Estuaries at the global scale are significant but highly uncertain sources of atmospheric nitrous oxide (N₂O), which is an intense greenhouse gas and ozone depletion agent. As the largest estuary in the United States, the Chesapeake Bay is suggested to be a spatially and temporally variable source and sink of N₂O. However, limited observations of N₂O cycling preclude us from estimating and predicting its net N₂O flux. To improve our mechanistic understanding of the processes that control the N₂O flux at the point of production, we applied multiple ¹⁵N tracers (${}^{15}{\mathrm{NH}}_4^{+}$, ¹⁵N-urea, ${}^{15}{\mathrm{NO}}_2^{-},$ and ${}^{15}{\mathrm{NO}}_3^{-}$) to separately track N₂O production from nitrification and denitrification under in situ and manipulated O₂ concentrations in the Chesapeake Bay. Nitrification was the major N₂O production pathway in oxic waters (up to 7.5 nmol N₂O L⁻¹ d⁻¹). In contrast, denitrification dominated N₂O production from hypoxic/anoxic waters (up to 20 nmol N₂O L⁻¹ d⁻¹). N₂O production from urea was observed for the first time in estuarine waters. The contribution from urea was small, but interestingly, showed a depth pattern distinct from other N₂O precursors. Experimentally lowering the O₂ concentration substantially enhanced N₂O production. Therefore, the expansion of hypoxic and anoxic zones in the Chesapeake Bay under climate change as suggested by some climate models may favor the production of N₂O, potentially providing positive feedback on warming. Overall, our study provides mechanistic constraints on N₂O dynamics that could benefit modeling studies to better estimate the N₂O flux in the Chesapeake Bay and other coastal environments.

中文翻译：

---

切萨皮克湾的一氧化二氮生产

全球范围的河口是大气中一氧化二氮 (N ₂ O) 的重要但高度不确定的来源，它是一种强烈的温室气体和臭氧消耗剂。作为美国最大的河口，切萨皮克湾被认为是空间和时间可变的 N 2 O 源和汇_{。然而，对 N 2} O 循环的有限观察使我们无法估计和预测其净 N ₂ O 通量. _{为了提高我们对在生产点控制 N 2} O 通量的过程的机械理解，我们应用了多个¹⁵ N 示踪剂（${}^{15}{\mathrm{NH}}_4^{+}$, ¹⁵ N-尿素,${}^{15}{不}_2^{-},$和${}^{15}{不}_3^{-}$) 分别跟踪切萨皮克湾原位硝化和反硝化产生的N _{2 O 和操纵的 O 2}浓度。硝化是含氧水中的主要 N ₂ O 生产途径（高达 7.5 nmol N ₂ O L ^-1 d ^-1）。相反，反硝化作用在缺氧/缺氧水的 N ₂ O 生产中占主导地位（高达 20 nmol N ₂ O L ^-1 d ^-1）。在河口水域首次观察到尿素产生N _{2 O。}尿素的贡献很小，但有趣的是，显示出与其他 N _{2不同的深度模式}O前体。通过实验降低O ₂浓度显着提高了N ₂ O的产生。因此，一些气候模型表明，气候变化下切萨皮克湾缺氧区和缺氧区的扩大可能有利于 N ₂ O 的产生，从而可能为变暖提供积极的反馈。总体而言，我们的研究提供了对 N ₂ O 动力学的机械约束，这有助于建模研究更好地估计切萨皮克湾和其他沿海环境中的 N ₂ O 通量。