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 N-15 tracers (NH4+15$$ {}<^>{15}{\mathrm{NH}}_4<^>{+} $$, N-15-urea, NO2-15,$$ {}<^>{15}{\mathrm{NO}}_2<^>{-}, $$ and NO3-15$$ {}<^>{15}{\mathrm{NO}}_3<^>{-} $$) to separately track N2O production from nitrification and denitrification under in situ and manipulated O-2 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 O-2 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.