase as the heavy metal level rising, while chemical forms of Ni are related chiefly with its content. Thermal properties and mechanism of heavy metals in urban soils related to not only their own properties but also the pH value of solution at adsorption balance. The pH value of adsorption reaction system covers the impact of the soils'Ph value on the process of adsorption. According to experiments, heavymetal adsorption in soils agrees the following order: Pb > Cu > Cd > Zn > Ni. It is the properties of soils, sorts of heavy metals, adsorption condition and mechanism that determine the optimal model to describe the heavy metal adsorption in soils. Heavy metal ions existing together would alter the process of adsorption and being result in the difference of optimal models between single and competitive element adsorption. Comparing to single element, the adsorption content of most heavy metals were lower in competitive system, and affinity of Pb to soils increases while the affinity of other metals decreases greatly. At the balance point of single element and complex factors, the order of Pb, Cd, Zn and Ni in chemical form distribution are: the changeable phase > carbonate phase > Fe-Mn oxide phase > organic phase > residual phase. The differences of the main factors, which influenced by physicochemical properties of soils, between the form distribution of heavy metals in urban soils and the form distribution at adsorption balance point are visible. The adsorption process of heavy metals in soils accompanies with the release of calcium ion and the release rate relates to cohesion between metals and soils. The dynamic processes of adsorption to heavy metals include quick-reaction and slow-reaction periods, and the time required to come to adsorption balance for different metals is different. Using Elovich formulation to simulate the dynamic processes of adsorption of Pb, Cd and Cu could get good effectiveness while it is better to use twin-constant speed rate formulation for Zn and Ni, in addition, both formulations mentioned above are suited to describe quick-reaction of adsorption. pH values of soils, contents of calcium carbonate and sand grain are main influencing factors for reaction rate of adsorption;content of calcium carbonate is the chief factor for Cd, Zn and Ni. Process of desorption to heavy metals in soils can be divided to quick-stage and slow-stage and different heavy metals need different time to reach desorption balance, and the metals would be desorpted easily if the adsorption capability of heavy metals is weak. There was heterogeneity in microbiologic community structure, biochemical reaction intensity and microbiological parameters as enzymatic activity in urban soils due to the heterogeneity of urban soil environment.
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