Plastic-film mulching and microbial inoculant jointly shape soil microbial community assembly during biological greenhouse solar disinfection

Plastic film mulching and microbial inoculants are widely used agronomic practices, yet their interactive effects on soil microbial communities remain unclear. A fully factorial field experiment (2 mulching levels × 5 inoculant treatments including Trichoderma , Bacillus , yeast, composite bioagent, and control; n = 3) was conducted, and bacterial (16S rRNA) and fungal (ITS) communities were characterized by amplicon sequencing. Bacterial and fungal communities responded oppositely to mulching. Mulching reduced bacterial Chao1 richness (−23.39%, p = 0.0029) and Shannon diversity (−8.50%, p = 0.011) uniformly across inoculants, but increased fungal PD (+46.48%, p = 0.0008), Shannon (+44.13%, p < 0.001), and Simpson (+9.68%, p < 0.001) diversity. Inoculant identity determined fungal richness ( p = 0.0070), with Bacillus mulch highest (279.67 ASVs) and yeast mulch lowest (111.67 ASVs). Significant mulching×inoculant interactions occurred for bacterial PD and fungal diversity indices ( p ≤ 0.033). Mulching decreased Actinobacteriota and increased Gemmatimonadota in bacteria, and increased Ascomycota while decreasing Mortierellomycota in fungi. Bacterial β-diversity was structured by mulching ( R 2 = 7.6–24.5%) and inoculant identity ( R 2 = 15.5–29.6%) with no dispersion heterogeneity ( p ≥ 0.10). Fungal β-diversity exhibited significant effects of both factors, but mulching increased within-group dispersion ( p = 0.0001), indicating elevated spatial heterogeneity under plastic film. Plastic mulching imposes divergent selective pressures on soil bacteria and fungi, with inoculant effects contingent on mulching status, highlighting the need for integrated microbial management strategies in mulched soil disinfestation systems.