Impact of Atmospheric Nitrogen Deposition on Ocean Carbon Export Controlled by Changes in Plankton Community...

Atmospheric deposition is an important source of nitrogen (N) fueling marine productivity, yet its full impact on ocean carbon export remains poorly constrained. Here, we use a biogeochemical model ensemble for the South China Sea to assess this impact. Our model quantifies that the regenerative recycling of atmospherically deposited N significantly amplifies its fertilizing effect, enhancing annual primary production in offshore waters by ∼22% (∼35 g C m^-2 yr^-1), which is approximately six times the increase predicted by conventional stoichiometric estimates (∼5.9 g C m^-2 yr^-1). Beyond production, atmospheric N deposition shifts plankton communities toward larger size classes, increasing annual carbon export at a 100 m depth by 28% and enhancing carbon export efficiency (e-ratio) by 4.4% in offshore waters. Sensitivity experiments further show that the export and efficiency changes are highly sensitive to sinking and remineralization rates of detritus: the e-ratio response can range from a 10.6% decrease to a 5.8% increase, depending entirely on variations in the detritus parameters. These findings, based on the South China Sea, highlight the need for targeted observations to constrain detritus properties and demonstrate that accurate modeling of plankton community structure and detritus dynamics is essential for predicting ocean responses to atmospheric deposition and other nutrient perturbations in similar oligotrophic systems.