Using species sensitivity distributions to assess the toxicity of pesticides on nitrifiers and broader microbial groups: From single-species tests to soil amplicon sequencing

Soil microorganisms are essential for ecosystem functioning, yet their responses to pesticides are inadequately captured by conventional ecotoxicological tests. Recent studies identify ammonia-oxidizing microorganisms (AOM) as soil microbial indicators of chemical stress. Building on previous single-species tests with AOM, we investigated the effects of pyraclostrobin, glyphosate and metsulfuron-methyl on nitrification. The more consistent impact of metsulfuron-methyl led us to focus on its effects on the abundance and diversity of AOM and broader microbial groups. An integrated approach combining amplicon sequencing with species sensitivity distributions (SSDs) was applied to derive thresholds relevant for risk assessment. Metsulfuron-methyl showed negative effects on AOM and prokaryotes, while fungi were more tolerant. SSDs based on single-species AOM data and soil microcosm amplicon sequencing yielded hazard concentration 5% (HC 5 ) values of 0.001 and 0.06 mg kg -1 , respectively, verifying the conservative nature of the former. SSDs constructed from amplicon sequencing provided HC 5 values of 0.042 and 0.1 mg kg -1 for prokaryotes and fungi, respectively. Calculated predicted no-effect concentrations (PNEC) suggested a potential risk only for AOM when based on single-species data, whereas no risk was indicated using soil-derived data. These findings support the complementary use of SSDs derived from single-species and soil-based approaches in pesticide risk assessment for soil microbes.