Microplastic composition-dependent effects on N2O emissions driven by changes in soil N process and microbial communities

Biodegradable microplastics (MPs) are increasingly promoted as alternatives to conventional plastics to mitigate agricultural plastic pollution. However, their effects on soil microbial functions and nitrogen (N) cycling remain poorly understood. Here, we investigated the effects of biodegradable (polylactic acid and polybutylene adipate-co-terephthalate; PLA and PBAT) and conventional (polyethylene, PE) MPs on plant growth, nitrous oxide (NO) emissions, and microbial communities in an 80-day coriander (Coriandrum sativum L.) microcosm experiment. The composition of MPs altered microbial α diversity and functional structure in the plastisphere compared to bulk soil, with PBAT inducing the most distinct changes. PBAT-driven microbial shifts extended to the surrounding soil, stimulating NAGase enzyme activity and increasing ammonium availability by 27.1 %, which increased plant biomass. Both biodegradable and conventional MPs (0.05 % w/w) significantly reduced NO emissions (PBAT: 16.5 %, PLA: 19.1 %, PE: 31.5 %) but via different mechanisms: PE increased nosZ abundance, promoting complete denitrification; PLA reduced NO via nirK suppression in a dose-dependent manner (39.7 % at 1 % w/w); PBAT lowered amoA and nirK/S gene abundance inhibiting nitrification and denitrification potential. Unlike prior studies in plant-free soils that observed MP-stimulated denitrification, our plant-soil system showed suppressed nitrification and substrate-limited denitrification. A meta-analysis of 14 plant-soil studies confirmed that MPs reduce NO emissions in vegetated systems by an average of 21.9 %. These findings highlight composition- and dose-dependent effects of MPs on microbial N cycling and gas fluxes, with implications for sustainable agriculture and safe biodegradable plastic design.