Mixed Archaeal Production and Nitrifier Denitrification Dominate N2O Production in the East China Sea: Insights From Isotopocule and Hydroxylamine Analyses.

Oceans are identified as potent sources of atmospheric nitrous oxide (N2O), while the magnitude of its flux and microbial production mechanisms remain uncertain in highly perturbed coastal zones. Here, the first analyses of N2O isotopocule signatures in the East China Sea (ECS) are presented, along with hydroxylamine (NH2OH) and N2O concentrations, to clarify the dominant N2O production processes in coastal water. In the ECS in October 2015, N2O ranged from 6.3 to 33.1 nmol L−1, equivalent to 99%–251% saturation, leading to air‐sea fluxes of 1.6–10.5 μmol m−2 d−1 (4.8 ± 2.5 μmol m−2 d−1) using the W2014 formula. The coexistence of high levels of NH4+, NH2OH, and NO2− indicated the potential for nitrification and/or hybrid N2O formation. In the shallow water (<300 m), the concentration (∼9.3 nmol L−1), δ15Nbulk–N2O (∼6.8‰), δ18O–N2O (∼45.1‰), and 15N site preference (SP, ∼14.8‰) of N2O were close to the isotopic signatures in atmospheric N2O, whereas values in the deep water increased with depth, with N2O reaching maxima of 33.1 nmol L−1, 8.6‰, 54.7‰, and 18.7‰, respectively. From the dual N2O isotopocule mapping approach, almost equal contributions of archaeal N2O production (archaeal nitrification and/or hybrid mechanism, ∼47%) and nitrifier denitrification (or denitrification) (∼53%) to total in situ N2O production were identified for the shallow water, but archaeal nitrification was responsible for ∼83% of the deeper N2O production. Moreover, the far‐field lateral advection from other areas served as a potential physical supply of deeper N2O. Our findings enhance the understanding of N2O dynamics in coastal waters. Plain Language Summary: Nitrous oxide (N2O) is a powerful climatically trace gas, of which the global warming potential per mole is nearly 300 times that of carbon oxide, and it also acts as a significant ozone depletion agent. Coastal seas, which are influenced by anthropogenic perturbations, are identified as potent sources of N2O. Based on the isotopic analysis of dissolved N2O and other environmental parameters in the ECS, a northwestern Pacific marginal sea, we can obtain a further understanding of the N2O biogeochemical dynamics in coastal waters. In the ECS, N2O production in the shallow water was attributed to a mixed pool of archaeal N2O production and nitrifier denitrification (or denitrification) with almost equal contributions, while archaeal nitrification represented the major source of deeper N2O production. The advected supply of N2O‐enriched waters from other regions and air‐sea gas exchange were important physical processes in coastal N2O cycling. Deciphering coastal N2O biogeochemical dynamics would further constrain and predict global climate change. Key Points: Archaeal N2O production (archaeal nitrification and/or hybrid formation) and nitrifier denitrification contributed equally to shallow N2OArchaeal nitrification dominated deep N2O productionAir‐sea gas exchange and water mass mixing are important physical processes influencing coastal N2O cycling [ABSTRACT FROM AUTHOR]