Applications of fipronil (Adonis 3UL) and Metarhizium acridum for use against locusts have minimal effect on litter decomposition and microbial functional diversity in Australian arid grassland

Litter and microbes are key drivers of nutrient cycles, particularly in arid ecosystems where decomposition rates are low. Locust control in arid regions represents a potentially important hazard to microbes, because local taxa are unlikely to have adapted to pesticide exposure and operations often occur during times of high microbial activity. We monitored the response of aboveground litter decomposition and soil bacteria functional diversity to aerial applications of fipronil (a chemical pesticide) barrier treatments and Metarhizium acridum (a fungal biopesticide) blanket treatments. Decomposition was monitored over 2 years (before and after treatments) using a replicated litter-bag experiment, whereas changes in bacteria functional diversity were measured over 1 month. Analysis of litter mass loss indicated there were no pesticide treatment effects relative to control. Less litter decomposed in small than large mesh bags, and less litter decomposed during the second year of the study. Litter had higher mean nitrogen (N) and carbon (C), and a lower C:N ratio, during the first year of the study. In contrast, within-treatment site analysis revealed a significant increase in litter mass remaining in bags at M. acridum-treated subsites. However, these values were only 4% different from control sites, suggesting that the effect detected may not be biologically significant. There appeared to be no pesticide treatment effect on bacterial community functional diversity and no significant temporal variation. The lack of large-scale pesticide treatment effects suggests that arid zone fungi and bacteria are resilient to such disturbances. Differences in decomposition was explained by differences in the activity of arthropods and in the shade provided by the two mesh sizes, and an annual decline could be attributed to lower litter C and N content and lower annual rainfall in Year 2. Results show the temporal variation possible in decomposition and microbe community measures in arid systems.