Estuarine and coastal sediments receive increasing inputs of both reactive nitrogen and microplastics, yet their combined effects on microbial nitrogen cycling and greenhouse gas production remain insufficiently understood. In this study, we investigated how common microplastic polymers, including polyethylene terephthalate, polyethylene, and polypropylene, affect nitrous oxide (N₂O) emissions and nitrifying microbial communities in sediments from the Yangtze River estuary and adjacent coastal areas.Using sediment microcosms, functional gene analysis, amplicon sequencing, and genome-resolved metagenomics, we found that microplastic addition enhanced N₂O production and influenced nitrifying microbial communities. This response was mainly associated with the selective stimulation of ammonia-oxidizing bacteria, particularly Nitrosomonas-like populations, rather than ammonia-oxidizing archaea or comammox Nitrospira. Genome-resolved analysis further suggested that enriched ammonia-oxidizing bacteria carry putative genes related to plastic interaction or degradation, which may provide an ecological advantage in microplastic-associated niches. These findings suggest that microplastics can influence coastal nitrogen cycling by favoring specific nitrifying microorganisms and enhancing microbially mediated N₂O production. This work highlights an underrecognized link between marine microplastic pollution and coastal greenhouse gas emissions, with implications for assessing the ecological effects of microplastics in estuarine and marine environments.