Are botanical pesticides not toxic to non-target organisms: Studying the effects of azadirachtin on soil microbes using advanced culture-independent approaches

Azadirachtin is probably the most popular botanical insecticide registered for use in conventional and organic farming. It is characterized by low toxicity to mammals and low environmental persistence. The biological origin of azadirachtin has created a general perception that is safe for not-target organisms including soil microbes. However this remains to be verified by experimental data. In order to fill this gap, a microcosm study was employed where an agricultural soil was exposed to four dose levels of azadirachtin (x0, x1, x10 and x100 the recommended dose). The impact of the insecticides to soil microbial activity was determined via high throughput enzyme activity screening while diversity effects were determined via Illumina-based assay of the 16S rRNA bacterial gene. In parallel the dissipation of azadricahtin was also determined. The degradation of azadirachtin in soil was quite rapid, with DT50 of 1.3 days at the x100 dose rate. Azadirachtin had a significant positive effect on the activities of betaglucosidase, chitinase, phosphomonoesterase, leucineaminopeptidase and on potential nitrification (PN), with the latter two parameters showing the most significant correlations. Distance based redundancy analysis indicated a small but significant effect of the applied dose rate on microbial activities with dose level, as a main factor, explaining 9.3 % of the total observed variance. This was mainly due to differences between the non-treated (x0) and the treated sample (x1, x10 and x100), with PN contributing mostly to these differences. The latter showed a rapid (4-7 days) positive response to azadirachtin applications at all dose levels. Diversity assessment of the bacterial communities is on the way and the complete dataset will be presented during the conference.