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

   https://doi.org/10.1029/2018GB005887  

   Ji, Qixing ; Buitenhuis, Erik ; Suntharalingam, Parvadha ; Sarmiento, Jorge L. ; Ward, Bess B. ( December 2018 , Global Biogeochemical Cycles)

   The ocean is estimated to contribute up to ~20% of global fluxes of atmospheric nitrous oxide (N₂O), an important greenhouse gas and ozone depletion agent. Marine oxygen minimum zones contribute disproportionately to this flux. To further understand the partition of nitrification and denitrification and their environmental controls on marine N₂O fluxes, we report new relationships between oxygen concentration and rates of N₂O production from nitrification and denitrification directly measured with¹⁵N tracers in the Eastern Tropical Pacific. Highest N₂O production rates occurred near the oxic‐anoxic interface, where there is strong potential for N₂O efflux to the atmosphere. The dominant N₂O source in oxygen minimum zones was nitrate reduction, the rates of which were 1 to 2 orders of magnitude higher than those of ammonium oxidation. The presence of oxygen significantly inhibited the production of N₂O from both nitrification and denitrification. These experimental data provide new constraints to a multicomponent global ocean biogeochemical model, which yielded annual oceanic N₂O efflux of 1.7–4.4 Tg‐N (median 2.8 Tg‐N, 1 Tg = 10¹² g), with denitrification contributing 20% to the oceanic flux. Thus, denitrification should be viewed as a net N₂O production pathway in the marine environment.

    

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2. Microbial N2O consumption in and above marine N2O production hotspots

   https://doi.org/10.1038/s41396-020-00861-2  

   Sun, Xin ; Jayakumar, Amal ; Tracey, John C. ; Wallace, Elizabeth ; Kelly, Colette L. ; Casciotti, Karen L. ; Ward, Bess B. ( May 2021 , The ISME Journal)

   null (Ed.)

   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. 

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3. A critical review of the 15 N 2 tracer method to measure diazotrophic production in pelagic ecosystems

   https://doi.org/10.1002/lom3.10353  

   White, Angelicque E. ; Granger, Julie ; Selden, Corday ; Gradoville, Mary R. ; Potts, Lindsey ; Bourbonnais, Annie ; Fulweiler, Robinson W. ; Knapp, Angela N. ; Mohr, Wiebke ; Moisander, Pia H. ; et al ( March 2020 , Limnology and Oceanography: Methods)

   Dinitrogen (N₂) fixation is an important source of biologically reactive nitrogen (N) to the global ocean. The magnitude of this flux, however, remains uncertain, in part because N₂fixation rates have been estimated following divergent protocols and because associated levels of uncertainty are seldom reported—confounding comparison and extrapolation of rate measurements. A growing number of reports of relatively low but potentially significant rates of N₂fixation in regions such as oxygen minimum zones, the mesopelagic water column of the tropical and subtropical oceans, and polar waters further highlights the need for standardized methodological protocols for measurements of N₂fixation rates and for calculations of detection limits and propagated error terms. To this end, we examine current protocols of the¹⁵N₂tracer method used for estimating diazotrophic rates, present results of experiments testing the validity of specific practices, and describe established metrics for reporting detection limits. We put forth a set of recommendations for best practices to estimate N₂fixation rates using¹⁵N₂tracer, with the goal of fostering transparency in reporting sources of uncertainty in estimates, and to render N₂fixation rate estimates intercomparable among studies.

    

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4. Quantifying Nitrous Oxide Cycling Regimes in the Eastern Tropical North Pacific Ocean With Isotopomer Analysis

   https://doi.org/10.1029/2020GB006637  

   Kelly, Colette L. ; Travis, Nicole M. ; Baya, Pascale A. ; Casciotti, Karen L. ( February 2021 , Global Biogeochemical Cycles)

   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.

    

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5. Nitrification and Nitrous Oxide Production in the Offshore Waters of the Eastern Tropical South Pacific

   https://doi.org/10.1029/2020GB006716  

   Santoro, Alyson E. ; Buchwald, Carolyn ; Knapp, Angela N. ; Berelson, William M. ; Capone, Douglas G. ; Casciotti, Karen L. ( February 2021 , Global Biogeochemical Cycles)

   Marine oxygen deficient zones are dynamic areas of microbial nitrogen cycling. Nitrification, the microbial oxidation of ammonia to nitrate, plays multiple roles in the biogeochemistry of these regions, including production of the greenhouse gas nitrous oxide (N₂O). We present here the results of two oceanographic cruises investigating nitrification, nitrifying microorganisms, and N₂O production and distribution from the offshore waters of the Eastern Tropical South Pacific. On each cruise, high‐resolution measurements of ammonium ([NH₄⁺]), nitrite ([NO₂⁻]), and N₂O were combined with¹⁵N tracer‐based determination of ammonia oxidation, nitrite oxidation, nitrate reduction, and N₂O production rates. Depth‐integrated inventories of NH₄⁺and NO₂⁻were positively correlated with one another and with depth‐integrated primary production. Depth‐integrated ammonia oxidation rates were correlated with sinking particulate organic nitrogen flux but not with primary production; ammonia oxidation rates were undetectable in trap‐collected sinking particulate material. Nitrite oxidation rates exceeded ammonia oxidation rates at most mesopelagic depths. We found positive correlations between archaealamoAgenes and ammonia oxidation rates and betweenNitrospina‐like 16S rRNA genes and nitrite oxidation rates. N₂O concentrations in the upper oxycline reached values of >140 nM, even at the western extent of the cruise track, supporting air‐sea fluxes of up to 1.71 μmol m⁻² day⁻¹. Our results suggest that a source of NO₂⁻other than ammonia oxidation may fuel high rates of nitrite oxidation in the offshore Eastern Tropical South Pacific and that air‐sea fluxes of N₂O from this region may be higher than previously estimated.

    

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