Enhanced Microplastic Burial in China’s Coastal Blue Carbon Ecosystems: Drivers and Potential Roles in Climate Change Mitigation

Blue carbon ecosystems (BCEs) are important hotspots of microplastics. However, the magnitude and drivers of microplastic storage across the coastline, together with their roles in climate change mitigation, remain largely unknown. The results showed that microplastic stocks in the surface 12 cm sediments along the Chinese coast ranged from 2170 to 42,240 particles m^-2, corresponding to 0.01-104.4 kg C ha^-1 of microplastic-derived carbon in BCEs. BCEs enhanced the trapping of microplastics, with 1.3-3.8-fold higher stocks than bare flats, while their contribution to organic carbon stocks was negligible (<1.4%). Assuming that microplastics would be decomposed if not stored in BCEs, we estimated that microplastic burial in BCEs contributes to 0.3-45.3 Gg CO_{2 eq} of avoided emissions across China. Microplastic abundance declined with depth in mangrove sediments, indicating accelerated accumulation in recent decades (<i>R</i>² = 0.86), faster than in seagrass or bare flats. Urban area, distance from shore, BCE type, and latitude were identified as the major environmental and human activity factors driving microplastic accumulation in BCEs by a multivariable linear regression model (Microplastic stock = exp(6.20 + 1.13 × Ln(Urban area) - 0.23 × Ln(Distance from shore) + 0.48 × BCEs type - 0.05 × Latitude), <i>R</i>² = 0.85). Our study provides new insights into the emerging carbon-pollution interactions in BCEs and offers a scalable modeling approach to inform coastal microplastic management.