Zinc Phytotoxicity As Affected By Soil Properties And Development Of A Predictive Model

In recent years, the situation of soil zinc contamination is serious. However, the soilenvironmental quality standards implemented in China can not fulfill the needs of practicalapplication. In view of this, the present study including:1) The dose-response curves and the species sensitivity distributions (SSD) of added Znin two soils to8different plant species were investigated using pot experiment. The thresholdvalues of phytotoxicity of zinc to plant species in soils were analyzed using Log-logisticdistribution model and the species sensitivity distribution was performed with BurrlizШmodel.The growth of plants was used as endpoints to assess the phytotoxicity of Zinc addedto soils.2) According to the distribution of soil pH and organic matter, the specimens of topsoilwere collected from16sites in China. The samples were treated by leaching after ZnCl2saltwas spiked into soils, and Barley root elongation, the growth of tomato and pakchoi wereused as endpoints. The threshold value of added Zn concentration was gained from alog-logistic curve. Main controlling factors in soil and the relationship between the thresholdvalue of Zn and soil properties were determined. Finally, the models to predict thephytotoxicity of Zn added to soils to the plants were developed.The results indicated that(1) The hormesis occurred on the tested plant species when zinc were added with lowlevel (<100mg kg-1) in the soils, however, the toxicity of zinc to plants were observed whenadded with high levels. Significant differences of threshold values (ECx) of zinc to plantspecies were observed due to the variety of plant species and the soil properties. Theaccumulated frequency of threshold values of zinc species varied significantly among theplant species, in general, leaf vegetables (i.e. the Chinese cabbage and mustard, etc) weremore sensitive to the toxicity of zinc in soils than plants of the grass family (e.g. the maize),no relationship of the species sensitivity distributions with soil properties were observed inthis study.(2)It has a good dose-response curve between zinc concentration added to soils andbiomass of plants in16tested soils. hormesis effect was found in this test, it is that biomass of plants is greater than the control in low concentrations.The value of EC50in barley rootelongation test ranged from534to4723mg·kg-1in unleached soils and from302to4520mg·kg-1in leached soils, respectively, which represented8.8-and15.0-fold variation amongsoils. Based on the predictable models of barley root elongation as follows: Log EC50=0.241+0.224pH+0.627OC+0.502CEC(R2=0.75, unleached）, Log EC50=1.874+0.431pH+0.614OC+0.135CEC（R2=0.82, leached）.(3) The value of EC50in the dry weight of tomato ranged from450to1320mg·kg-1inunleached soils and from293to1820mg·kg-1in leached soils, respectively, whichrepresented2.9-and6.2-fold variation among soils. The predictable models of the dry weightof tomato: Log EC50=2.054+0.403pH+0.432OC+0.509CEC (R2=0.70, unleached）, LogEC50=3.482+0.135pH+0.464OC+0.138CEC（R2=0.72, leached）.(4) The value of EC50in the dry weight of pakchoi ranged from417to1042mg·kg-1inunleached soils and from319to990mg·kg-1in leached soils, respectively, which represented2.5-and3.1-fold variation among soils.The predictable models of the dry weight of tomato:Log EC50=4.132+0.160pH+0.632OC+0.566CEC (R2=0.76, unleached）, LogEC50=3.917+0.636pH+0.486OC+0.375CEC (R2=0.76, leached).(5) The results showed that soil pH was a main factor controlling the toxicity of Zn insoils; soil organic carbon content and cation exchange capacity was the second factors.Compared with unleached soil, the leached soil can significantly reduce the toxicity of zinc,especially soil’s pH>8(except Langfang). So leaching should be considered in the riskevaluation of Zn in soil. Furthermore, with soil pH raised, the value of distribution coefficient(Kd) increased. The soil pH was also a main factor controlling the value of Kd.In conclusion, the test provides lots of based research data and experience for the studyof ecological risk assessment of zinc in soil and supplies a scientific basis in the formulationof environmental quality standards.