Using amplicon sequencing in an ecotoxicological context: Applying species sensitivity distributions to assess the toxicity of biopesticides and a synthetic pesticide on soil microbiota

Biopesticides are gaining popularity due to their perceived environmental safety. However, their natural biocidal properties may still pose risks to soil microbial communities. Currently assessment of the toxicity of pesticides on soil microorganisms relies on the OECD 216 N mineralization test, while no tests are available to detect effects on soil microbial diversity. Amplicon sequencing provides a high-resolution analysis of soil microbial diversity, but it has not been used in an ecotoxicological context. We hypothesized that selected biopesticides (dihydrochalcone, isoflavone, aliphatic phenol, spinosad, pyrethrins) would exert lower adverse effects on bacterial, fungal, and protist communities than the synthetic compound chlorothalonil. To further quantify these effects, we used the amplicon sequencing data to construct species sensitivity distributions (SSDs) and derive ecotoxicological threshold values (HC₅ - hazard concentration 5 %). All pesticides exhibited low persistence in soil. Chlorothalonil showed the most prominent effects on the bacterial and protist communities, followed by pyrethrins, while the other biopesticides exhibited little effects. SSD-derived HC₅ values for chlorothalonil ranged from 2.8 to 18.6 μg g^-1 for bacteria and 7.9 μg g^-1 for protists, substantially lower than the HC₅ of pyrethrins for protists (43.0 μg g^-1) and dihydrochalcone for bacteria (45.8 μg g^-1). Our findings verified the higher toxicity of chlorothalonil to soil microbial communities compared to biopesticides with their effects adequately captured by amplicon sequencing and quantified by SSDs. We advocate for the potential exploitation of amplicon sequencing data to derive robust ecotoxicological thresholds to inform environmental risk assessment of pesticides.