Assessing the toxicity of biopesticides and a synthetic pesticide on the soil microbiota using amplicon sequencing and species sensitivity distributions

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 could identify even subtle effects on soil microbial communities. However, it is still not clear how these results could be used in an ecotoxicological context. We tested, in two contrasting soils, the hypothesis that biopesticides (dihydrochalcone, isoflavone, aliphatic phenol, spinosad, and pyrethrins) exert lower adverse effects on the bacterial, fungal and protist communities compared to a model synthetic compound (chlorothalonil). To further quantify these effects, we used the amplicon sequencing data to construct species sensitivity distributions (SSDs), from which we derived ecotoxicological threshold values (HC5 - hazard concentration 5 %). All pesticides exhibited low persistence in the studied soils. Chlorothalonil showed the highest toxicity to bacterial and protist communities, followed by pyrethrins, while the other biopesticides exhibited little effects on the soil microbiota. The effects of chlorothalonil, pyrethrins, and dihydrochalcone were quantified using SSDs. HC5 values of chlorothalonil ranged from 2.6 to 15.2 ng g-1 for bacteria and 3.2 ng g-1 for protists, being lower than the HC5 value of pyrethrins for protists (32.6 ng g-1) and of dihydrochalcone for bacteria (45.8 ng 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.