Nitrogen cycle in the Northwest Pacific: from source-sink structure to multi-interface fluxes

Nitrogen is a key limiting nutrient for ocean primary productivity and it regulates the biological carbon pump and, ultimately, carbon sequestration. The sources, transformations, and sinks of nitrogen are thus crucial for marine ecosystem health and global climate. The nitrogen cycle is undergoing profound and complex perturbations owing to rapid global marine environmental changes. The Northwest Pacific Ocean acts as a pivotal regulator of global climate by absorbing substantial amounts of carbon and heat and is among the fastest warming and acidifying ocean regions under global change. However, the source-sink dynamics and associated fluxes of the nitrogen cycle in this region remain poorly constrained, hindering the ability to accurately assess its response to these changes. Here, we synthesize the current knowledge on key nitrogen cycling processes in the Northwest Pacific, including atmospheric deposition, nitrogen fixation, nitrification, and nitrogen removal, to examine their spatiotemporal variations, source-to-sink fluxes, and underlying mechanisms. Our results show that atmospheric deposition (4–6 Tg N a–1) and nitrogen fixation (~6 Tg N a–1) contribute comparably to new nitrogen inputs; however, they exhibit distinct spatial and seasonal patterns. Nitrogen deposition decreases from coastal regions to the open ocean and peaks in winter–spring, whereas nitrogen fixation is highest in the center of the subtropical gyre in summer. The contribution of subsurface nitrate and dissolved organic nitrogen to primary production is expected to be high. However, their fluxes and spatiotemporal patterns remain unresolved. In the euphotic zone, intense nitrogen cycling, driven by low export and high regeneration rates, sustains vigorous nitrification (~180 Tg N a–1) and nitrous oxide production. This nitrous oxide flux accounts for nearly half of its sea-to-air exchange (0.29 Tg N a–1), offsetting a substantial portion of the carbon sequestration benefit provided by the biological pump. The close spatial coupling between the primary production and nitrification rates indicates that organic matter production and its subsequent regeneration as a key regulator of nitrification in the ocean. Although organic nitrogen burial in sediments is the primary removal pathway in this region, the flux remains poorly constrained owing to limited observations. Furthermore, we summarize how warming and acidification affect biological nitrogen fixation, phytoplankton nitrogen uptake, nitrification, and nitrous oxide production. These findings highlight the profound yet complex influence of global change on the marine nitrogen cycle. However, the synergistic effects of multiple environmental drivers remain largely unclear. Based on these findings, we propose the following priorities for advancing research on the nitrogen cycle in the Northwest Pacific: (1) clarify the spatiotemporal variability and long-term trends of nitrogen deposition and nitrogen fixation to determine how these external nitrogen sources regulate marine productivity and carbon sequestration; (2) investigate the nitrogen dynamics across spatial scales with a particular focus on submesoscale and mesoscale processes; (3) quantify the nitrogen fluxes across key interfaces and their coupling with other elemental cycles; and (4) advance the predictive understanding of nitrogen cycling and its climate feedbacks in a changing Northwest Pacific through interdisciplinary frameworks that integrate multiple methodologies.