Origin and fluxes of nitrous oxide along a latitudinal transect in western North Pacific: Controls and regional significance

Abstract

Nitrous oxide (N 2 O) is an atmospheric trace gas playing an important role in both radiative forcing and stratospheric ozone depletion. The oceans are the second most important natural source of N 2 O. The magnitude of the flux of this source is poorly constrained. Moreover, the relative importance of the microbial processes leading to the formation or the consumption of N 2 O in oceans remains unclear. We present here fluxes and isotope and isotopomer signatures of N 2 O measured at three stations located along a latitudinal transect in subtropical and subarctic western North Pacific. These results indicate that about 30% to 55% of the oceanic flux of N 2 O to the atmosphere originates from the deep euphotic and shallow aphotic zones. The sea-to-air fluxes of N 2 O calculated using an isotope mass balance model indicate that the emission rate of N 2 O in subarctic waters is about 2 times higher than in oligotrophic subtropical waters suggesting that nutrient-rich water coming from the western subarctic gyre stimulates the N 2 O production. Moreover, isotopomer analysis has revealed that in shallow water N 2 O originates from nitrification and nitrifier denitrification processes, and its distribution in the water column is partly controlled by the incident solar radiation. The results of this study contribute to better constrain the global N 2 O budget and provide important information to better predict the future evolution of the oceanic emissions of N 2 O.

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

“…The ranges of δ values measured for the studied region are comparable to those measured for the East China Sea (Zhang et al, unpublished data) and the Station ALOHA in the subtropical North Pacific gyre (Ostrom et al, 2000;Popp et al, 2002); however, SP and δ 18 O spanned a narrower range than the subtropical and subarctic western North Pacific (Breider et al, 2015;Toyoda et al, 2002), especially the eastern tropical North Pacific (Yamagishi et al, 2007) and the Peruvian coastal upwelling (Bourbonnais et al, 2017). Previous research shows that N 2 O production mechanisms are different at various water depth and locations in the ocean (Breider et al, 2015;Frame et al, 2014;Fujii et al, 2013). To elucidate the relative importance of the roles of nitrification and nitrifier-denitrification in forming N 2 O at different depths, we divided the water column into the shallow water (0-120 m), the intermediate water (120-1,000 m), and deep water (>1,000 m).…”

“…Nitrate and N 2 O concentrations increased rapidly with the steady decline of DO, reaching maximum at the depth of 700 m together with the DO minimum (Figure ), which is consistent with N 2 O maximum (~28 nM) at 600–800 m reported by Tseng et al () for this region. An N 2 O maximum within the O 2 minimum has also been observed in various regions of the North Pacific, including the WPS (Tseng et al, ), the subtropical western North Pacific (Breider et al, ; Han & Zhang, ), the subtropical North Pacific gyre (Popp et al, ), and the subarctic western North Pacific (Toyoda et al, ), but the maximum N 2 O concentrations for the SCS (~23–29 nM) were slightly higher than the WPS (~23 nM) and lower than the North Pacific gyre and the western North Pacific (Figure ). A higher maximum N 2 O concentration in the SCS relative to the WPS is likely because the North Pacific water carried by the Kuroshio intruding westward from the WPS into this layer (100–750 m) within the SCS through the Luzon Strait.…”

“…Comparison of N 2 O, δ 15 N bulk , SP and δ 18 O at the water column of the SCS and various regions in the North Pacific (WPS (Tseng et al, ); SWNP (Breider et al, ); NPIW [Han & Zhang, ]; ALOHA (Popp et al, ); and ETNP (Yamagishi et al, )). SCS = South China Sea; WPS = West Philippines Sea; SWNP = subtropical western North Pacific; NPIW = North Pacific intermediate water; ETNP = eastern tropical North Pacific.…”

“…Isotopic composition of N 2 O observed in the SCS and comparison with those reported for other marine environments. Data sources: the western North Pacific (Breider et al, ; Yoshikawa et al, ), the ETNP (Yamagishi et al, ); the troposphere (Yoshida & Toyoda, ), ALOHA (Popp et al, ), the East China Sea (Zhang et al, ); and the South Atlantic (Frame et al, ), Peruvian coastal upwelling system (Bourbonnais et al, ). The rectangles show the reported range of isotopomer ratios of N 2 O produced by nitrification driven by AOA (solid pink boxes) and AOB (dotted pink boxes), denitrification (solid black boxes), and nitrifier denitrification (dashed blue boxes).…”

“…In the shallow water from the surface to the depth of isotopic minimum (~120 m), N 2 O increased from 7.1 to 13.4 nM, while ammonium was around 0.06-0.1 μM at most depths but show peaks of 0.1-0.26 μM at the (Breider et al, 2015;Yoshikawa et al, 2016), the ETNP (Yamagishi et al, 2007); the troposphere , ALOHA (Popp et al, 2002), the East China Sea (Zhang et al, 2010); and the South Atlantic (Frame et al, 2014), Peruvian coastal upwelling system (Bourbonnais et al, 2017). The rectangles show the reported range of isotopomer ratios of N 2 O produced by nitrification driven by AOA (solid pink boxes) and AOB (dotted pink boxes), denitrification (solid black boxes), and nitrifier denitrification (dashed blue boxes).…”