Urbanization processes affect the accumulation of heavy metals in urban soils. Effects of urbanization on heavy metal accumulation in soils were studied using Beijing as an example. It has been suggested that the ecological function of vegetation covers shifting from natural to agricultural settings and then to urban greenbelts could increase the zinc（Zn） concentrations of soils successively. The Zn concentration of urban soils was significantly correlated to the percentage of the impervious land surface at the500 m × 500 m spatial scale. For urban parks, the age or years since the development accounted for 80% of the variances of cadmium（Cd） and Zn in soils. The population density,however, did not affect the heavy metal distributions in urban soils. To conclude, the urban age turned out to be a notable factor in quantifying heavy metal accumulation in urban soils.
It is widely acknowledged that a simplified and robust approach to evaluating thecombined effects of chemical mixtures is critical for ecological risk assessment(ERA)of contaminated soil.The earthworm(Eisenia fetida)was used as a model to study the combined effects of polymetallic contamination and the herbicide siduron in field soil using a microcosm experiment.The responses of multiple biomarkers,including the activities of catalase(CAT),superoxide dismutase(SOD),glutathione reductase(GR)and acetylcholine esterase(AChE),the concentrations of glycogen,soluble protein(SP),malonaldehyde(MDA),and metallothionein(MT),and the neutral red uptake test(NRU),were investigated.Multivariate analysis,Principal Component Analysis(PCA)and Spearman’s Rank Correlations analysis(BVSTEP)revealed that the activities of AChE and CAT and the NRU content were the prognostic biomarkers capturing the minimum data set of all the variables.Internal Cd(tissue Cd)in earthworms was closely related to the health status of worms under combined contamination of heavy metals and siduron.The integrated effect(E mix)calculated based on the activities of AChE and CAT and NRU content using the stress index method had significantly linear regression with internal Cd(p<0.01).E mix(10),E mix(20),and E mix(50)were then calculated,at 1.27,1.63 and 2.71 mg/kg dry weight,respectively.It could be concluded that a bioassaybased approach incorporating multivariate analysis and internal dose was pragmatic and applicable to evaluating combined effects of chemical mixtures in soils under the guidance of the top-down evaluation concept of combined toxicity.
