A pot experiment is conducted to evaluate the bioavailability and toxicity of free Cd ion .The biomass and the Cd absorption of tomato shoot and root are studied.In this study, Cd2 + concentration in soil solution is measured by a modified soil column -Donnam membrance technique.In addition, WinHumicV is used to calculate Cd 2 + concentration in soil solution and the calculated free Cd concentration is compared with the measure.The results show that the calculated Cd2 + concentrations are only slightly higher than the measured values, and free Cd ion is the dominant species in soil solution for all Cd treatments .Its propor-tion in total Cd is nearly constant (1%).Conclusion shows that the Cd uptake and tomato growth endpoints correlate very well with Cd2 + concentration(R2 >0.95).% 采用盆栽实验，研究番茄地上部和根部的生物量及其对镉的吸收量与土壤溶液中自由态镉离子浓度的关系，探索土壤溶液中自由态镉的生物有效性和毒性.在本研究中，土壤自由态镉浓度采用改进的土柱-道南膜技术进行测定，并利用化学平衡模型 WinHumicV 对镉浓度进行验证.结果表明：测定的自由态镉与模型计算结果具有较高的一致性，占土壤总镉比重约1%的自由态镉是土壤溶液中具有生物有效性镉的主要形态.结论：番茄的生物量及其对镉的吸收量都与土壤溶液中自由态镉离子浓度具有很好的相关性(R2>0.95).
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