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

Natural products present an environmentally attractive alternative to synthetic pesticides which have been impli- cated in the off-target effect. Currently, the assessment of pesticide toxicity on soil microorganisms relies on the OECD 216 N trans- formation 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, iso- flavone, 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 concentra- tions, 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).