Impacts of polyethylene microplastics on soil microbial communities and metabolism in broomcorn millet (Panicum miliaceum L.).

Abstract

Polyethylene (PE) microplastics (MPs) are the predominant MP pollutants in agricultural soil ecosystems. To investigate the underlying mechanisms associated with high-load exposure, a controlled pot experiment was conducted to evaluate the effects of pristine PE MPs (1%, 2%, and 3% w/w, ∼100 μm) on soil physicochemical properties, enzyme activities, bacterial communities, and metabolomic profiles in broomcorn millet-planted (P) and unplanted (NP) soils. The results showed that PE treatment significantly increased soil organic matter and inorganic nitrogen content in a dose-dependent manner, while inhibiting enzymes involved in carbon cycling. PE treatment significantly increased Shannon diversity in P soil by 4.1%-4.5% (p < 0.05), whereas network analysis revealed that PE treatment reduced overall co-occurrence connectivity (e.g., the M2 network exhibited the lowest average degree). In P soil, PE treatments (2%-3%) significantly increased the relative abundances of Lysobacter, Sphingomonas, Altererythrobacter, and Flavisolibacter. Metabolomic profiling revealed that PE treatment induced 686 differentially abundant metabolites in NP soil compared to 404 in P soil. PE primarily influenced lipid metabolism in NP soils, whereas carbohydrate-related pathways were more prominent in P soils. Mantel tests and structural equation modeling (SEM) analyses suggested that soil enzyme activity was associated with nutrient availability, microbial biomass, bacterial community structure, and specific metabolites. This study elucidated the mechanistic responses of soil metabolism to high-load pristine MP exposure and highlights plant presence as an important contextual factor for interpreting MP-induced changes.