Cordgrass invasion amplifies microplastic hazard through polymer-selective retention in coastal wetlands

Coastal wetlands are key repositories and pathways for microplastics (MPs), a role modulated by vegetation. The global spread of the nonnative cordgrass Spartina alterniflora is thus likely to alter MP deposition and fate, yet its effects on MP accumulation and related ecological risks are not well quantified. Integrating field surveys along an 18,000-km latitudinal transect (∼20°) of China's coastline with an ensemble of three machine-learning models, we produced a 500 m-resolution map of the MP polymer hazard index (PHI) in sediments (0-40 cm depth) and compared MP stocks and risks among native marshes, S. alterniflora-invaded marshes, and unvegetated mudflats. While MP abundance did not differ significantly between native and invaded marshes, PHI in invaded marshes was 20.0 % higher than in native marshes and 57.1 % higher than in mudflats, indicating a shift toward more hazardous polymers. This shift led to a marked expansion of high-risk coverage (increasing by 26.8 % compared with native marshes and 386.1 % relative to mudflats). Climatic variables (mean annual temperature and precipitation) emerged as dominant predictors of PHI across habitats, while plant invasion further accentuated habitat-specific effects mediated via microbial mediation. These results demonstrate that plant invasion amplifies coastal plastic pollution risk through both enhanced retention and selective accumulation of more hazardous polymers, underscoring the need to incorporate invasion control into coastal MP management strategies.