PE microplastics altered microbial resource limitation and C/N use efficiency in cotton rhizosphere soil

Polyethylene microplastics (PE MPs) increasingly influence soil ecosystem functions, but we lack mechanistic understanding of their effects on rhizosphere microbial metabolism and resource partitioning. We examined how PE MPs (varying in size and concentration) alter C and N cycling, enzymes, and microbial traits in cotton rhizosphere soils. The results demonstrated that PE MPs disrupted soil C:N stoichiometry: 2 ± 0.3 mm/2 % treatments increased SOC/DOC (12.6 %/20.0 %). They also reduced STN, NH⁺₄, and NO^-₃ availability, causing C:N and C:P imbalances. Microbial metabolism was predominantly N limited. While low PE concentrations intensified N limitation by sequestering available N, high concentrations mitigated C limitation through C release. MPs particle size significantly regulated carbon use efficiency (CUE) and nitrogen use efficiency (NUE). Smaller MPs enhanced CUE but suppressed β-glucosidase (BG) activity and denitrification gene expression, thereby reducing NUE. In contrast, larger MPs decreased microbial biomass (MBC/MBN) through physical obstruction. Structural Equation Modeling identified soil physical traits (pH, EC), extracellular enzymes (BG, AKP), and microbial diversity as pivotal determinants of metabolic efficiency. MPs reconfigured the tripartite "resource, enzyme, and community" network in a size and concentration dependent manner. Milliscale MPs altered the C/N nutrient dynamics in the rhizosphere soil, while microscale MPs affected the C/N metabolism of rhizosphere soil microorganisms. This study uncovers dual mechanisms of MPs in regulating rhizosphere metabolic efficiency by altering soil stoichiometric balance and modulating microbial functional gene expression, providing a theoretical basis for ecological risk assessment of agricultural microplastics and the development of precision management strategies.