106 / 2024-09-08 17:17:51

Dissolved organic nitrogen in oligotrophic oceans regulated by nitrogen fixation

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

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.