Study Of Zn-toxicity Thresholds In Soils With Different Bioassay Endpoints, Its Predictive Models And Field Validation

Soil Zn pollution ecological risk has received extensive attention, and the lack of research onecological risk threshold of Zn has become the bottleneck of the prevention of soil Zn pollution inChina.Sixteen different soils, representative of pH and organic carbon distribution regularity of Chinesesoils, were selected and sampled for extraneous Zn toxicity bioassays. The samples were amended withZnCl₂to obtain a range of eight concentrations. The toxicity thresholds （EC_x） of Zn determined usingthe dose-response curves based on four different toxicity bioassay endpoints such as potentialnitrification rate （PNR）, the barley root elongation, tomato and bok choy growth toxicity etc. Thequantitative relationship between the ECxand the soil physic-chemical properties was studied toestablish the Zn toxicity threshold predictive model. Then the effects of aging time on the speciationtransformation and toxicity thresholds of added Zn in the soils were investigated. The predictedZn-toxicity model was developed based on the aging factors and the main soil properties, and it couldused to validate ECxmeasured under laboratory condition. Collected Zn toxicity data of Chinese soilsused SSD （Species Sensitivity Distribution） to obtain ecological thresholds （hazardous contrations ofprotecting5%species）. In the procedure of deriving the toxicity threshold predictive models and theaging factors were utilized to normalize toxicity data. The predictive models of zinc ecologicalthresholds based on soil properties were obtained. The quantitative equation of total predicted no effectconcentration （PNECtotal） based on total Zn were developed in order to provide the recommendation forupdating current Chinese soil environmental quality standard.The toxicity thresholds ECxbased on PNR, barley root elongation, bok choy and tomato growthvaried significantly. In general, the sensitivity of the bioassay endpoints follows the order: PNR>tomato growth> bok choy growth> the barley root elongation, while the stability order reversed. ThepH was the most important factor affecting the Zn toxicity threshold in soils and the developedregression model based on pH、CEC and organic carbon contents of soil can well predict the Zn toxicitythreshold.The effects of aging time （14,180,360,450and540days） on the speciation transformation andtoxicity thresholds （EC_x） of added Zn in the soils were investigated in this study, the main affectingfactors were also determined. The measured aging factors （AF_s）, AF₁₀and AF₅₀ranged from1.077¹.743and1.174¹.441, respectively, which increased with the aging time. The aging of Zn in thesoils was positively correlated with pH, CEC and organic carbon （OC） with soil pH being the mostsignificant controlling factor, followed by CEC and organic carbon. The predicted Zn-toxicity modelwas developed based on the aging factors and main soil properties, and it could be well validated themeasured ECxunder laboratory condition.In order to validate the toxicity thresholds of Zn determined in lab condition, two sites wereselected for tests under field condition: the acid red soil located in Qiyang, Hunan province and theneutral paddy soil located in Jiaxing, Zhejiang province. The field validation result showed the coincidences between the toxicity thresholds of Zn obtained in lab and field conditions varied indifferent tested soils, the closer EC10and EC50were observed in the paddy soil than those in the red soil.The predictive models of Zn toxicity thresholds established based on the lab test data can predict wellthe toxicity values of Zn for rice in paddy soil in field condition, the determined toxicity thresholds ofZn in the paddy soil in field condition fall in the two folds of the prediction interval, however, in termsof acid red soil, the predictive model underestimated the ecological risk of Zn in the soil.Fitting21species of Zn toxicological data from Chinese soil after normalization with BurrIII, tocreate SSD distribution curves in different soil conditions. The overall sensitivity order of species wasnot influenced by soil properties: phosphatase, lettuce, rice and ryegrass were less sensitive to Zntoxicity than cucumber, romaine and PNR. The soil pH and OC had significant impacts on thedistributions of Zn SSD curves and the SSD curves moved to the direction of higher concentration withpH or OC increasing. The impact of CEC on the SSD curves distribution could be negligible. The5%hazardous concentration for Zn was derived from the SSD curves and the aged HC5, the ecologicalthresholds for Zn in soil were obtained by correcting the HC5values with aging factors. The two-factorpredictive model based on soil pH and OC can well predict the ecological threshold （HC5） for Zn. Thequantitative equations of predicted no effect concentration （PNECtotal） based on total Zn were obtainedbased on ecological threshold models. The PNECtotalfor Zn calculated with the equations were morespecific and more scientific than the current soil quality standard for Zn since the effects of soilproperties on Zn toxicity were taken into account.The results of this research provide the basis for the evaluation of soil Zn in China ecological riskand revision of environmental quality standards.