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The mobility and availability of heavy metals in the soil to plants depend not only on thetotal concentration of heavy metals, but also on their speciations. This research was based onthe soil sampled from the green belts of Xi’an South2Ring Road to investigate thedistribution regularities of heavy metal speciations at various depths by determining the totalheavy metal concentration and studying the speciations of heavy metals achieved by modifiedBCR sequential extraction method. Simultaneously, the linear relation between heavy metalforms and total concentration was discussed by the analysis of correlation coefficient andregression simulation. Furthermore, the major effects and mechanism of soil physical andchemical properties content to heavy metal forms were also studied. Results show that:1. The mean concentrations of Cd, Cr, Cu, Ni, Pb and Zn in the soil samples were1.06mg/kg,85.95mg/kg,53.18mg/kg,29.45mg/kg,73.74mg/kg and147.43mg/kg. According to themodified individual pollution index, Cd, Cu, Pb and Zn polluted in various respectivelydegrees and Cr, Ni was relatively clean. Cd was the most serious contamination which worthto receive attentions. Otherwise, Nemerow synthetical pollution index showed that: due to theintegrated pollution which has reached medium level, the potential environmentalcontamination risks should take attentions.2. Cd、Cr、Cu、Ni、Pb、Zn were existed in residual forms. The sequence of Cd differentfractions was residual fraction (B4)> acid-soluble fraction (B1)> reducible fraction (B2)>oxidizable fraction (B3). The sequence of different fractions of Cr and Ni was B4>B3>B2>B1.The sequence of different fractions of Cu, Pb and Zn was B4>B2>B3>B1. Cd occurred moreabundantly in mobile fraction than did the other elements. Cd is most likely to be absorbed byplants, easy to transfer, and the bioavailability is the highest. The existing forms of Cr is morestable, with little damage to the environment. The concentration of reducible fraction Pb ishigh, if the soil environment has changed(such as pH, redox potential, etc.),Pb will bereleased.3. The concentration of “un-residual form” was the highest in topsoil (depth from0to20cm) except Cr, which indicated that the potential availability of heavy metal in topsoil washigher than deep soil. The proportion of the “un-residual form” of Cd and Cr was equal in thetop soil and deep soil (depth from20to30cm) and both of them are higher than soil whichwas from30to40cm. For Cr and Ni, the proportion of unsteady state was similar in all thedepths of soil. The ratio of the unsteady state over the total amount of Cu decreased with thedepth. While, the ratio of the unsteady state over the total amount of Zn in (depth from20to30cm) soil was higher than the top soil and the soil from30to40cm. After all, the residual condition was the main heavy metal speciation of different soil profiles.4. According to RAC (Risk Assessment Code), Cd was medium risky in all samples, Cuhad low risk, while Cr and Pb had no risk. Some samples of Ni and Cu had low risk, whileothers had no risk. According to RSP (Secondary Phase Compared to The Native ValueMethod), the unstable degree of risks of each heavy metal to ecological environmental had theorder as Zn>Pb> Ni>Cd>Cr&Cu. In general，Ni, Pb and Zn has potential availabilities, whenthe environment changes, they might be harmful to the environment, while Cd, Cr and Cuwere more stable.5. The statistical data showed that for all six heavy metals, there was a close correlationbetween residual fraction (B4) and total amount (B). The correlation coefficient of Ni isrB4-B=0.734(P≤0.05), the correlation coefficient of Cd, Cr, Cu, Pb, Zn is rB4-B=0.987(P≤0.01),rB4-B=0.870(P≤0.01), rB4-B=0.906(P≤0.01), rB4-B=0.896(P≤0.01), rB4-B=0.961(P≤0.01).6. The correlational analysis between different heavy metal showed that there were closecorrelations between Cr and other metals, the concentrations of chemical forms of Cr mayimpact others. In addition, negative correlations were found between Zn and other metals. Aremarkable correlation existed between Ni and Cr, Cr also had a significant linear relationshipwith Ni on residual fraction, oxidizable fraction and even the total amount, which indicatedthat Cr and Ni may come from the same source of pollution.7. The principal component analysis showed that speciations of heavy metals contentvariation in soil were mainly affected by salinity, electrical conductivity, pH and particlediameter. The mechanism was that the adsorption, desorption, precipitation effect betweensoil factors and heavy metal could inhibit or promote the fixation and migration of heavymetal in soil.
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