Nitrous oxide production in the Chesapeake Bay

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 ((Formula presented.), ¹⁵N-urea, (Formula presented.) and (Formula presented.)) 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.