Abstract: 【Objective】Although the attention has been paid for decades to the ecological risk of Cd to rice in soils, most studies focused on its healthy risk in terms of the food standard instead of its ecological risk and toxicity thresholds in soils. The toxicity thresholds (ECx, x=10, 50) and its predicted models of Cd to rice cultivars in various soils in China were determined using ISO 11269-1 root-elongation endpoints with the aim of providing fundamental data for the revision of soil environmental quality standards of Cd in soils of China.【Method】Eight different soils with various properties and three rice cultivars were selected in this study, the dose-response curves and the toxicity thresholds were investigated using Log-Logistic distribution models based on the ISO 11269-1 root-elongation test in soils, the predicted models for Cd toxicity was also developed in this study. 【Result】The results indicated that the relative root elongation (%) decreased with the Cd concentrations increment in soils. The 10% (EC10) and the half inhibiting concentration (EC50) of Cd to rice cultivars varied significantly among the tested soils and the rice species. In general, the toxicity thresholds (ECx, x=10, 50) of Cd to rice species decreased with it’s sensitiveness to Cd stress in soils, i.e. the ECx followed the order of T167＞L28＞X45. The significant differences of the toxicity thresholds (ECx, x=10, 50) of Cd to rice species were also observed in this study as determined with single rice species, e.g. in terms of the Cd-stress-sensitive cultivar of T167, the 10% inhibiting concentration (EC10) of Cd to root-elongation varied form 1.40-5.25 mg•kg-1 with the maximum variety of 275.0%, the half inhibiting concentration (EC50) varied form 17.83-46.93 mg•kg-1 with the maximum variety of 163.2%, respectively. However, for the Cd-stress-tolerance cultivar of X45, the 10% inhibiting concentration (EC10) of Cd to root-elongation varied form 1.72-8.22 mg•kg-1 with the maximum variety of 377.9%, the EC50 varied form 26.96-68.16 mg•kg-1 with the maximum variety of 152.8%, respectively. The multiple regression analysis showed that there was a significant positive correlation (P＜0.05) between soil pH, organic carbon (OC), cation exchange capacity (CEC) and the toxicity thresholds (ECx, x=10, 50) of Cd to rice cultivars in soils. A predicted model was developed and validated for the toxicity thresholds (ECx, x=10, 50) of Cd to rice with the measured ECx fell within the range of predicted values±2 standard deviations. 【Conclusion】 Significant difference of the toxicity thresholds (ECx, x=10, 50) of Cd to rice cultivars was observed in soils as determined by root-elongation test, and a significant positive correlation (P＜0.05) was also observed between soil pH, OC, CEC and the toxicity thresholds. The ECx of Cd toxicity were affected by the variance of the tested rice cultivars in soils, the ecological toxicity of Cd to different test endpoints should be considered for the future revision of the soil environmental quality standards of Cd in soils of China.

Key words: rice , cadmium , toxicity threshold , predicted model