Significant Seasonal N2O Dynamics Revealed by Multi‐Year Observations in the Northern South China Sea

Abstract

AbstractThe coastal ocean and marginal sea play a disproportionally important role in the release of nitrous oxide (N2O) into the atmosphere. The spatial and temporal distribution of N2O in these important source regions remains highly uncertain due to the scarcity of N2O measurements. Here we present a large data set of N2O concentrations and fluxes obtained from 10 cruises covering four seasons in the Northern South China Sea (NSCS). The study area is overall a net source of atmospheric N2O with an annual flux of 1.9 ± 1.2 × 108, 0.8 ± 0.5 × 108 and 1.2 ± 0.7 × 108 mol N2O yr−1 in the shelf, slope and basin regions, respectively. In terms of global warming potentials, the N2O emissions offset 27.8% of the CO2 sink on the shelf, and are equivalent to 3.5 and 0.2‐fold of the CO2 emission in the slope and basin of the NSCS. On the seasonal time scale, N2O flux was significantly higher in autumn and winter than in the warm seasons. The spatial variability was contrastingly less pronounced. The seasonality of N2O distribution in the shelf region was modulated by the riverine discharge, while intrusion of the Kuroshio Current exerted profound control on N2O distribution in the open waters of the NSCS. The variable relationships between N2O excess, apparent oxygen utilization, and nitrate in the shelf, NSCS basin and the Luzon Strait indicated a regional difference in N2O cycling pathways along with the impact of water mass mixing. Our study establishes a robust baseline to understand N2O distribution and flux in the NSCS.