Plastic particles driving cadmium mobility and nitrous oxide emissions: Revealing microbial Fe-N interactions in...

Urban wetlands act as pollutant interfaces where microplastics (MPs) and cadmium (Cd) often coexist, however, the underlying biogeochemical mechanisms mediating their interactions remain poorly understood. Here, we investigated the effects of polypropylene MPs (PP-MPs) on Cd mobility and nitrous oxide (N₂O) emissions in flooded wetland soils planted with Acorus calamus using a two-month in-situ soil column experiment. PP-MPs significantly enhanced N₂O emissions by 45 %, accompanied by a 28 % increase in exchangeable Cd fractions, whereas plant Cd uptake decreased by 56 %, indicating that PP-MPs altered Cd bioavailability and cycling. These changes were closely associated with shifts in microbial community composition, particularly the enrichment of iron-reducing and nitrate-dependent Fe (II)-oxidising bacteria that mediate iron-nitrogen (Fe-N) redox coupling processes. Enhanced Fe-N interactions under PP-MPs exposure promoted the formation of Fe-Mn oxide plaques on root surfaces, facilitating Cd immobilisation and reducing its phytoavailability. Moreover, PP-MPs stimulated microbial denitrification by upregulating narG, nirS, and norB while suppressing nosZ, thereby increasing N₂O production and inhibiting its reduction to N₂. These findings demonstrate that PP-MPs modulate Cd speciation and greenhouse gas emissions via Fe-N-coupled microbial pathways, revealing their overlooked functional roles and ecological risks in wetland biogeochemistry.