Abstract: 【Objective】With the continuous requirements for the management of contaminated soil, intensive studies on the ecological risk assessment of contaminated soil are increasingly important. At present, the toxicity test based on species sensitivity distribution for ecological risk assessment of regional contaminated soil has been becoming more and more important compared with the toxicity test based on single species. The toxicity assay should include not only the test for throughout the life cycle, but also the sensitivity of different test endpoints. Toxicity thresholds of Zn determined with different test endpoints are important for evaluation of the environmental risk of Zn in polluted soils. Root ecology plays an important role in the ecological risk assessments and the derivation of toxicity thresholds of heavy metals in polluted soils. The Zn toxicity thresholds (EC_x, x=10, 50) of barley as determined by total root length, number of root tips, root surface area and the average diameter of barley root and the quantitative relationships between the EC_x and soil properties were studied with the aim to provide basic data for environmental risk evaluation of Zn polluted soils. 【Method】 Eight kinds of field soils with various properties were collected around China, Zn (ZnCl₂) was applied and the treated soils were used in a pot experiment after 4 weeks of incubation. Different micro morphological endpoints of barley roots were measured with root scanner (STD1600 Epson), the Zn toxicity thresholds (EC_x, x=10, 50) of barley were determined based on the dose-responsive curve with Log-logistic model, the prediction models of Zn toxicity thresholds were developed based on the total Zn concentrations and soil properties (e.g. pH, CEC and org-C etc).【Result】The results indicated that toxicity thresholds of Zn varied significantly among the different micro morphological endpoints of barley roots. The average EC₁₀ and EC₅₀ determined with total root length, number of root tips, root surface area and the average diameter of the root were 228, 295, 335, 261 mg·kg^-1 and 702, 779, 837, 739 mg·kg^-1, respectively. The smallest EC_x were observed with the test of number of root tips while the maximum EC_x were observed with endpoint of root surface area, which indicated that the growth of root tips is the most sensitive endpoint among the root micro morphological endpoints of barley when it is stressed by Zn pollution in soil. Meanwhile, the variation coefficient of EC₁₀ (43.4%) was generally higher than that of EC₅₀ (21.6%). A positive correlation was found between EC_x (x=10, 50) and the background Zn contents, soil properties (pH, OC contents), and correlation coefficient of pH between EC_x reached was very significant (P＜0.01).【Conclusion】It was found that the growth of root tips is more sensitive than other root micro morphological endpoints when stressed by Zn pollution in soil. The toxicity thresholds of Zn to plant can be predicted very well based on soil pH, OC, and soil Zn background concentration.

Key words: Zn, toxicity threshold, barley, root micro morphology, predict models