The effect of natural products used as pesticides on the soil microbiota: OECD 216 nitrogen transformation test fails to identify effects that were detected via q-PCR microbial abundance measurement.

BACKGROUND: Natural products present an environmentally attractive alternative to synthetic pesticides which have been implicated in the off-target effect. Currently, the assessment of pesticide toxicity on soil microorganisms relies on the OECD 216 N transformation assay (OECD stands for the Organisation Economic Co-operation and Development, which is a key international standard-setting organisation). We tested the hypotheses that (i) the OECD 216 assay fails to identify unacceptable effects of pesticides on soil microbiota compared to more advanced molecular and standardized tests, and (ii) the natural products tested (dihydrochalcone, isoflavone, aliphatic phenol, and spinosad) are less toxic to soil microbiota compared to a synthetic pesticide compound (3,5-dichloraniline). We determined the following in three different soils: (i) ammonium (NH4 +) and nitrate (NO3 -) soil concentrations, as dictated by the OECD 216 test, and (ii) the abundance of phylogenetically (bacteria and fungi) and functionally distinct microbial groups [ammonia-oxidizing archaea (AOA) and bacteria (AOB)] using quantitative polymerase chain reaction (q-PCR). RESULTS: All pesticides tested exhibited limited persistence, with spinosad demonstrating the highest persistence. None of the pesticides tested showed clear dose-dependent effects on NH4 + and NO3 - levels and the observed effects were <25% of the control, suggesting no unacceptable impacts on soil microorganisms. In contrast, q-PCR measurements revealed (i) distinct negative effects on the abundance of total bacteria and fungi, which were though limited to one of the studied soils, and (ii) a significant reduction in the abundance of both AOA and AOB across soils. This reduction was attributed to both natural products and 3,5-dichloraniline. CONCLUSION: Our findings strongly advocate for a revision of the current regulatory framework regarding the toxicity of pesticides to soil microbiota, which should integrate advanced and well-standardized tools. © 2024 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.