As the world's largest marginal sea, biogeochemical cycling of nitrite (NO₂⁻) in the South China Sea (SCS) is still unclear. NO₂⁻ concentration generally shows a peak at or near the base of the sunlit euphotic zone, which is called primary NO₂⁻ 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 NO₂⁻ are used for the first time to explore the NO₂⁻ 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 (NH₃) oxidation. A steady-state model is used to estimate the biogeochemical cycling rates of NO₂⁻ in the NSCS. Our results show that both the oxidation rate of NH₃ and the assimilation rate of NO₂⁻ are significantly reduced from the shelf to the basin. Notably, the role of NO₂⁻ oxidation in the basin is not negligible, although the assimilation of NO₂⁻ mainly represents the fate of NO₂⁻ in the NSCS. The residence time of NO₂⁻ implies that the cycle of NO₂⁻ is dynamic and the PNM is an active signal in the NSCS. Our results further demonstrate the importance of NH₃ 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.

中文翻译：

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双同位素表明南海北部亚硝酸盐循环

作为世界上最大的边缘海，南海（SCS）亚硝酸盐（NO ₂ ^-）的生物地球化学循环尚不清楚。NO ₂ ^-浓度通常在阳光照射的透光区底部或附近显示一个峰值，称为初级NO ₂ ^-最大值（PNM）。PNM在南海广泛出现，但其形成机制尚未阐明。在这项研究中，首次使用NO ₂ ^-双同位素来探索南海北部（NSCS）的NO ₂ ^-循环和PNM形成。我们认为 NSCS 中 PNM 的形成主要由氨（NH ₃) 氧化。稳态模型用于估计NSCS中 NO ₂ ^-的生物地球化学循环速率。我们的结果表明，从陆架到盆地，NH ₃的氧化速率和NO ₂ ^-的同化速率都显着降低。值得注意的是，NO ₂ ^-氧化在盆地中的作用不可忽略，尽管NO ₂ ^-的同化主要代表了NSCS中NO ₂ ^-的归宿。NO的停留时间₂ ^-意味着没有周期₂ ^-是动态的，PNM 是 NSCS 中的活动信号。我们的研究结果进一步证明了 NH ₃氧化在边缘海 PNM 形成中的重要性，这可以应用于全球边缘海以更深入地了解海洋氮 (N) 循环。