Abstract: Six typical China soils with different properties were selected and added with seven concentrations of ZnCl₂ to study the effects of different aging time (14, 90， 180, 360, and 540 days) on the form transformation and eco-toxicity threshold (EC_x) of added Zn in the soils, with the main affecting factors analyzed. The results indicated that with the increase of aging time, the fraction of 0.01 mol·L^-1 CaCl₂extracted Zn in the soils decreased sharply initially, then slowed down, and reached the dynamic balance after 540 d incubation. The ecotoxicity thresholds (EC_x, x=10, 50) of Zn to bok choy increased significantly with aging time (P<0.05), which implied the marked decrease of the phytotoxicity of Zn. The measured aging factors AF₁₀ and AF₅₀ of Zn ranged from 1.077-1.743 and 1.174-1.441, respectively, and increased with aging time. The balanced concentration of Zn in the soils was significantly negatively correlated with soil pH, CEC, and organic carbon (Org-C) content, and soil pH was the most important controlling factor, followed by CEC and Org-C. It took shorter time to reach Zn balance in the soils with higher pH. The prediction model of the EC_x of Zn was developed based on the aging factors and the main soil properties, and could be well validated by the measured EC_x under field condition. This study would provide theoretical basis for the normalization of the eco-toxicity thresholds of added Zn in different soils and the formulation of the environmental criterion of Zn in China soils.

Key words: zinc, aging factor, form, eco-toxicity threshold, soil property.