Atmospheric nitrogen deposition fluxes into coastal wetlands and their impacts on ecosystem carbon sequestration in East Asia

Abstract. Coastal wetlands serve as critical sinks for both carbon and nitrogen within regional ecosystems, playing an essential role in mitigating atmospheric greenhouse gases and nutrient enrichment. This study integrates high-resolution wetland type data, AIS-based ship emission inventories, and regional nitrogen deposition simulations to quantify nitrogen inputs to East Asian coastal wetlands from the perspective of source–sink coupling. Firstly, the atmospheric nitrogen deposition fluxes in coastal wetland areas of East Asia were simulated and evaluated with WRF-CMAQ. Nitrogen deposition fluxes were spatially coupled with classified wetland maps in ArcGIS. Net primary productivity (NPP) was estimated using a modified CASA light-use efficiency model, incorporating solar radiation and FPAR from remote sensing. Carbon sequestration and oxygen release were then quantified using stoichiometric relationships based on NPP. The results indicate that total nitrogen deposition across East Asian coastal wetlands follows a general gradient of “high in the south, low in the north” and “strong in urban-industrial clusters, weak in remote coastal zones.” On average, ship emissions contribute 10.13 % and 15.22 % to NO₃^--N and NH₄⁺-N deposition, respectively, while their contribution to gaseous NH₃-N is negligible. Among wetland types, salt marshes receive the highest nitrogen input per unit area (654.99 mg NO₃^--N·m^-2·yr^-1), although tidal flats dominate total regional nitrogen input due to their extensive spatial coverage. Dry and wet deposition exhibit significant seasonal variation: wet deposition consistently prevails during the spring and summer months due to frequent rainfall, while dry deposition becomes increasingly prominent in autumn and winter. For instance, in the Korean Peninsula, the wet-dry gap in nitrate deposition reaches 0.17 g N·m^-2·yr^-1, while the Yangtze River Delta exhibits relatively balanced ammonium inputs (dry-wet difference of only 0.05 g N·m^-2·yr^-1). Carbon sequestration capacity shows strong spatial and temporal coupling with nitrogen deposition. Mangrove forests exhibit the highest annual NPP (~776.16 g C·m^-2·yr^-1 in summer), supported by high FPAR and solar radiation (1749.29 MJ·m^-2), followed by salt marshes and tidal flats. Seasonal patterns reveal a summer peak in carbon uptake across all wetland types, with mangrove NPP in summer being two times higher than winter values. Nitrogen deposition primarily enhances carbon sequestration during warm seasons; for instance, in the mangroves of the Pearl River Delta, nitrogen inputs increase summer CO₂ fixation by 6.85 g C·m^-2, while the effect is negligible in winter (<0.06 %) or in nitrogen-saturated regions. These findings provide a scientific foundation for understanding how coastal ecosystems respond to anthropogenic activities and long-range nitrogen transport. Furthermore, the results serve as an important reference for wetland conservation, nitrogen cycle management, and the development of regional carbon neutrality strategies.