Dissolve organic nitrogen (DON) is the largest fixed nitrogen reservoir in oligotrophic upper oceans (< 200 m). The cycling of DON is crucial for nitrogen turnover and related biogeochemical processes in the upper ocean; however, its controls remain unclear. We measured the concentration and nitrogen isotope ratio of DON ([DON] and δ¹⁵N-DON) in the upper 200 m for three seasons in the North Pacific Subtropical Gyre (NPSG), a hot spot of N₂ fixation. Higher [DON] were found in the upper 100 m, suggesting an accumulation of DON in the sunlit layer. The accumulated DON showed lower δ¹⁵N-DON in the NPSG center than in the regions near the North Equatorial Current, possibly due to higher nitrogen fixation rates. In addition, we also infer that the accumulated δ¹⁵N-DON could be associated with the diazotroph communities. By integrating globally reported DON and nitrogen fixation rate data, we find that the accumulated δ¹⁵N-DON signals significantly correlate with depth-integrated nitrogen fixation rates. Our results provide a novel diagnosis, demonstrating the DON characteristics regulated by diazotroph-derived nitrogen in oligotrophic upper oceans.

dissolved organic nitrogen,nitrogen fixation,oligotrophic ocean,stable nitrogen isotope