Abstract: Microplastics (MPs) have attracted increasing attention due to their potential threat to soil ecosystems, while the mechanisms by which MPs affect nitrogen cycling within agricultural soil-microbe systems remain insufficiently understood. In this study, two mass concentrations (0.01% and 0.1%, w/w) of polyethylene (PE) and polylactic acid (PLA) MPs were applied in a 45-day maize (Zea mays L.) pot experiment to systematically evaluate the combined effects of MP type and concentration on maize growth, soil aggregate stability, nitrogen nutrients, microbial community composition, and nitrogen-cycling functions. The results show that the 0.1% PLA treatment significantly reduces maize shoot biomass by 11.7% compared with the control (P<0.05), whereas no significant differences are observed for the other treatments. In addition, both PE and 0.1% PLA treatments lead to a significant reduction in root biomass. All MP treatments markedly decrease the geometric mean diameter (GMD) of soil aggregates, and PLA treatment also significantly reduces the mean weight diameter (MWD). In terms of nitrogen dynamics, MPs do not significantly affect total nitrogen (TN) and ammonium nitrogen (NH₄⁺-N) contents, but 0.1% PE and PLA significantly decrease nitrate nitrogen (NO₃⁻-N) levels. PE and 0.01% PLA significantly reduce bacterial Shannon diversity, leading to an increased abundance of Acidobacteriota and a decline in key taxa such as Nitrospirota. PE significantly suppresses the abundance of denitrification genes such as narG and nosZ, while 0.01% PLA upregulates nitrogen fixation genes nifH and nifD. This study highlights the potential disruptive effects of MPs on plant growth and nitrogen cycling in farmland soils, providing a theoretical basis for ecological risk assessment in agroecosystems.