Biodegradable polylactic acid microplastics pose greater field nitrous oxide emission risks than conventional polyethylene by reshaping microbial production-reduction pathways in a wheat-maize rotation system

Microplastic contamination in agroecosystems is increasing, yet the field-scale effects of biodegradable microplastics on soil nitrous oxide (N₂O) emissions remain unclear. We conducted a wheat-maize rotation field experiment to compare conventional polyethylene microplastics (PE-MPs) and biodegradable polylactic acid microplastics (PLA-MPs) at three application levels. Measurements included N₂O fluxes, soil physicochemical properties, N-transformation enzyme activities, potential nitrification and denitrification rates, functional gene abundances, N₂O/(N₂O+N₂) product ratios, 15N-18O-based N₂O production pathways. Two-way ANOVA showed that microplastic type was the most consistent determinant of key responses, whereas concentration and interaction effects varied among indicators and seasons. PE-MPs caused weak and variable changes in soil N cycling and cumulative N₂O emissions. In contrast, PLA-MPs increased cumulative N₂O emissions by 38.9-61.1%, increased dissolved organic carbon, lowered pH, suppressed ammonia monooxygenase activity and nitrification, and stimulated nitrite reductase activity, potential denitrification, and nirK/nirS abundances. Increased N₂O/(N₂O+N₂) and (nirK+nirS)/nosZ ratios indicated a stronger imbalance between N₂O production and nosZ-mediated reduction. Dual-isotope tracing further showed that microplastics reduced nitrifier nitrification and shifted N₂O production toward nitrifier denitrification, coupled nitrification-denitrification, and heterotrophic denitrification, with the strongest shift under PLA-MPs. These results demonstrate PLA-MPs pose greater N₂O emission risks than PE-MPs, biodegradability does not necessarily imply environmental safety.