【Objective】Plastic film mulching and nitrogen fertilization are widely adopted in agriculture; yet the interactive effects of biodegradable microplastics and nitrogen on soil microbial composition and ecological functions in farmland soils remain unclear. 【Methods】Four treatments were established: control (CK), urea addition (N), urea combined with polylactic acid microplastics (NPL), and urea combined with polybutylene adipate terephthalate microplastics (NPB). Soil physicochemical properties and microbial characteristics were analyzed to evaluate the combined effects of biodegradable microplastics and nitrogen fertilizer on carbon and nitrogen transformation processes as well as soil ecological functions. 【Results】The NPL and NPB treatments increased soil dissolved organic carbon content (DOC) and mitigated the pH decline induced by urea. Variations in physicochemical properties among treatments drove shifts in bacterial community structure, with NPL and NPB exhibiting similar influences on microbial community composition. Urea and microplastic amendments selectively enriched taxa involved in nitrogen transformation (phylum Nitrospirota) and organic matter decomposition (genera Arthrobacter and Rhodanobacter). Moreover, NPL and NPB treatments altered the abundance of genes associated with carbon and nitrogen metabolism. Key functional genes related to denitrification (nirS, nosZ), carbon fixation (rbcL), and methanogenesis (mcr) were upregulated. In contrast, genes associated with nitrification (amoA, amoB, amoC, hao) and methane oxidation (pmoA, mmoX) were significantly suppressed. The soil multifunctionality index (SMI) was lower in the N, NPL, and NPB treatments compared to CK. Enzyme activities and the abundance of carbon- and nitrogen-cycling genes exerted positive direct effects on SMI, while changes in soil nutrients availability drove shifts in SMI indirectly by regulating enzyme activities and the abundance of functional genes involved in carbon and nitrogen cycling.【Conclusion】The coexistence of biodegradable microplastics and urea alters soil physicochemical properties and reshapes microbial community composition, thereby influencing soil ecological functions.