Atmospheric nitrogen deposition is a crucial external source of nitrogen to surface oceans, significantly affecting marine biogeochemical cycles. Since the onset of industrialization, human activities have caused a more than threefold increase in nitrogen deposition over open oceans. Simultaneously, global warming has intensified ocean stratification, hindering the vertical transport of deep-sea nutrients. Understanding the trends of nitrogen deposition and its impact on marine productivity under the combined influences of human emissions and climate change is particularly important, especially in the Northwest Pacific, a region heavily affected by East Asian continental outflows. China, as a major contributor of anthropogenic nitrogen emissions in East Asia, transports approximately 30% of its land-based reactive nitrogen emissions to the downwind Northwest Pacific. Since the 2000s, ammonia (NH₃) and nitrogen oxides (NOₓ) emissions in China have undergone notable changes driven by economic growth and policy regulations, yet the response of atmospheric nitrogen deposition in the Northwest Pacific remains unclear. In the field of impacts on marine, existing studies often assume the full bioavailability of deposited nitrogen, converting it into carbon fluxes based on the Redfield ratio, while overlooking intermediate responses such as changes in sea surface nitrate and chlorophyll concentrations, particularly on monthly timescales.

In this study, the Goddard Earth Observing System-Chemical transport (GEOS-Chem) Model was used to simulate atmospheric nitrogen deposition in the Northwest Pacific from 2005 to 2015, and the impacts of anthropogenic emission changes on marine atmospheric deposition trends were analyzed. The results showed that the average total nitrogen deposition was 3.56 kg N ha^-1 yr^-1, with wet deposition accounting for 64%. The contribution of oxidized nitrogen to the deposition was greater than that of reduced nitrogen. From 2005 to 2013, total nitrogen deposition in the Northwest Pacific remained relatively stable, while there was a rapid decline from 2013 to 2015. Changes in China’s nitrogen emissions predominantly influenced annual variations of nitrogen deposition in the Northwest Pacific, while seasonal variability of nitrogen deposition was largely driven by the East Asian monsoon. Additionally, monthly nitrogen deposition data were correlated with sea surface nitrate and chlorophyll-a concentrations to assess marine responses. The correlation between monthly nitrogen deposition and sea surface nitrate concentration, as well as chlorophyll-a concentration, was relatively stronger in mid-latitude (30°-40°N) and subtropical gyre regions, respectively, providing insights into areas where marine productivity is more sensitive to nitrogen deposition.
 

atmospheric nitrogen deposition, trends, Northwest Pacific, marine primary productivity