Effect of Biodegradable PLA-Based and Conventional LDPE Mulch Films on Pathogenic and Functional Soil Microbial Communities

Plastics and microplastics are widespread in the environment, yet knowledge about their impact on agricultural soils, including their microbiological properties, remains limited. Therefore, this study addressed the research question regarding the impact of secondary microplastics, biodegradable poly(lactic acid) (PLA) mulch film, and low-density polyethylene (LDPE) film on the abundance, structure, and functions of soil bacteria, with particular emphasis on the presence of bacterial pathogens. PLA and LDPE were applied to the soil at a dose of 4 g kg−1 d.m. of soil. The aim of the experiment was to evaluate and compare the effectiveness of soil bioaugmentation with the Pseudomonas umsongensis strain and its biostimulation with humic acids in mitigating the negative effects of microplastics. The response of culturable bacteria revealed high sensitivity of organotrophic bacteria to both microplastics, with a stronger inhibitory effect from PLA, as well as stimulation of actinomycetes. 16S rRNA gene amplicon sequencing indicated that the materials differentially influenced the bacterial response. PLA most strongly stimulated Actinobacteriota and favored the dominance of Bacillus and Limnochorda, whereas LDPE promoted the growth of Actinobacteriota and Chloroflexota as well as genera KD4-96 and 1921-2. Both microplastics were colonized by potential pathogens, including Bacillus, Mycobacterium, Ralstonia, and Cupriavidus. PLA additionally stimulated the proliferation of Leifsonia sp. and Curtobacterium sp., while both PLA and LDPE reduced the abundance of Enterobacter sp. and Herbaspirillum sp. Bioaugmentation using the Pseudomonas umsongensis strain was more effective in restoring the balance of the soil microbiome than biostimulation with humic acids. The results indicate that microbial preparations based on Pseudomonas umsongensis may serve as an important tool in restoring the balance of soil exposed to microplastics.