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

Abstract

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 μmol 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.

Nitrite Cycle Indicated by Dual Isotopes in the Northern South China Sea

“…Due to the influence of rainwater and runoff, the surface salinity is usually less than 34 in the NSCS. The vertical changes of temperature and salinity in the upper 300 m water column at each station are similar and show a reversed “S” characteristic (Figure 2), which is consistent with other studies (Zhang et al., 2019). Previous studies have shown that the intrusion of Kuroshio Water (KW) affects the characteristics of temperature and salinity in the upper water column in the SCS, with a subsurface maximum of salinity (Wong et al., 2007).…”

“…In the process of nitrification, nitrous oxide (N 2 O) is produced as a by‐product, which has an impact on global greenhouse effect (Ostrom et al., 2000; Santoro et al., 2011; Ward, 2008). Recently, nitrifier denitrification with NO 2 − as a substrate has captured more attention (e.g., Frame & Casciotti et al., 2010; Sutka et al., 2006; Wrage et al., 2001; Zhang et al., 2019), which further illustrates the importance of understanding the NO 2 − cycle.…”

“…In addition, the northern SCS (NSCS) is affected by the intrusion of Kuroshio, a subtropical western boundary current of the North Pacific (Figure 1, Du et al., 2013; Xu et al., 2018). A recent study has shown that NO 2 − peaks appear in the euphotic zone of the NSCS (Zhang et al., 2019). Intriguingly, the shelf and the slope of the NSCS is a net source of atmospheric N 2 O, which is produced in the euphotic zone by nitrifier denitrification (Zhang et al., 2019).…”

“…A recent study has shown that NO 2 − peaks appear in the euphotic zone of the NSCS (Zhang et al., 2019). Intriguingly, the shelf and the slope of the NSCS is a net source of atmospheric N 2 O, which is produced in the euphotic zone by nitrifier denitrification (Zhang et al., 2019). This suggests that the formation of the PNM in the NSCS is related to the process responsible for N 2 O production, although solid evidence is needed.…”

“…as a substrate has captured more attention (e.g., Frame & Casciotti et al, 2010;Sutka et al, 2006;Wrage et al, 2001;Zhang et al, 2019), which further illustrates the importance of understanding the NO 2 − cycle.…”