Distribution of Concentration and Stable Isotopic Composition of N2O in the Shelf and Slope of the Northern South China Sea: Implications for Production and Emission

Oceans are an important natural source of the greenhouse gas nitrous oxide (N2O). The isotopomer signature of N2O provides a useful tool to differentiate the production processes of N2O in the oceans. Here we present the distribution of the concentration and stable isotopic composition of dissolved N2O in the water column of the shelf and slope region of the northern South China Sea (SCS) in June 2015. Dissolved N2O concentrations in surface waters ranged from 6.9 to 9.1 nM with an average of 7.7 0.6 nM (136 10% saturation). Higher N2O was found at the region influenced by coastal water entrained by eddies. Vertical profiles of dissolved N2O showed a general increase with depth below the mixed layer and reached a broad peak (23-29 nM) at around 700 m coinciding with the nitrate maximum and oxygen minimum. The SP values measured for N2O ranged between 10.2 and 18.8, suggesting that dissolved N2O in the water column had been produced from both nitrification (ammonium oxidation) and nitrifier denitrification (nitrite reduction). Nitrification dominated in the intermediate water (120-1,000 m) while nitrifier denitrification dominated in the euphotic zone. The sea-to-air fluxes of N2O were estimated to be 7.0 +/- 6.1 and 6.9 +/- 6.5 mu mol m(-2) day(-1) using two different gas transfer relationships. N2O emissions from the shelf and slope regions of the northern SCS were estimated to be 0.25 Tg N2O year(-1), suggesting that coastal areas like the SCS are net sources of N2O to the atmosphere.