Forty-Eight Years of Maize Monoculture Under Contrasting Tillage Affect Short-Term Microbial Responses to Azoxystrobin and Vermicompost

Azoxystrobin is a widely used strobilurin fungicide, yet how soil microorganisms respond to overexposure in long-term monocultures, and whether organic amendments can mitigate that response, remains poorly understood. We ran a 30-day greenhouse experiment with soil from a maize monoculture maintained since 1975 under conventional tillage (LC) and no-tillage (SD). Azoxystrobin was applied at the recommended (DR1X, 5 mg kg−1) and tenfold (DR10X) doses, with or without vermicompost (VC, 50 g kg−1). Analyses included 16S rRNA gene sequencing, fatty acid methyl ester (FAME) profiling and partial least squares structural equation modeling (PLS-SEM). DR10X significantly reduced the relative abundance of Gram-positive biomarkers in both systems and increased the fungi/bacteria ratio by 50% under LC, without reducing total microbial biomass. VC increased pH, electrical conductivity and, without fungicide, richness, and reversed the DR10X increase in the fungi/bacteria ratio under LC, but Chloroflexi and Planctomycetota reached their lowest relative abundances in VC-amended fungicide treatments under LC. Shannon diversity was unaffected, whereas Pielou’s evenness increased only under LC with DR10X, with or without VC. Soil chemistry significantly predicted FAME biomarkers only under SD; most other PLS-SEM paths were not significant. Microbial responses to azoxystrobin depended on soil management, and VC effects were taxon-specific rather than protective.