Nitrous oxide production in the Chesapeake Bay

Abstract

Estuaries at the global scale are significant but highly uncertain sources of atmospheric nitrous oxide (N 2 O), 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 N 2 O. However, limited observations of N 2 O cycling preclude us from estimating and predicting its net N 2 O flux. To improve our mechanistic understanding of the processes that control the N 2 O flux at the point of production, we applied multiple 15 N tracers ( 15 NH þ 4 , 15 N-urea, 15 NO À 2 , and 15 NO À 3 ) to separately track N 2 O production from nitrification and denitrification under in situ and manipulated O 2 concentrations in the Chesapeake Bay. Nitrification was the major N 2 O production pathway in oxic waters (up to 7.5 nmol N 2 O L À1 d À1 ). In contrast, denitrification dominated N 2 O production from hypoxic/anoxic waters (up to 20 nmol N 2 O L À1 d À1 ). N 2 O 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 N 2 O precursors. Experimentally lowering the O 2 concentration substantially enhanced N 2 O 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 N 2 O, potentially providing positive feedback on warming. Overall, our study provides mechanistic constraints on N 2 O dynamics that could benefit modeling studies to better estimate the N 2 O flux in the Chesapeake Bay and other coastal environments.

Nitrous Oxide Consumption in Oxygenated and Anoxic Estuarine Waters

“…Relatively homogeneous vertical distributions of N 2 O that were nearly equilibrated with the atmosphere were observed at downstream station CB3 on both the CB2019 and CB2020 cruises (Figure 1b). N 2 O concentrations at upstream CB2 in 2019 fall were supersaturated (up to 160%), across the water column suggesting net N 2 O production, which is confirmed by the direct measurement of N 2 O production (Tang et al, 2022). In contrast, N 2 O concentrations at CB1.5 in 2020 summer showed a distinct pattern compared to the other stations.…”

“…Here we conducted the first direct measurements of N 2 O reduction rates in estuarine water, using the largest estuary in the United States, the Chesapeake Bay, as a model system to evaluate N 2 O reduction dynamics. When combined with concurrent measurements of N 2 O production by Tang et al, 2022, a comprehensive picture of N 2 O cycling is revealed. The in situ anoxic waters were observed to actively reduce N 2 O and the N 2 O reduction rate was higher than the N 2 O production rate, leading to N 2 O undersaturation in anoxic waters.…”

Nitrous Oxide (N2O) Emissions Decrease Significantly under Stronger Light Irradiance in Riverine Water Columns with Suspended Particles

“…36 Some researchers indicated that nitrification primarily dominated N 2 O production in oxic natural waters, such as oceans and lakes, whereas denitrification prevailed in hypoxic/anoxic waters. 37,38 As such, we speculate that from clear to turbid waters, increased concentrations of suspended particles caused more suboxic microsites for denitrifiers to produce N 2 O. This result implies that denitrification might dominate N 2 O production in turbid water columns, where the occurrence of denitrification has been widely reported.…”

All about Nitrite: Exploring Nitrite Sources and Sinks in the Eastern Tropical North Pacific Oxygen Minimum Zone

“…All data were first normalized around zero before calculating the Pearson's correlation coefficient. Gene abundances (nirS and amoA) used for the RDA and correlation analyses were measured as previously described (Peng et al, 2015;Jayakumar et al, 2009;Tang et al, 2022). In the results and discussion sections, results are classified as shallow boundary or ODZ core waters according to a previously published threshold (Babbin et al, 2020) (Babbin et al, 2020).…”