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

Abstract

AbstractOceans 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.