Significant Seasonal N<sub>2</sub>O Dynamics Revealed by Multi‐Year Observations in the Northern South China Sea
Xianhui Wan, Hua Lin, Bess B. Ward, Shuh‐Ji Kao, Minhan Dai
Abstract
Abstract The coastal ocean and marginal sea play a disproportionally important role in the release of nitrous oxide (N 2 O) into the atmosphere. The spatial and temporal distribution of N 2 O in these important source regions remains highly uncertain due to the scarcity of N 2 O measurements. Here we present a large data set of N 2 O 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 N 2 O with an annual flux of 1.9 ± 1.2 × 10 8 , 0.8 ± 0.5 × 10 8 and 1.2 ± 0.7 × 10 8 mol N 2 O yr −1 in the shelf, slope and basin regions, respectively. In terms of global warming potentials, the N 2 O emissions offset 27.8% of the CO 2 sink on the shelf, and are equivalent to 3.5 and 0.2‐fold of the CO 2 emission in the slope and basin of the NSCS. On the seasonal time scale, N 2 O flux was significantly higher in autumn and winter than in the warm seasons. The spatial variability was contrastingly less pronounced. The seasonality of N 2 O distribution in the shelf region was modulated by the riverine discharge, while intrusion of the Kuroshio Current exerted profound control on N 2 O distribution in the open waters of the NSCS. The variable relationships between N 2 O excess, apparent oxygen utilization, and nitrate in the shelf, NSCS basin and the Luzon Strait indicated a regional difference in N 2 O cycling pathways along with the impact of water mass mixing. Our study establishes a robust baseline to understand N 2 O distribution and flux in the NSCS.