The influence of soil organic matter content on the toxicity of pesticides to the springtail Folsomia candida.

Abstract

The European environmental risk assessment (ERA) of pesticides to soil invertebrates applies a correction factor (CF) of 2 to endpoints derived from toxicity tests with lipophilic pesticides (log Kow > 2) to correct for differences in organic matter (OM) content between artificial soil (AS) and natural soils. Because the applicability of this CF to springtails has never been assessed, this study investigated the influence of soil OM content on the toxicity of five pesticide active substances differing in lipophilicity to the springtail Folsomia candida. Toxicity tests following Organisation for Economic Cooperation and Development guideline 232 were conducted in AS containing 10%, 5%, and 2.5% peat and a natural soil (LUFA 2.2) with 4.5% OM. For all pesticides, toxicity (median lethal and median effect concentrations [LC50s, EC50s]) differed significantly between soils and strongly negatively correlated with soil OM content in AS (r2 > 0.88). Utilizing the regression equations derived from the data, LC50s and EC50s were calculated for soils with 10% and 5% OM. For EC50s, the differences in model-estimated toxicity between these soils ranged from 1.85 to 3.23, sometimes exceeding the CF of 2. To identify differences between species, data from a sister paper on the earthworm Eisenia andrei was used. Although ratios between model-estimated EC50s in soils containing 10% and 5% OM were similar between species (2.08-3.24 for earthworms), pesticide-specific toxicity-OM relationships differed in some cases. Non-lipophilic pesticides were influenced by soil OM content in a similar manner as the lipophilic pesticides, showing that the influence of soil properties on non-lipophilic pesticides may currently be overlooked. Overall, this study shows that the CF of 2 suffers from erroneous assumptions concerning lipophilicity, OM content, and toxicity. Further research is required to improve our mechanistic understanding of the relationship between toxicity and soil OM content, ultimately increasing the ecological relevance of CFs used in ERAs.