The impacts of polypropylene microplastics on carbon and nitrogen cycling in coastal wetlands: Field evidence

Coastal wetlands are hotspots for carbon and nitrogen cycling and serve as sinks for microplastics (MPs). Although MP effects on these biogeochemical cycles have been investigated under laboratory experiments, field-based evidence is lacking. This study presents the first field investigation of MP impacts, specifically polypropylene (PP), on sediment carbon and nitrogen cycling in intertidal and supratidal zones of a coastal wetland, employing in-situ culture over three months. Regarding carbon cycling, PP MPs exerted similar effects across zones and showed distinct concentration-dependent variations. 0.2 % (w/w) PP MPs had no significant effect on carbon cycling in either zone, whereas 2 % (w/w) PP MPs significantly stimulated carbon-degrading enzyme activity but did not affect total organic carbon content. For nitrogen cycling, the impact of PP MPs exhibited pronounced spatial heterogeneity. In the intertidal zone, both concentrations had no effect on ammonium and nitrate, whereas in the supratidal zone, they significantly altered inorganic nitrogen levels, with ammonium responses varying by MP concentration. Compared to existing laboratory findings, our field study suggests MP impacts on carbon and nitrogen cycling in coastal wetlands, particularly in the intertidal zone, are less pronounced. These findings provide a more practical understanding of the ecological impacts of MPs in coastal wetlands.