The effect of clay type on the toxicity of carbendazim and imidacloprid to the earthworm Eisenia andrei in artificial soils

In Europe, the risk assessment of pesticides to soils organisms is based on standardized laboratory toxicity tests using artificial soil containing kaolin clay. However, kaolin is not the most representative clay type for European agricultural soils, and its use may affect the bioavailability and toxicity of pesticides, potentially leading to an underestimation of the actual risk to soil organisms. In this study, reproduction toxicity tests with the earthworm Eisenia andrei following OECD guideline 222 were performed in artificial soils prepared with kaolin or bentonite clay, using the pesticides carbendazim and imidacloprid. The results showed that the OECD guideline 222 quality criteria could be met in soils prepared with bentonite clay. EC₅₀ reproduction values (and 95% CIs) in soils prepared with kaolin and bentonite clay were 1.80 (1.02-2.57) and 4.19 (-10.4-18.8) mg kg^-1 for carbendazim, and 0.71 (0.06-1.36) and 2.27 (-0.26-4.80) mg kg^-1 for imidacloprid. For both pesticides, toxicity (LCx, ECx biomass, ECx reproduction) was higher in soils prepared with kaolin clay, although the differences were not always statistically significant. Differences in toxicity between the soils were likely due to a combination of the bentonite's larger interlayer distance, providing space for the pesticides to enter in between the clay sheets, and the higher cation exchange capacity (7.30 and 22.8 cmol_c kg^-1 for kaolin and bentonite soil, respectively) leading to increased pesticide sorption. Overall, these findings suggest that kaolin is a suitable clay type for standardized artificial soil, as it exhibited the highest toxicity, and thus provided a "worst-case" scenario.