Microplastics and co-occurring nitrogen synergistically accelerate blue carbon loss

Blue carbon ecosystems (BCEs)-including seagrass meadows, mangroves, and salt marshes-store vast amounts of organic carbon, yet their carbon sink function is increasingly threatened by hazardous microplastics and co-occurring nitrogen pollution. Here, we used a controlled microcosm experiment and multi-omics analysis (16S rRNA amplicon sequencing, Fourier Transform Ion Cyclotron Resonance Mass Spectrometry, and Ultra-Performance Liquid Chromatography-Mass Spectrometry) to investigate how two common microplastics-polyethylene terephthalate (PET) and polylactic acid (PLA)-interact with nitrogen fertiliser (N-dominated fertiliser with co-delivered nutrients) to influence seagrass soil biogeochemistry. We showed that PLA combined with N-fertiliser increased CO₂ emissions from seagrass soil by 106% relative to nitrogen alone and by 195% compared to controls (p < 0.01), while PET addition had negligible effects. PLA degradation released carboxylic acid and derivatives, supporting putative sulphate-reducing and fermentative bacteria, and increasing the formation of organic-oxygen compounds (e.g., disaccharides, o-glycosyl compounds). N-fertiliser addition further enriched putative organic matter-degrading microbial taxa, particularly Clostridia and Bacteroidia, and elevated microbial metabolic potential across pathways. Combined PLA and nitrogen treatments resulted in the lowest (-37% vs control) residual dissolved organic carbon concentrations, indicating accelerated carbon loss. These findings suggest that microplastics, especially PLA biopolymer, and nitrogen act as hazardous co-contaminants that enhance microbial mineralisation of soil organic matter, potentially weakening blue carbon storage. Plastic waste and fertiliser inputs should therefore be considered together in future risk assessments of coastal carbon stocks and in the development of strategies to safeguard BCEs.