Pesticide effects on nitrogen cycle related microbial functions and community structure

The extensive application of pesticides in agriculture raises concerns about their potential negative impact on soil microorganisms, being the key drivers of nutrient cycling. We investigated the effect of 20 commercial pesticides, applied at their recommended dose and five times their recommended dose, on soil microbiota, with a particular focus on microbial nitrogen (N) cycling in three different agricultural soils from southern Australian. Functional effects were determined by measuring soil enzymatic activities, potential nitrification (PN), and the abundance of functional genes involved in N cycling (amoA and nifH). Effects on the diversity of nitrifiers were determined with amplicon sequencing. Overall, the effect of pesticides on N microbial cycling was dose-independent and soil specific. The fungicides flutriafol and azoxystrobin, the herbicide chlorsulfuron and the insecticide fipronil induced a significant reduction in PN and beta-1,4-N-acetylglucosaminidase activity (P < 0.05) (NAG) in the alkaline loam soil with low organic carbon content i.e. a soil with properties which typically favors pesticide bioavailability and therefore potential toxicity. In the microbial community level, significant increment was shown on the abundance of the most dominant Nitrososphaeraceae (ammonia-oxidizing archaea; AOA) whereas an overall reduction was observed on the less dominant AOA and other nitrifiers (ammonia oxidizing bacteria; AOB and nitrite oxidizing bacteria; NOB). The inhibiting effects were more evident in the soil samples treated with fungicides. Our findings provide crucial evidence for the sensitivity of nitrifiers to pesticides, reinforcing their potential use as indicators of pesticide microbial ecotoxicity.