Ecological Threshold For Cadmium In Soils Based On Earthworm Bio Toxicity

In the present study, the acute toxicity and chronic toxicity of cadmium to Earthworm（E. fetida） in 18 Cd spiked soils was investigated by employing the methods of Organization for Economic Co-operation and Development（OECD）.The toxicity thresholds of Cd determined were based on the different test ends（LCx and ECx）. The quantitative relationship between the toxicity thresholds of Cd and the soil physic-chemical properties was studied to establish the Cd toxicity threshold predictive model based on E. fetida biological toxicity.（1） Acute toxicity test results showed that the soil type has great influence on the acute toxicity of Cd. The toxicity of cadmium in red soil of Yingtan is the strongest, and in yellow brown soil of Guiyang is the lowest. The toxicity threshold of Cd for ₅₀% lethal concentrations(LC₅₀) of E. Fetida varied from 440.7 to 1520.4 mg/kg. Stepwise multiple regressions are carried out to determine the relative contributions of soil properties to cadmium LC₅₀. Significant positive correlations were observed between soil pH, organic matter content and cadmium LC₅₀. The accumulation of Cd in E. fetida and available Cd in different types of soils were also studied, significant positive correlations were observed between the two. By influencing the available Cd, soil physico-chemical properties impacted on the bioavailability and toxicity of cadmium to E. fetida.（2） Chronic toxicity test results showed that E. fetida growth development（weight variety） was less sensitive to Cd, and the obvious inhibition effect on earthworm weight only occurred when the concentration of cadmium in soil to reach a higher level（200 mg/kg）. Meanwhile, the cocoon production volume reflecting E. fetida reproductive testing end was very sensitive to Cd. The cocoon production volume differed in different soils, and the toxicity threshold of Cd for ₅₀% of maximal inhibition effect(EC₅₀) of cocoon production of E. fetida varied from 66.5 to 263.5 mg/kg. Stepwise multiple regressions are carried out to determine the relative contributions of soil properties to cadmium toxicity threshold EC₅₀. Significant positive correlations were observed between soil pH, organic matter content and cadmium EC₅₀, and significant negative correlation was observed between clay content and cadmium EC₅₀. The accumulation of cadmium in E. fetida in different types of soils was also studied, the bioavailability of Cd in higher pH, higher soil organic matter was much lower, which was in accordance with the results of the inhibition of cocoon production experiments.（3） The influences of organic matter（low molecular organic acids, for example） as one of important environmental characteristics on the prediction model of ecological threshold of soil cadmium was studied in simulated soil solution, the main objective of this study is to explore the effects of organic matter on the toxicity and biological validity of Cd to E. fetida, and to elucidate the possible influence mechanism. The results showed that Organic matter and heavy metal cadmium ion formed organic metal complexes, which reduced the free metal cadmium ion in soil solution and then reduced the toxicity of heavy metal cadmium. The detoxifying capability depend on the complexing constant of organic metal complexes, the bigger the complexing constant is, the lower the toxicity of cadmium is.In this paper the results of the study could provide the basis for evaluating the ecological risk of soil Cd and establishing ecological threshold of soil Cd.