Microplastics transport and impact on nitrogen cycling and N₂O emissions in estuaries

Microplastic pollution in estuarine ecosystems disrupts nitrogen cycling and enhances nitrous oxide (N₂O) emissions, reinforcing the role of estuaries as greenhouse gas (GHG) hotspots. This review integrates mechanisms that modulate microplastic-induced disruptions to nitrogen cycling processes and transform estuarine biogeochemistry. It elucidates key mechanistic pathways whereby microplastic dynamics influence microbial nitrogen transformations and alter GHG fluxes. Microplastics affect nitrogen cycling through multiple mechanisms, including adsorption of nitrogenous compounds, restructuring of microbial communities, and modulation of enzymatic processes that control nitrogen transformations. Within plastisphere biofilms, microplastics foster microbial interactions that promote incomplete denitrification and nitrifier-driven N₂O production, intensifying N₂O fluxes from estuarine sediments and waters. The review synthesizes recent findings on microplastic degradation, genetic drift, and horizontal gene transfer, which may further reshape nitrogen cycling capacity over time. Recent advancements in microplastic characterization, including aptamer-based sensors, flow cytometry, and improved extraction methods, enhance the ability to quantify and trace microplastic impacts in estuarine environments. This review proposes an integrative conceptual model for microplastic-mediated amplification of N₂O emissions in estuaries and identifies critical research and policy directions. Addressing microplastic-induced disruptions of nitrogen cycling and GHG dynamics will require integrated mitigation strategies, targeted regulatory interventions, and interdisciplinary research to support sustainable estuarine management.