The influence of soil organic matter content on the toxicity of pesticides to the enchytraeid Enchytraeus crypticus

This study investigated the influence of soil organic matter (OM) content on the toxicity of five pesticides to Enchytraeus crypticus to assess the suitability of the correction factor (CF) of 2 currently applied to lipophilic (log K_ow > 2) pesticides to correct for differences in bioavailability and toxicity between natural and artificial soil (AS) due to differences in OM content. Toxicity tests were performed following standardized guidelines using AS containing 10%, 5%, and 2.5% peat, and Landwirtschaftliche Untersuchungs-und Forschungsanstalt (LUFA) 2.2 soil (4.5% OM). Likelihood-ratio tests showed that soil type significantly influenced the toxicity of non-lipophilic and lipophilic pesticides, and linear regression analyses revealed that toxicity strongly correlated with soil OM content in AS (R² ≥ 0.96). Pesticide toxicity in LUFA 2.2 soil could not always be accurately predicted based on the OM content. Utilizing the obtained toxicity-OM regression equations, pesticide toxicity in soils containing 10.0% and 5.0% OM were modelled to assess the CF of 2. The differences in model-estimated toxicity between these soils ranged from 1.56 to 1.96 for EC₅₀ values, and between 1.33 and 3.40 for EC₁₀ values. EC₅₀ values were compared with data from a sister paper on Eisenia andrei to identify differences between species. Toxicity ratios were always higher for earthworms (2.08–3.21) than for enchytraeids. This study shows that the CF of 2 suffers from erroneous assumptions concerning lipophilicity, OM content and toxicity. Moreover, it highlights the importance of assessing CFs for new test species before introducing them into European soil environmental risk assessment.