Abstract
Nitrous oxide (N 2 O) is an important greenhouse gas emitted in significant volumes by the Pacific Ocean. However, the relationship between N 2 O dynamics and environmental drivers in the subtropical western North Pacific Ocean (STWNPO) remains poorly understood. We investigated the distribution of N 2 O and its production as well as the related mechanisms at the surface (0–200 m), intermediate (200–1500 m), and deep (1500–5774 m) layers of the STWNPO, which were divided according to the distribution of water masses. We applied the transit time distribution (TTD) method to determine the ventilation times, and to estimate the N 2 O equilibrium concentration of water parcels last in contact with the atmosphere prior to being ventilated. In the surface layer, biologically derived N 2 O (ΔN 2 O) was positively correlated with the apparent oxygen utilization (AOU) (R 2 = 0.48), suggesting that surface N 2 O may be produced by nitrification. In the intermediate layer, ΔN 2 O was positively correlated with AOU and NO 3 − (R 2 = 0.92 and R 2 = 0.91, respectively) and negatively correlated with nitrogen sinks (N * ) (R 2 = 0.60). Hence, the highest ΔN 2 O value in the oxygen minimum layer suggested N 2 O production through nitrification and potential denitrification (up to 51% and 25% of measured N 2 O, respectively). In contrast, the deep layer exhibited a positive correlation between ΔN 2 O and AOU (R 2 = 0.92), suggesting that the N 2 O accumulation in this layer may be caused by nitrification. Our results demonstrate that the STWNPO serves as an apparent source of atmospheric N 2 O (mean air−sea flux 2.0 ± 0.3 μmol m -2 d -1 ), and that nitrification and potential denitrification may be the primary mechanisms of N 2 O production in the STWNPO. We predict that ongoing ocean warming, deoxygenation, acidification, and anthropogenic nitrogen deposition in the STWNPO may elevate N 2 O emissions in the future. Therefore, the results obtained here are important for elucidating the relationships between N 2 O dynamics and environmental changes in the STWNPO and the global ocean.