Plant presence fundamentally alters soil microbial and biogeochemical responses to polylactic acid microplastics:...

Incomplete degradation of polylactic acid (PLA) generates microplastics (MPs) that accumulate in agricultural soils. Although plant-microbe interactions are central to soil biogeochemical processes, their role in shaping soil responses to biodegradable MPs remains poorly understood. Here, we conducted a microcosm experiment to examine the effects of PLA MPs on soil carbon and nitrogen metabolisms in the presence and absence of maize (Zea mays L.). PLA MPs increased soil dissolved organic carbon and NH₄⁺ -N but reduced NO₃^--N across treatments. In unplanted soils, PLA MPs enhanced large macroaggregate formation and aggregate stability, stimulated β-1,4-glucosidase and N-acetyl-β-D-glucosaminidase activities, and enriched bacterial Pseudomonadota and fungal Ascomycota and Basidiomycota. In contrast, these structural, enzymatic, and microbial responses were suppressed or reversed in planted soils. Functional predictions indicated that PLA MPs enhanced microbial carbon and nitrogen metabolic pathways in unplanted soils, but suppressed these processes in planted systems. Structural equation modeling corroborated that plant presence fundamentally restructured the linkages among microbial enzyme activities, functional gene abundances, and soil nutrient pools. Overall, our findings demonstrated that plants act as a key regulator of MPs-driven microbial and biogeochemical responses, highlighting the necessity of incorporating plant-soil interactions when evaluating the risks of biodegradable MPs in agricultural ecosystems.