| Soil environmental quality standards are the bases for the risk identification and assessment,prevention and remediation of soil heavy metal pollution.Establishing safety threshold of heavy metals is very important for the development and revision of soil environmental quality standards.Zinc(Zn)is beneficial for living organisms in appropriate dosages,but can be harmful at excess concentration.Soil ecological thresholds for Zn in Chinese soils have been sparsely reported.In the present study,ecological thresholds for Zn were derived based on ecotoxicity data for terrestrial organisms in Chinese soils,the effect of soil properties on Zn ecotoxicity,differences in species sensitivity,and differences between laboratory and realistic field conditions.Since the ecological thresholds based on total concentration might overestimate the risk of regional soils under the condition of high heavy metal contents,the ecological thresholds based on extractable Zn were also derived,in order to provide a more scientific basis for the improvement of soil environmental quality standards with respect to the ecological risk of Zn contamination.The main findings were as follows:(1)The ecotoxicity data for 19 terrestrial organisms in Chinese soils were collected,and the ecotoxicity data were normalized by using Zn ecotoxicity predictive models to eliminate the effect of soil properties on Zn ecotoxicity,and corrected the data with leaching and aging factors to minimize the differences in Zn ecotoxicity under the laboratory and field conditions.Then,species sensitivity distribution(SSD)curves were generated with a BurrⅢ function based on corrected ecotoxicity data.The concentration of Zn in soil that provides ecological safety for(100-x)%of species(HCx),was calculated from the SSD curve.Finally,we developed the predictive models for HCx by quantifying the relationship between the Zn HCx and soil properties.Results showed that leaf vegetables(such as Chinese cabbage,lettuce and cabbage)were more sensitive to Zn toxicity in soil than grain crops(such as wheat,barley and rice).The HC5 values calculated from SSD curves were 38.3(acidic soil),142.7(neutral soil),263.3(alkaline calcareous soil)and 262.2(alkaline non-calcareous soil)mg/kg,respectively,with a 7-fold variation between the four scenarios.Both the two-factor(soil pH and OC)and three-factor(soil pH,OC and CEC)models could predict the corresponding HCx values well,with determination coefficients(R2)of 0.941～0.959 and 0.978～0.982,respectively.Soil pH was the most crucial factor affecting Zn HCx values,and the Zn HCx increased with increasing soil pH,and the crucial effect of soil pH on soil Zn criteria raised at higher protective levels(2)Two different extractants(0.05 mol/L Na2-EDTA(EDTA-Ⅰ)and 1%NaHCaEDTA+1 mol/L NH4O-Ac(EDTA-Ⅱ))extractable concentrations were used to indicate extractability of added Zn in 20 Chinese soils.The influences of soil properties on extractrability were evaluated by stepwise regression.Results showed that the extraction rates of EDTA-I and EDTA-Ⅱwere(83.4±2.2)%and(62.1±3.5)%.The models with only one factor,total added concentration,could predict the extractrable Zn concentration well.(3)The ecotoxicity data based on EDTA extractable Zn were obtained based on the ecotoxicity data for 19 terrestrial organisms in Chinese soils and the relationship between EDTA extractable Zn and total exogenous Zn.And the new dose-toxicity relationships between phytotoxicity response and extractable concentration were established,incorporating the soil properties.Then the ecological thresholds for extractable Zn were derived based on extractable ecotoxicity data for terrestrial organisms in Chinese soils,ecotoxicity predictive models based on EDTA extractable Zn,leaching and aging factors,and the differences in species sensitivity.Results showed that the HC5 values based on EDTA-Ⅰ extractable Zn were 33.5(acidic soil),118.1(neutral soil),216.8(alkaline calcareous soil)and 223.5(alkaline non-calcareous soil)mg/kg,respectively;the HC5 values based on EDTA-Ⅱextractable Zn were 24.9(acidic soil),75.9(neutral soil),149.2(alkaline calcareous soil)and 130.4(alkaline non-calcareous soil)mg/kg,respectively.Soil pH,OC and CEC are the main factors affecting extractable Zn HCx values,controlled>96%of the variation in HCx.The coefficients of pH in HCx-EDTA-I predictive models were higher than those in in HCx-EDTA-Ⅱpredictive models. |
