Spatial patterns of microplastic accumulation and microbial degradation function along coastal wetland tidal gradients

Coastal wetlands are key transition zones for microplastics (MPs) transporting from land to sea, where MPs can accumulate and persist, posing long-term ecological risks. This study examined MPs abundance patterns and microbial degradation-related functional potential across high-tidal (including vegetation area), middle-tidal, and low-tidal zones in the Qiantang River estuary, a pronounced dynamic tidal system, connecting the mainland to the East China Sea. MPs abundance increased with tidal elevation (low < middle < high-tidal zone), with peak concentrations in topsoil (low- and high-tidal zones) or the 10-20 cm layer (middle-tidal zone). MPs were mainly fragments (72.45%-79.47%) and 10-100 μm in size (81.89%). Partial least squares path modeling (PLS-PM) identified soil physicochemical properties, particularly nitrogen-related parameters (NH₄⁺-N and NO₃^--N), as the main factors associated with MPs distribution, followed by spatial factors (tidal zone and depth). Microbial degradation-related functional potential inferred from microbial community composition exhibited a significant negative association with MPs abundance. Vegetation within the tidal zone also influenced MPs patterns: rhizosphere soil (RS) had significantly higher MPs abundance (2633-3284 n g^-1) and smaller particle sizes than bulk soils (1009-1536 n g^-1), coinciding with increased MPs-degrading bacteria (e.g., Xanthomonadales, especially Stenotrophomonas) in the RS. These findings highlight the combined effects of tidal dynamics and rhizosphere interactions on MPs fate within coastal wetlands, providing crucial insights for the management of MPs pollution in these ecologically sensitive areas.