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The El Niño‐Southern Oscillation (ENSO) is a natural climate phenomenon that alters the biogeochemical and physical dynamics of the Eastern Tropical Pacific Ocean. Its two phases, El Niño and La Niña, are characterized by decreased and increased coastal upwelling, respectively, which have cascading effects on primary productivity, organic matter supply, and ocean‐atmosphere interactions. The Eastern Tropical South Pacific oxygen minimum zone is a source of nitrous oxide (N₂O), a potent greenhouse gas, to the atmosphere. Here, we present the first study to directly compare N₂O sources during opposing ENSO phases using N₂O isotopocule analyses. Our data show that during La Niña, N₂O accumulation increased six‐fold in the upper 100 m of the water column, and N₂O fluxes to the atmosphere increased up to 20‐fold. N₂O isotopocule data demonstrated substantial increases in δ¹⁸O up to 60.5‰ and decreases in δ¹⁵N^βdown to −10.3‰ in the oxycline, signaling a shift in N₂O cycling during La Niña compared to El Niño. During El Niño, N₂O production was primarily due to ammonia‐oxidizing archaea, whereas during La Niña, N₂O production by incomplete denitrification supplemented that from ammonia‐oxidation, with N₂O consumption likely maintaining the high site preference values (up to 26.7‰). Ultimately, our results illustrate a strong connection between upwelling intensity, biogeochemistry, and N₂O flux to the atmosphere. Additionally, they highlight the combined power of N₂O isotopocule analysis and repeat measurements in the same region to constrain N₂O interannual variability and cycling dynamics under different climate scenarios.

 

Obtaining nitrous oxide isotopocule measurements with isotope ratio mass spectrometry (IRMS) involves analyzing the ion current ratios of the nitrous oxide parent ion (N₂O⁺) as well as those of the NO⁺fragment ion. The data analysis requires correcting for “scrambling” in the ion source, whereby the NO⁺fragment ion obtains the outer N atom from the N₂O molecule. While descriptions exist for this correction, and interlaboratory intercalibration efforts have been made, there has yet to be published a package of code for implementing isotopomer calibrations.

We developed a user‐friendly Python package (pyisotopomer) to determine two coefficients (γandκ) that describe scrambling in the IRMS ion source, and then used this calibration to obtain intramolecular isotope deltas in N₂O samples.

With two appropriate reference materials,γandκcan be determined robustly and accurately for a given IRMS system. An additional third reference material is needed to define the zero‐point of the delta scale. We show that IRMS scrambling behavior can vary with time, necessitating regular calibrations. Finally, we present an intercalibration between two IRMS laboratories, using pyisotopomer to calculateγandκ, and to obtain intramolecular N₂O isotope deltas in lake water unknowns.

Given these considerations, we discuss how to use pyisotopomer to obtain high‐quality N₂O isotopocule data from IRMS systems, including the use of appropriate reference materials and frequency of calibration.

 

Biological dinitrogen fixation is the major source of new nitrogen to marine systems and thus essential to the ocean’s biological pump. Constraining the distribution and global rate of dinitrogen fixation has proven challenging owing largely to uncertainty surrounding the controls thereon. Existing South Atlantic dinitrogen fixation rate estimates vary five-fold, with models attributing most dinitrogen fixation to the western basin. From hydrographic properties and nitrate isotope ratios, we show that the Angola Gyre in the eastern tropical South Atlantic supports the fixation of 1.4–5.4 Tg N.a⁻¹, 28-108% of the existing (highly uncertain) estimates for the basin. Our observations contradict model diagnoses, revealing a substantial input of newly-fixed nitrogen to the tropical eastern basin and no dinitrogen fixation west of 7.5˚W. We propose that dinitrogen fixation in the South Atlantic occurs in hotspots controlled by the overlapping biogeography of excess phosphorus relative to nitrogen and bioavailable iron from margin sediments. Similar conditions may promote dinitrogen fixation in analogous ocean regions. Our analysis suggests that local iron availability causes the phosphorus-driven coupling of oceanic dinitrogen fixation to nitrogen loss to vary on a regional basis.

 

Abstract The ocean is a net source of N 2 O, a potent greenhouse gas and ozone-depleting agent. However, the removal of N 2 O via microbial N 2 O consumption is poorly constrained and rate measurements have been restricted to anoxic waters. Here we expand N 2 O consumption measurements from anoxic zones to the sharp oxygen gradient above them, and experimentally determine kinetic parameters in both oxic and anoxic seawater for the first time. We find that the substrate affinity, O 2 tolerance, and community composition of N 2 O-consuming microbes in oxic waters differ from those in the underlying anoxic layers. Kinetic parameters determined here are used to model in situ N 2 O production and consumption rates. Estimated in situ rates differ from measured rates, confirming the necessity to consider kinetics when predicting N 2 O cycling. Microbes from the oxic layer consume N 2 O under anoxic conditions at a much faster rate than microbes from anoxic zones. These experimental results are in keeping with model results which indicate that N 2 O consumption likely takes place above the oxygen deficient zone (ODZ). Thus, the dynamic layer with steep O 2 and N 2 O gradients right above the ODZ is a previously ignored potential gatekeeper of N 2 O and should be accounted for in the marine N 2 O budget. 

Nitrous oxide (N 2 O) is a potent greenhouse gas and an ozone destroying substance. Yet, clear step-by-step protocols to measure N 2 O transformation rates in freshwater and marine environments are still lacking, challenging inter-comparability efforts. Here we present detailed protocols currently used by leading experts in the field to measure water-column N 2 O production and consumption rates in both marine and other aquatic environments. We present example 15 N-tracer incubation experiments in marine environments as well as templates to calculate both N 2 O production and consumption rates. We discuss important considerations and recommendations regarding (1) precautions to prevent oxygen (O 2 ) contamination during low-oxygen and anoxic incubations, (2) preferred bottles and stoppers, (3) procedures for 15 N-tracer addition, and (4) the choice of a fixative. We finally discuss data reporting and archiving. We expect these protocols will make 15 N-labeled N 2 O transformation rate measurements more accessible to the wider community and facilitate future inter-comparison between different laboratories. 

Nitrous oxide (N₂O) is a powerful greenhouse gas, and oceanic sources account for up to one third of the total natural flux to the atmosphere. In oxygen‐deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N₂O can be produced and consumed by several biological processes. In this study, the concentration and isotopocule ratios of N₂O from a 2016 cruise in the ETNP were analyzed to examine sources of and controls on N₂O cycling across this region. Along the north‐south transect, three distinct biogeochemical regimes were identified: background, core‐ODZ, and high‐N₂O stations. Background stations were characterized by smaller variations in N₂O concentration and isotopic profiles relative to the other regimes. Core‐ODZ stations were characterized by co‐occurring N₂O production and consumption at anoxic depths, indicated by high δ¹⁸O‐N₂O (>90‰) and low δ¹⁵N₂O^β(<−10‰) values, and confirmed by a time‐dependent model, which indicated that N₂O production via denitrification was significant and may occur with a nonzero site preference. High‐N₂O stations, located at the periphery of a mesoscale eddy, were defined by N₂O reaching 126.07 ± 12.6 nM and low oxygen concentrations expanding into near‐surface isopycnals. At these stations, model results indicated significant N₂O production from ammonia‐oxidizing archaea and denitrification from nitrate at the N₂O maximum within the oxycline, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N₂O production mechanisms to a mesoscale eddy through isotopocule measurements, suggesting the importance of eddies to spatiotemporal variability in N₂O cycling and emissions from this region.

 

Marine oxygen‐deficient zones represent a natural source of nitrous oxide (N₂O), a potent greenhouse gas and ozone‐depleting agent. To investigate controls on N₂O production, the responses of ammonia oxidation (AO) to nitrite () and N₂O with respect to oxygen (O₂), ammonium () and concentrations were evaluated using tracer incubations in the Eastern Tropical North Pacific. Within the oxycline, additions of and O₂stimulated N₂O production according to Michaelis–Menten kinetics, indicating that both substrates were limiting, and that N₂O production, even if the exact mechanisms remain uncertain, is mediated by predictable kinetics. Low half‐saturation constants for (12–28 nM) and O₂(460 ± 130 nM) during N₂O production indicate that AO communities are well adapted to low concentrations of both substrates. Hybrid N₂O formation (i.e., from one and one unlabeled nitrogen (N) source, e.g., , NO) accounted for ~ 90% of the N₂O production from and was robust across the different O₂, , and conditions. Lack of response to variable substrate concentrations implies that the unlabeled N source was not limiting for N₂O production. Although both O₂and were key modulators of N₂O production rates, N₂O yield (N₂O produced per produced) seemed to be controlled solely by O₂. The N₂O yield increased when O₂concentrations dropped below the half‐saturation concentration for AO to (< 1.4 μM), the range where production decreased faster than N₂O production. Our study shows that O₂control on N₂O yield from AO is robust across stations and depths.

 

Nitrous oxide (N₂O), a potent greenhouse gas, is produced disproportionately in marine oxygen deficient zones (ODZs). To quantify spatiotemporal variation in N₂O cycling in an ODZ, we analyzed N₂O concentration and isotopologues along a transect through the eastern tropical North Pacific (ETNP). At several stations along this transect, N₂O concentrations reached a near surface maximum that exceeded prior measurements in this region, of up to 226.1 ± 20.5 nM at the coast. Above theσ_θ = 25.0 kg/m³isopycnal, Keeling plot analysis revealed two sources to the near‐surface N₂O maximum, with different δ¹⁵N₂O^αand δ¹⁵N₂O^βvalues, but each with a site preference (SP) of 6‰–8‰. Given expected SPs for nitrification and denitrification, each of these sources could be comprised of 17%–26% nitrification (bacterial or archeal), and 74%–83% denitrification (or nitrifier‐denitrification). Below theσ_θ = 25.0 kg/m³isopycnal, box model analysis indicated that the observed 46‰–50‰ SPs in the anoxic core of the ODZ cannot be reproduced in a steady state context without an SP for N₂O production by denitrification, and may indicate instead a transient net consumption of N₂O. Furthermore, time‐dependent model results indicated that while δ¹⁵N₂O^αand δ¹⁸O‐N₂O reflect both N₂O production and consumption in the anoxic core of the ODZ, δ¹⁵N₂O^βpredominantly reflects N₂O sources. Finally, we infer that the high (N₂O) observed at some stations derive from a set of conditions supporting high rates of N₂O production that have not been previously encountered in this region.