Purpose <p>The continuous accumulation of microplastics (MPs) in soil poses a significant threat to the nitrogen cycle and associated ecosystem functions. This study aimed to investigate and compare the effects of three prevalent MPs—polyethylene (PE), polylactic acid (PLA), and tire wear particles (TWPs)—on soil nitrogen dynamics, key enzyme activities, and the structure of microbial communities.</p> Methods <p>A soil incubation experiment was conducted by amending soil with PE, PLA, or TWPs at concentrations of 0.1% and 5% (w/w). Over 35 days, changes in soil physicochemical properties (e.g., bulk density, pH, DOC), concentrations of ammonium (NH₄⁺-N) and nitrate (NO₃^--N), activities of urease and nitrate reductase, and bacterial community composition (via 16&#xa0;S rRNA gene sequencing) were analyzed.</p> Results <p>High-concentration MPs (5%) exerted a stronger inhibitory effect on soil NH₄⁺-N, NO₃^--N, and available nitrogen (AN), concurrently suppressing nitrification rates while enhancing the activities of nitrogen-transforming enzymes. The magnitude of the response varied among microplastic types, with PLA generally exhibiting a stronger response pattern under the tested conditions. The addition of MPs significantly reduced bacterial alpha-diversity and altered community structure, with TWPs notably increasing the relative abundance of Proteobacteria. Both PLA and TWPs enriched bacterial taxa putatively associated with nitrogen fixation, nitrification, and denitrification.</p> Conclusion <p>MPs, particularly biodegradable PLA under the tested conditions, were associated with marked changes in soil nitrogen availability and transformation processes. This disruption is mediated through alterations in soil physicochemical properties, modulation of key enzyme activities, and shifts in the structure and function of the soil microbial community. These findings underscore the potential risk of MPs pollution to soil nitrogen cycling and ecosystem health.</p> Graphical Abstract <p></p>