Nitrite Cycle Indicated by Dual Isotopes in the Northern South China Sea

As the world's largest marginal sea, biogeochemical cycling of nitrite (NO 2 − ) in the South China Sea (SCS) is still unclear. NO 2 − concentration generally shows a peak at or near the base of the sunlit euphotic zone, which is called primary NO 2 − maximum (PNM). The PNM appears widely in the SCS, but its formation mechanism has not yet been realized. In this study, the dual isotopes of NO 2 − are used for the first time to explore the NO 2 − cycle and PNM formation in the northern SCS (NSCS). We propose that the formation of the PNM in the NSCS is mainly driven by ammonia (NH 3 ) oxidation. A steady‐state model is used to estimate the biogeochemical cycling rates of NO 2 − in the NSCS. Our results show that both the oxidation rate of NH 3 and the assimilation rate of NO 2 − are significantly reduced from the shelf to the basin. Notably, the role of NO 2 − oxidation in the basin is not negligible, although the assimilation of NO 2 − mainly represents the fate of NO 2 − in the NSCS. The residence time of NO 2 − implies that the cycle of NO 2 − is dynamic and the PNM is an active signal in the NSCS. Our results further demonstrate the importance of NH 3 oxidation in the formation of PNM in the marginal sea, which can be applied in global marginal seas to gain a deeper understanding of marine nitrogen (N) cycle.