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

The 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.