Regional distribution and environmental regulation mechanism of nitrous oxide in the Bohai Sea and North Yellow Sea: A preliminary study.

Nitrous oxide is one of the most powerful greenhouse gases and can destroy the ozone layer through photochemical reactions. In 2019, we conducted three cruises to study the spatial and temporal variability of N2O distribution and emissions in the Bohai Sea (BS) and North Yellow Sea (NYS), and analyzed the regional sources and sinks. The maximum average N2O concentrations were observed in the summer, followed by autumn, while the minimum was observed in the spring. The N2O concentration decreased in a gradient from the estuary to the continental shelf, particularly in summer, which indicated that the riverine input from the estuary was a strong source of N2O in the Bohai Sea. Due to the vertical mixing of the water column, the vertical distribution of N2O was moderate in autumn, while the bottom remained a hotspot for N2O emissions in spring and summer. The generalized additive model (GAM) showed that the temperature, salinity, DO and pH were strong predictors of N2O in the BS and NYS. Excess N2O concentrations were positively linearly correlated with the apparent oxygen utilization and NO3- concentrations, which suggested that nitrification was the dominant process of in situ N2O production in the BS and NYS. The mixing of water masses, especially DW (diluted water) and BCW (Bohai Sea coastal water), provided a significant amount of N2O to the entire shelf area of the BS. In addition, the coastal input was a dominate pusher of N2O emissions in the estuarine region. Overall, the annual N2O emissions from BS and NYS were approximately 1.72 × 10-2 Tg yr-1, which accounted for 0.51% of the annual global marine N2O emissions, but only 0.04% of the total area of the world's oceans. Hence, both the BS and NYS acted as N2O sources to the atmosphere.