Global estuaries are large but highly uncertain sources of nitrous oxide (N₂O) to the atmosphere. Estuaries have been severely perturbed by excess nitrogen input, deoxygenation and warming, with potential impacts on N₂O emissions. Here we use the Chesapeake Bay – the largest estuary in the United States - to investigate the response of N₂O production to deoxygenation and warming. Using four ¹⁵N tracers (¹⁵NH₄⁺, ¹⁵N-urea, ¹⁵NO₂^-, ¹⁵NO₃^-), we measured N₂O production from nitrification and denitrification under manipulated oxygen and temperature conditions. At >10 µM O₂, nitrification was the major N₂O production pathway, while denitrification’s contribution was negligible. When O₂ was experimentally reduced to <5 µM, N₂O production from nitrification increased from 0.1 to 0.4 nmol N₂O L^-1 d^-1, due to a combination of increased N₂O yield and a decreased nitrification rate. In contrast, N₂O production from denitrification increased exponentially from ~0 to 15 nmol N₂O L^-1 d^-1 when O₂ was lowered from 25 to near 0 µM; the maximum N₂O yield occurred at ~5 µM O₂. When the temperature was raised from 15ºC to 35ºC (ambient temperature ~26ºC), N₂O production from nitrification and denitrification rose by 2-10 fold, due to an increase in both rates but with relatively constant N₂O yields. Due to the projected expansion of hypoxia and warming in future estuaries, N₂O production and emission may therefore increase, in turn enhancing greenhouse warming and ozone depletion. These new observations provide a benchmark for modeling and predicting N₂O production under future climate change.

Primary Presenter: Weiyi Tang, Princeton University (weiyit@princeton.edu)

Authors:

John Tracey, Lamont-Doherty Earth Observatory  ()

Naomi Intrator, Princeton University  ()

Moriah Kunes, Princeton University  ()

Jenna Lee, Princeton University  ()

Amal Jayakumar, Princeton University  ()

Bess Ward, Princeton University  ()