Azadirachtin and trifloxystrobin had no inhibitory effects on key soil microbial functions even at high dose rates

Synthetic pesticides may have non-target effects on soil microorganisms which have been identified as a specific protection goal in pesticide environmental risk assessment. Most studies to date have focused on the effects of synthetic pesticides on soil microorganisms, whereas little is known about the response of the soil microbial community to the so-called low-risk pesticide classes. The main objective of this study was to assess the impact of a botanical (azadirachtin) and a low-dose pesticide (trifloxystrobin), applied as commercial formulations, on soil microbial functions. In a microcosm study, pesticides were applied in soil at increasing dose rates (up to ×100 the recommended dose rate). Their dissipation and the formation of the major transformation product (TP) of trifloxystrobin were determined. Enzymatic activities and the abundance of key functional microbial groups were measured via fluorometric assays, potential nitrification (PNT) and q-PCR. Trifloxystrobin and azadirachtin did not persist in soil at all dose rates with DT50s of 1.1–1.4 and 1.3 days respectively. No pesticide dose-dependent inhibitory effects on the activity of soil enzymes, the abundance of ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA), sulfur-oxidizing bacteria (SOB) and on PNT were seen. Instead the application of both pesticides at all dose rates stimulated PNT. Trifloxystrobin was transformed to trifloxystrobin acid (TFSA), which persisted in soil. However its formation did not correlate with adverse effects on soil microbial functions. We provide first evidence that trifloxystrobin and azadirachtin do not impose unacceptable effects on soil microbial functions even at high dose rates.