N₂O production and consumption from stable isotopic and concentration data in the Peruvian coastal upwelling system

Abstract

The ocean is an important source of nitrous oxide (N 2 O) to the atmosphere, yet the factors controlling N 2 O production and consumption in oceanic environments are still not understood nor constrained. We measured N 2 O concentrations and isotopomer ratios, as well as O 2 , nutrient and biogenic N 2 concentrations, and the isotopic compositions of nitrate and nitrite at several coastal stations during two cruises off the Peru coast (~5-16°S, 75-81°W) in December 2012 and January 2013. N 2 O concentrations varied from below equilibrium values in the oxygen deficient zone (ODZ) to up to 190 nmol L À1 in surface waters. We used a 3-D-reaction-advection-diffusion model to evaluate the rates and modes of N 2 O production in oxic waters and rates of N 2 O consumption versus production by denitrification in the ODZ. Intramolecular site preference in N 2 O isotopomer was relatively low in surface waters (generally À3 to 14‰) and together with modeling results, confirmed the dominance of nitrifier-denitrification or incomplete denitrifier-denitrification, corresponding to an efflux of up to 0.6 Tg N yr À1 off the Peru coast. Other evidence, e.g., the absence of a relationship between ΔN 2 O and apparent O 2 utilization and significant relationships between nitrate, a substrate during denitrification, and N 2 O isotopes, suggest that N 2 O production by incomplete denitrification or nitrifier-denitrification decoupled from aerobic organic matter remineralization are likely pathways for extreme N 2 O accumulation in newly upwelled surface waters. We observed imbalances between N 2 O production and consumption in the ODZ, with the modeled proportion of N 2 O consumption relative to production generally increasing with biogenic N 2 . However, N 2 O production appeared to occur even where there was high N loss at the shallowest stations.

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

“…Cross plots of SP versus δ 18 O‐N 2 O and δ 15 N α ‐N 2 O versus δ 18 O‐N 2 O showed similar positive slope, whereas 15 N β and 18 O had a slightly negative slope in this study (Figure ). A similar trend was also observed by Bourbonnais et al () at the Peruvian coastal upwelling system and seen as the clear mechanistic evidence for removal of N 2 O by denitrification. Hence, our study may suggest that N 2 O reduction occurred in the shelf and slope sediment.…”

“…δ 15 N bulk ‐N 2 O, δ 15 N α , SP at the intermediate water and deep water correlated positively with δ 18 O‐N 2 O, while δ 15 N β correlated negatively with δ 18 O‐N 2 O (Figure c). This is thought to be a clear mechanistic evidence for removal of N 2 O by denitrification (Bourbonnais et al, ) or a sign of shift in the N 2 O production and consumption processes (Yamagishi et al, ). Furthermore, unlike the shallow and intermediate waters, no correlations between ∆N 2 O, AOU, and nitrate were observed for the deep waters in this study (Figure ).…”

“…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).…”

Global Nitrous Oxide Production Determined by Oxygen Sensitivity of Nitrification and Denitrification

“…In the anoxic and the peripheral suboxic waters (generally <10 μM O 2 ), our 15 N tracer incubation experiments showed that denitrification is the dominant source of N 2 O production, because rates of N 2 O production from NO 2 − and NO 3 − reduction were tenfold to 100‐fold higher than N 2 O production from NH 4 + oxidation. Similarly, using N 2 O natural abundance isotopes, a recent study concluded that incomplete denitrification caused N 2 O supersaturation in the Peruvian OMZ (Bourbonnais et al, ). Highest rates of N 2 O production from NO 2 − and NO 3 − reduction occurred just below the oxic‐anoxic interface, where O 2 inhibition of denitrification is relieved and availability of organic matter promotes denitrification.…”

Remote Western Arctic Nutrients Fuel Remineralization in Deep Baffin Bay

“…N 2 O isotopomer analyses were performed post‐cruise at the University of Massachusetts Dartmouth using a GV IsoPrime continuous flow isotope ratio mass spectrometer, interfaced with a multicollector and customized purge‐and‐trap system (Bourbonnais et al, ). N 2 O measurements were standardized and corrected for the scrambling effect (Westley et al, ) using a set of reference N 2 O gases (EMPA CB08976, EMPA CB09715, EMPA CB09766 and EMPA 53504; Mohn et al, ) of known bulk and site‐specific isotopic composition.…”