Repeated applications of fipronil, propyzamide and flutriafol affect soil microbial functions and community composition: A laboratory-to-field assessment.

Pesticides play an important role in conventional agriculture by controlling pests, weeds, and plant diseases. However, repeated applications of pesticides may have long lasting effects on non-target microorganisms. Most studies have investigated the short-term effects of pesticides on soil microbial communities at the laboratory scale. Here, we assessed the ecotoxicological impact of fipronil (insecticide), propyzamide (herbicide) and flutriafol (fungicide) on (i) soil microbial enzymatic activities, (ii) potential nitrification, (iii) abundance of the fungal and bacterial community and key functional genes (nifH, amoA, chiA, cbhl and phosphatase) and (iii) diversity of bacteria, fungi, ammonia oxidizing bacteria (AOB) and archaea (AOA) after repeated pesticide applications in laboratory and field experiments. Our results showed that repeated applications of propyzamide and flutriafol affected the soil microbial community structure in the field and had significant inhibitory effects on enzymatic activities. The abundances of soil microbiota affected by pesticides recovered to levels similar to the control following a second application, suggesting that they might be able to recover from the pesticide effects. However, the persistent pesticide inhibitory effects on soil enzymatic activities suggests that the ability of the microbial community to cope with the repeated application was not accompanied by functional recovery. Overall, our results suggest that repeated pesticide applications may influence soil health and microbial functionalities and that more information should be collected to inform risk-based policy development.

Impact of twenty pesticides on soil carbon microbial functions and community composition.

Pesticides are known to affect non-targeted soil microorganisms. Still, studies comparing the effect of multiple pesticides on a wide range of microbial endpoints associated with carbon cycling are scarce. Here, we employed fluorescence enzymatic assay and real-time PCR to evaluate the effect of 20 commercial pesticides, applied at their recommended dose and five times their recommended dose, on soil carbon cycling related enzymatic activities (α-1,4-glucosidase, ß-1,4-glucosidase, ß-d-cellobiohydrolase and ß-xylosidase), and on the absolute abundance of functional genes (cbhl and chiA), in three different South Australian agricultural soils. The effects on cellulolytic and chitinolytic microorganisms, and the total microbial community composition were determined using shotgun metagenomic sequencing in selected pesticide-treated and untreated samples. The application of insecticides significantly increased the cbhl and chiA genes absolute abundance in the acidic soil. At the community level, insecticide fipronil had the greatest stimulating effect on cellulolytic and chitinolytic microorganisms, followed by fungicide metalaxyl-M and insecticide imidacloprid. A shift towards a fungal dominated microbial community was observed in metalaxyl-M treated soil. Overall, our results suggest that the application of pesticides might affect the soil carbon cycle and may disrupt the formation of soil organic matter and structure stabilisation.

Pesticide effects on nitrogen cycle related microbial functions and community composition.

The extensive application of pesticides in agriculture raises concerns about their potential negative impact on soil microorganisms, being the key drivers of nutrient cycling. Most studies have investigated the effect of a single pesticide on a nutrient cycling in single soil type. We, for the first time, investigated the effect of 20 commercial pesticides with different mode of actions, applied at their recommended dose and five times their recommended dose, on nitrogen (N) microbial cycling in three different agricultural soils from southern Australian. Functional effects were determined by measuring soil enzymatic activities of ß-1,4-N-acetyliglucosaminidase (NAG) and l-leucine aminopeptidase (LAP), potential nitrification (PN), and the abundance of functional genes involved in N cycling (amoA and nifH). Effects on nitrifiers diversity were determined with amplicon sequencing. Overall, the pesticides effect 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 ß-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. For the nitrifier community, the greatest pesticide effects were on the most dominant Nitrososphaeraceae (ammonia-oxidizing archaea; AOA) whose abundance increased significantly compared to the less dominant AOA and other nitrifiers. The inhibiting effects were more evident in the soil samples treated with fungicides. By testing multiple pesticides in a single study, our findings provide crucial information that can be used for pesticide hazard assessment.