N2O Cycling Research Articles

Sample preservation methods for nitrous oxide concentration and isotope ratio measurements in aquatic environments

AbstractThe nitrogen (N) and oxygen (O) stable isotope analysis of dissolved nitrous oxide (N2O) can provide important constraints on the sources and cycling of N2O in aquatic environments. The isotopic composition of aqueous N2O, both in field (natural abundance) or experimental (15N‐labeling) samples, however, may be altered by abiotic reactions involving nitrite () or hydroxylamine (NH2OH) and microbial activity during sample storage, if samples are not adequately preserved. Here we tested five different preservatives, mercuric chloride (HgCl2), copper sulfate (CuSO4), zinc chloride (ZnCl2), hydrochloric acid (HCl) mixed with sulfamic acid (SFA), and sodium hydroxide (NaOH), for fixing natural water samples from an estuary and a lake with different concentrations over a range of different storage times for N2O analyses. ZnCl2 and CuSO4 decreased the pH, and led to abiotic N2O production from , shifting the N2O isotopic composition significantly. Removal of with a mixture of SFA and HCl did not always prevent the alteration of the original N and O isotope composition of N2O, confirming the requirement for complete removal, and underscoring the biasing effects of at very low pH, even at trace levels. At low background, HgCl2 preservation led to robust and accurate results. However, in light of its toxicity and bioaccumulation potential in food webs, this fixative should be avoided. The addition of NaOH reduced abiotic side reactions involving , and yielded the most reliable and reproducible results for N2O concentration and isotope ratio measurements across tested settings and storage times.

High Abundances and Expression Levels of Atypical, Non‐Denitrifier N2O Reductases Drive Strong Microbial N2O Consumption Rates in a Minimally Impacted Mangrove Stand

AbstractKnowledge of the ecological mechanisms governing N2O cycling in marine sediments lags that of water columns and terrestrial soils, leaving much to be learned about how microbial community dynamics relate to variability in sediment N2O fluxes. The present study assesses these relationships across two distinct environments by focusing on the community structure and activity of N2O reducing microorganisms. The N2O sink capacity of minimally impacted Bermudian mangrove sediments was first estimated using trace‐level microsensors and profile interpretation modeling. Molecular data obtained from these sediments were then compared with those from the Northeast Subarctic Pacific (NESAP) outer continental margin, where previous measurements suggest considerable N2O effluxes. Net N2O uptake was observed for mangrove sediments under ambient and elevated dissolved inorganic nitrogen concentrations (−0.22 ± 0.15 to −0.30 ± 0.26 μmol N2O m−2 d−1), suggesting the microbial potential for N2O consumption exceeded the potential for production via combined nitrification and denitrification. Targeting of bacterial nosZI and nosII gene clusters for quantification using qPCR indicated higher abundance and expression of non‐denitrifier nosZII genes in mangrove sediments demonstrating net N2O uptake. Net N2O production in NESAP sediments was associated with higher abundance and expression of nosZI genes associated with canonical denitrifiers. These results suggest that organisms possessing atypical nosZII genes may act as important N2O scavengers in low‐nitrogen coastal sediments.

Thermodynamic, exergoeconomic evaluation and optimization of S–N2O/t-N2O nuclear power cycle for the construction of the lunar base

With the continuous development of deep space exploration, many planetary exploration schemes and development plans regard the construction of planetary bases as an essential goal, especially the exploration of the Moon. Supercritical and transcritical nitrous oxide (N2O) cycles are compact, low-cost, efficient, and lightweight for nuclear reactors to supply power to the bases on other planets. This paper presents the thermodynamic, exergoeconomic, and mass analyses of a combined cycle consisting of a supercritical N2O recompression Brayton cycle and a transcritical N2O cycle (S–N2O/t-N2O). It is shown that under the optimal conditions, the combined thermal efficiency and exergy efficiency are 45.52 % and 60.13 %, respectively. Based on the exergy analysis, the exergy destruction mainly occurs in the reactor and main compressor. A sensitivity study shows that the split ratio, pressure ratio in the supercritical N2O cycle, main compressor inlet pressure, turbine2 inlet temperature, and turbine2 inlet pressure have significant effects on the net output work, thermal efficiency, specific mass, and levelized cost of electricity. Furthermore, multi-objective optimizations are considered to obtain the Pareto frontier solutions for different multi-objectives, and the optimal design condition is found. These findings could improve the power cycle performance for the construction of the Lunar Base.

Seasonal ventilation controls nitrous oxide emission in the NW Iberian upwelling

Despite their small spatial extent, coastal upwelling systems are an important source of oceanic nitrous oxide (N2O) to the atmosphere. To date, hot-spot N2O emissions have been reported for low oxygen waters of the eastern boundary upwelling systems at their tropical latitudes, but there is a limited number of studies in their “oxygenated” temperate latitudes. This is the first study of the N2O cycle in the NW Iberian Upwelling system, where we investigated the seasonality of the N2O concentrations and their emissions to the atmosphere, along with the spatial differences in this coastal region in response to the upwelling. Monthly observations were collected from February 2017 to July 2018, in two hydrographic sections within the Ría of Vigo and Ría of A Coruña, two coastal embayments with contrasting response to the upwelling of the Eastern North Atlantic Central Water (ENACW) in the region. N2O concentrations ranged between 8.56 to 12.53 nmol kg−1 (94–121 % of saturation) in the shelf, and from 8.62 to 17.60 nmol kg−1 (94–203 % of saturation) inside the rías, with the highest N2O concentration at the bottom, which increase as the upwelling progress from April to October. The air-sea fluxes of N2O varied between −1.6 to 3.26 µmol m−2 d−1 in the shelf and −1.53 to 7.00 µmol m−2 d−1 inside the rías. Local differences on the ventilation and remineralization pattern drives the seasonality of N2O and differences between Ria of Vigo and Ria of A Coruña, being the higher values of N2O concentrations and air-sea fluxes registered in the inner Ria of Vigo. Our study reports the N2O emissions of an upwelling system in a temperate latitude, where the upwelling waters are central waters relatively well ventilated in terms of oxygen content, behaving as a moderate low net source of N2O to the atmosphere compared to tropical upwelling latitudes, characterised by a lower oxygen content.

Shifts in the Isotopic Composition of Nitrous Oxide Between El Niño and La Niña in the Eastern Tropical South Pacific

AbstractThe 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 δ15Nβ down 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.

Hydrologic and Organic Carbon Quality Controls on Nitrous Oxide Dynamics Across a Variably Confined Karst Aquifer

AbstractAtmospheric concentrations of the potent greenhouse gas nitrous oxide (N2O) have increased approximately 20% since the pre‐industrial era. Riverine systems represent an important atmospheric source of N2O however, the influence of varying hydrogeological conditions and dissolved organic carbon (DOC) quantity and quality on N2O dynamics remains poorly understood. We investigate effects of these processes on N2O dynamics in the Santa Fe River watershed in north‐central Florida, USA where a gradient in DOC quantity and quality is caused by varying surface water‐groundwater exchange. The watershed is underlain by the karstic Floridan aquifer, which is confined in the headwaters and contains terrigenous recalcitrant DOC, while the unconfined lower reaches contain protein‐like labile DOC substrates. Where extensive surface water‐groundwater exchange occurs at the boundary between the confined and unconfined aquifer, incomplete denitrification raises N2O concentration to 6.1 μg N‐N2O L−1, approximately 20 times greater than the concentration expected from equilibration with atmospheric N2O. These elevated concentrations create average local fluxes reaching ∼950 μg N‐N2O m−2 hr−1 within the mixing zone. Summation of spatially explicit emission estimates for upstream, mixing zone, and downstream river sections yield a total average emission rate of ∼3,758 kg N‐N2O yr−1. Despite having the smallest surface area across the river transect (∼8%), emissions from the mixing zone account for 26% of the total river emission rate. These findings highlight the potential contributions from karst landscapes to N2O cycling caused by varying redox conditions during surface water‐groundwater exchange, which creates hotspots of N2O production and emissions.

Impacts of vertical mixing and ice-melt on N2O and CH4 concentrations in the Canadian Arctic Ocean

The sources and sinks of methane (CH4) and nitrous oxide (N2O) in the Arctic Ocean are subject to significant uncertainty, due to a combination of high spatial and temporal variability, and limited observations over relevant scales. To address this, an underway ship-board system was developed to simultaneously and continuously measure surface water CH4 and N2O concentrations in the Canadian Arctic Ocean, providing high-resolution data over a range of hydrographic regimes. Across our survey region during late summer, 2021, CH4 and N2O saturation ranged from 94% to 231% and 96%–120%, respectively. Localized CH4 and N2O maxima were observed along the cruise track, including high concentrations over the continental shelf of eastern Baffin Island, and in some waters influenced by sea ice cover or recent glacier retreat. We use our high-resolution data to examine the relationship between N2O and CH4 concentrations and a variety of oceanographic variables, demonstrating both positive correlations with salinity, which reflects the likely influence of vertical mixing, and negative correlations with salinity, indicative of ice-melt and freshwater inputs. These results build upon previous discrete CH4 and N2O observations in the Canadian Arctic, providing new information on the fine-scale distribution of these gases, and the potential underlying biogeochemical factors driving their variability. Results from our work have implications for understanding the biogeochemical cycling of Arctic Ocean CH4 and N2O under a rapidly warming climate.

Marine N2O cycling from high spatial resolution concentration, stable isotopic and isotopomer measurements along a meridional transect in the eastern Pacific Ocean

Nitrous oxide (N2O) is a potent greenhouse gas and ozone depleting substance, with the ocean accounting for about one third of global emissions. In marine environments, a significant amount of N2O is produced by biological processes in Oxygen Deficient Zones (ODZs). While recent technological advances are making surface N2O concentration more available, high temporal and spatial resolution water-column N2O concentration data are relatively scarce, limiting global N2O ocean models’ predictive capability. We present a N2O concentration, stable isotopic composition and isotopomer dataset of unprecedently large spatial coverage and depth resolution in the broader Pacific, crossing both the eastern tropical South and North Pacific Ocean ODZs collected as part of the GO-SHIP P18 repeat hydrography program in 2016/2017. We complement these data with dissolved gases (nitrogen, oxygen, argon) and nitrate isotope data to investigate the pathways controlling N2O production in relation to apparent oxygen utilization and fixed nitrogen loss. N2O yield significantly increased under low oxygen conditions near the ODZs. Keeling plot analysis revealed different N2O sources above the ODZs under different oxygen regimes. Our stable isotopic data and relationships between the N2O added by microbial processes (ΔN2O) and dissolved inorganic nitrogen (DIN) deficit confirm increased N2O production by denitrification under low oxygen conditions near the oxycline where the largest N2O accumulations were observed. The slope for δ18O-N2O versus site preference (SP, the difference between the central (α) and outer (β) N atoms in the linear N2O molecule) in the eastern tropical North Pacific ODZ was lower than expected for pure N2O reduction, likely because of the observed decrease in δ15Nβ. This trend is consistent with prior ODZ studies and attributed to concurrent production of N2O from nitrite with a low δ15N or denitrification with a SP >0‰. We estimated apparent isotope effects for N2O consumption in the ETNP ODZ of 3.6‰ for 15Nbulk, 9.4‰ for 15Nα, -2.3‰ for 15Nβ, 12.0‰ for 18O, and 11.7‰ for SP. These values were generally within ranges previously reported for previous laboratory and field experiments.

Network analysis of 16S rRNA sequences suggests microbial keystone taxa contribute to marine N2O cycling

The mechanisms by which large-scale microbial community function emerges from complex ecological interactions between individual taxa and functional groups remain obscure. We leveraged network analyses of 16S rRNA amplicon sequences obtained over a seven-month timeseries in seasonally anoxic Saanich Inlet (Vancouver Island, Canada) to investigate relationships between microbial community structure and water column N2O cycling. Taxa separately broadly into three discrete subnetworks with contrasting environmental distributions. Oxycline subnetworks were structured around keystone aerobic heterotrophs that correlated with nitrification rates and N2O supersaturations, linking N2O production and accumulation to taxa involved in organic matter remineralization. Keystone taxa implicated in anaerobic carbon, nitrogen, and sulfur cycling in anoxic environments clustered together in a low-oxygen subnetwork that correlated positively with nitrification N2O yields and N2O production from denitrification. Close coupling between N2O producers and consumers in the anoxic basin is indicated by strong correlations between the low-oxygen subnetwork, PICRUSt2-predicted nitrous oxide reductase (nosZ) gene abundances, and N2O undersaturation. This study implicates keystone taxa affiliated with common ODZ groups as a potential control on water column N2O cycling and provides a theoretical basis for further investigations into marine microbial interaction networks.

Nitrous Oxide Consumption in Oxygenated and Anoxic Estuarine Waters

AbstractEstuaries emit a large but highly uncertain amount of Nitrous oxide (N2O) into the atmosphere. To better understand N2O cycling processes in estuaries, we provide the first direct observations of N2O consumption in the seasonally anoxic Chesapeake Bay, the largest estuary in the United States. N2O consumption rates in anoxic waters reached up to 3.3 nmol L−1 d−1 but were generally undetectable in oxygenated waters. However, N2O consumption rates were substantially enhanced when the oxygen concentration was experimentally decreased in initially oxygenated samples, indicating the potential of N2O consumption in oxygenated environments, for example, surface waters. These potential N2O consumption rates followed Michaelis‐Menten kinetics as a function of increasing N2O substrate concentration. N2O‐consuming microbes that predominantly contained the clade II nitrous oxide reductase gene were detected throughout the water column. These new observations of environmental controls on N2O consumption will benefit the modeling of N2O cycling and help to constrain the estuarine N2O flux.

Coupled abiotic-biotic cycling of nitrous oxide in tropical peatlands.

Atmospheric nitrous oxide (N2O) is a potent greenhouse gas thought to be mainly derived from microbial metabolism as part of the denitrification pathway. Here we report that in unexplored peat soils of Central and South America, N2O production can be driven by abiotic reactions (≤98%) highly competitive to their enzymatic counterparts. Extracted soil iron positively correlated with in situ abiotic N2O production determined by isotopic tracers. Moreover, we found that microbial N2O reduction accompanied abiotic production, essentially closing a coupled abiotic-biotic N2O cycle. Anaerobic N2O consumption occurred ubiquitously (pH 6.4-3.7), with proportions of diverse clade II N2O reducers increasing with consumption rates. Our findings show that denitrification in tropical peat soils is not a purely biological process but rather a 'mosaic' of abiotic and biotic reduction reactions. We predict that hydrological and temperature fluctuations differentially affect abiotic and biotic drivers and further contribute to the high N2O flux variation in the region.

Significant Seasonal N2O Dynamics Revealed by Multi‐Year Observations in the Northern South China Sea

AbstractThe coastal ocean and marginal sea play a disproportionally important role in the release of nitrous oxide (N2O) into the atmosphere. The spatial and temporal distribution of N2O in these important source regions remains highly uncertain due to the scarcity of N2O measurements. Here we present a large data set of N2O concentrations and fluxes obtained from 10 cruises covering four seasons in the Northern South China Sea (NSCS). The study area is overall a net source of atmospheric N2O with an annual flux of 1.9 ± 1.2 × 108, 0.8 ± 0.5 × 108 and 1.2 ± 0.7 × 108 mol N2O yr−1 in the shelf, slope and basin regions, respectively. In terms of global warming potentials, the N2O emissions offset 27.8% of the CO2 sink on the shelf, and are equivalent to 3.5 and 0.2‐fold of the CO2 emission in the slope and basin of the NSCS. On the seasonal time scale, N2O flux was significantly higher in autumn and winter than in the warm seasons. The spatial variability was contrastingly less pronounced. The seasonality of N2O distribution in the shelf region was modulated by the riverine discharge, while intrusion of the Kuroshio Current exerted profound control on N2O distribution in the open waters of the NSCS. The variable relationships between N2O excess, apparent oxygen utilization, and nitrate in the shelf, NSCS basin and the Luzon Strait indicated a regional difference in N2O cycling pathways along with the impact of water mass mixing. Our study establishes a robust baseline to understand N2O distribution and flux in the NSCS.

Land‐Use Intensity Increases Benthic N2O Emissions Across Three Sub‐Tropical Estuaries

AbstractEstuaries play an important role in regulating nitrous oxide (N2O) fluxes to the atmosphere, but little is known about how catchment land‐use changes influence benthic N2O cycling. We measured seasonal benthic N2O fluxes and constructed N2O budgets in three sub‐tropical estuaries draining catchments with contrasting levels of land‐use intensity. Benthic habitats were a net N2O sink in the minimally impacted Noosa River Estuary (−287 nmol m−2 h−1) and a net source of N2O in the highly impacted Brisbane River Estuary (126 nmol m−2 h−1). Vegetated habitats can act as an important sink of N2O with uptakes of −286 and −35 nmol m−2 h−1 in the Noosa and Maroochy River Estuaries, respectively. Benthic N2O fluxes were significantly correlated with benthic NO3− fluxes, suggesting NO3− availability was an important control on benthic N2O fluxes. Combining benthic flux data with surface water N2O emissions measurements showed that increased benthic N2O fluxes helped drive increasing water–air N2O emissions over the land‐use intensity gradient. Overall, our results show that land‐use driven changes to both the diversity and sediment composition of benthic habitats play an important role in regulating N2O dynamics in estuarine ecosystems. This highlights that both sediment quality and nitrogen loading need to be considered in order to reduce emissions of greenhouse gases in the coastal ecosystems.

Nitrous oxide production in the Chesapeake Bay

AbstractEstuaries at the global scale are significant but highly uncertain sources of atmospheric nitrous oxide (N2O), which is an intense greenhouse gas and ozone depletion agent. As the largest estuary in the United States, the Chesapeake Bay is suggested to be a spatially and temporally variable source and sink of N2O. However, limited observations of N2O cycling preclude us from estimating and predicting its net N2O flux. To improve our mechanistic understanding of the processes that control the N2O flux at the point of production, we applied multiple 15N tracers (, 15N‐urea, and ) to separately track N2O production from nitrification and denitrification under in situ and manipulated O2 concentrations in the Chesapeake Bay. Nitrification was the major N2O production pathway in oxic waters (up to 7.5 nmol N2O L−1 d−1). In contrast, denitrification dominated N2O production from hypoxic/anoxic waters (up to 20 nmol N2O L−1 d−1). N2O production from urea was observed for the first time in estuarine waters. The contribution from urea was small, but interestingly, showed a depth pattern distinct from other N2O precursors. Experimentally lowering the O2 concentration substantially enhanced N2O production. Therefore, the expansion of hypoxic and anoxic zones in the Chesapeake Bay under climate change as suggested by some climate models may favor the production of N2O, potentially providing positive feedback on warming. Overall, our study provides mechanistic constraints on N2O dynamics that could benefit modeling studies to better estimate the N2O flux in the Chesapeake Bay and other coastal environments.

In-depth characterization of denitrifier communities across different soil ecosystems in the tundra

BackgroundIn contrast to earlier assumptions, there is now mounting evidence for the role of tundra soils as important sources of the greenhouse gas nitrous oxide (N2O). However, the microorganisms involved in the cycling of N2O in this system remain largely uncharacterized. Since tundra soils are variable sources and sinks of N2O, we aimed at investigating differences in community structure across different soil ecosystems in the tundra.ResultsWe analysed 1.4 Tb of metagenomic data from soils in northern Finland covering a range of ecosystems from dry upland soils to water-logged fens and obtained 796 manually binned and curated metagenome-assembled genomes (MAGs). We then searched for MAGs harbouring genes involved in denitrification, an important process driving N2O emissions. Communities of potential denitrifiers were dominated by microorganisms with truncated denitrification pathways (i.e., lacking one or more denitrification genes) and differed across soil ecosystems. Upland soils showed a strong N2O sink potential and were dominated by members of the Alphaproteobacteria such as Bradyrhizobium and Reyranella. Fens, which had in general net-zero N2O fluxes, had a high abundance of poorly characterized taxa affiliated with the Chloroflexota lineage Ellin6529 and the Acidobacteriota subdivision Gp23.ConclusionsBy coupling an in-depth characterization of microbial communities with in situ measurements of N2O fluxes, our results suggest that the observed spatial patterns of N2O fluxes in the tundra are related to differences in the composition of denitrifier communities.

Identifying the Sources and Drivers of Nitrous Oxide Accumulation in the Eddy‐Influenced Eastern Tropical North Pacific Oxygen‐Deficient Zone

AbstractNitrous 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 δ15N2Oβ (<−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.

Clumped isotope signatures of nitrous oxide formed by bacterial denitrification

Multiply substituted isotopic species of nitrous oxide (N2O), referred to as clumped isotopes, represent a promising new tool for distinguishing production pathways of this potent greenhouse gas. This work presents the first determination of enrichment factors of N2O clumped isotopes during bacterial denitrification. Samples of N2O obtained after 1-, 3-, and 7-day incubations of a pure culture of the denitrifier Pseudomonas aureofaciens at 20 °C and 30 °C were analysed by the recently developed quantum cascade laser absorption spectroscopy (QCLAS) method. Enrichment factors εp/s of the cumulative product (p) relative to the substrate (s) were determined using a Rayleigh model for the seven most abundant isotopically substituted molecules (isotopocules) of N2O. Values of the enrichment factors εp/s (with uncertainty expressed as expanded standard uncertainty at the 95% confidence interval) at the two incubation temperatures (20 °C/30 °C) are:14N15N16O (456): ε456 = (−40.3 ± 2.6)‰/(−35.1 ± 0.7)‰15N14N16O (546): ε546 = (−38.1 ± 3.4)‰/(−31.2 ± 0.6)‰14N14N17O (447): ε447 = (21.3 ± 1.2)‰/(24.5 ± 0.5)‰14N14N18O (448): ε448 = (38.8 ± 1.5)‰/(46.4 ± 1.2)‰14N15N18O (458): ε458 = (−8.9 ± 2.0)‰/(−11.7 ± 0.6)‰15N14N18O (548): ε548 = (−3.4 ± 1.1)‰/(−1.8 ± 0.5)‰15N15N16O (556): ε556 = (−85.9 ± 1.5)‰/(−63.9 ± 1.4)‰Temporal evolutions of the abundances of singly substituted N2O isotopocules during nitrate reduction agree with previously published experiments: there is normal isotope effect associated with the production of 14N15N16O and 15N14N16O; i.e., intermediates leading to 14N14N16O react faster than intermediates leading to 14N15N16O and 15N14N16O. However, the production of 14N14N17O and 14N14N18O is associated with inverse isotope effect; i.e., intermediates leading to 14N14N16O react slower than intermediates leading to 14N14N17O and 14N14N18O due to preferential cleavage of 16O during nitrate reduction to N2O. Isotopic fractionation at the incubation temperature of 30 °C was significantly lower compared to 20 °C. We observed a large kinetic isotope effect of the 15N site preference (SP) and the 15N–18O site preference (SP18) at the onset of the reaction. SP18 was found to be closer to 0‰ than SP, which is thought to arise from similar rates of breakage of the 15N–O and 14N–O bonds in the reaction intermediates. The 15N–18O clumped isotope anomalies in two isotopic isomers (isotopomers) 14N15N18O and 15N14N18O (Δ458+548avg) follow a temporal trend similar to those of SP and SP18. The 15N–15N clumped isotope anomalies in 15N15N16O are greater than 0‰ and show no clear temporal trend or influence of incubation temperature, suggesting no strong combinatorial effects involved during the N–N bond formation. Overall, our data illustrate that clumped N2O isotopes may be used as independent tracers for reaction mechanisms of N2O conversion and may establish themselves as a worthwhile tool to study the biogeochemical cycle of N2O.

Dissolved Nitrous Oxide and Hydroxylamine in the South Yellow Sea and the East China Sea During Early Spring: Distribution, Production, and Emissions

Coastal marine systems are active regions for the production and emission of nitrous oxide (N2O), a potent greenhouse gas. Due to the inherently high variability in different coastal biogeochemical cycles, the factors and mechanisms regulating coastal N2O cycling remain poorly understood. Hydroxylamine (NH2OH), a potential precursor of N2O, has received less attention than other compounds in the coastal areas. Here, we present the spatial distribution of N2O and the first reported NH2OH distribution in the South Yellow Sea (SYS) and the East China Sea (ECS) between March and April 2017. The surface N2O concentrations in the SYS and the ECS varied from 5.9 to 11.3 nmol L–1 (average of 8.4 ± 1.4 nmol L–1) and were characterized by offshore and north–south decreasing gradients. NH2OH showed patchy characteristics and was highly variable, fluctuating between undetectable to 16.4 nmol L–1. We found no apparent covariation between N2O and NH2OH, suggesting the NH2OH pathway, i.e., nitrification (ammonium oxidation), was not the only process affecting N2O production here. The high NH2OH values co-occurred with the greatest chlorophyll-a and oxygen levels in the nearshore region, along with the relationships between NO2–, NO3–, and NH2OH, indicating that a “fresh” nitrifying system, favoring the production and accumulation of NH2OH, was established during the phytoplankton bloom. The high N2O concentrations were not observed in the nearshore. Based on the correlations of the excess N2O (ΔN2O) and apparent oxygen utilization, as well as ΔN2O vs. NO3–, we concluded that the N2O on the continental shelf was mainly derived from nitrification and nitrifier denitrification. Sea-to-air fluxes of N2O varied from −12.4 to 6.6 μmol m–2 d–1 (−3.8 ± 3.7 μmol m–2 d–1) using the Nightingale et al. (2000) formula and −13.3 to 6.9 μmol m–2 d–1 (−3.9 ± 3.9 μmol m–2 d–1) using the Wanninkhof (2014) formula, which corresponds to 75–112% in saturation, suggesting that the SYS and the ECS acted overall as a sink of atmospheric N2O in early spring, with the strength weakening. Our results reveal the factors and potential mechanisms controlling the production and accumulation of NH2OH and N2O in the SYS and the ECS during early spring.

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

AbstractNitrous 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 the σθ = 25.0 kg/m3 isopycnal, Keeling plot analysis revealed two sources to the near‐surface N2O maximum, with different δ15N2Oα and δ15N2Oβ values, 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 the σθ = 25.0 kg/m3 isopycnal, 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 δ15N2Oα and δ18O‐N2O reflect both N2O production and consumption in the anoxic core of the ODZ, δ15N2Oβ predominantly 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.

Nitrous oxide processing in carbonate karst aquifers

The increased environmental abundance of anthropogenic reactive nitrogen species (Nr = ammonium [NH4+], nitrite [NO2−] and nitrate [NO3−]) may increase atmospheric nitrous oxide (N2O) concentrations, and thus global warming and stratospheric ozone depletion. Nitrogen cycling and N2O production, reduction, and emissions could be amplified in carbonate karst aquifers because of their extensive global range, susceptibility to nitrogen contamination, and groundwater-surface water mixing that varies redox conditions of the aquifer. The magnitude of N2O cycling in karst aquifers is poorly known, however, and thus we sampled thirteen springs discharging from the karstic Upper Floridan Aquifer (UFA) to evaluate N2O cycling. The springs can be separated into three groups based on variations in subsurface residence times, differences in surface–groundwater interactions, and variable dissolved organic carbon (DOC) and dissolved oxygen (DO) concentrations. These springs are oxic to sub-oxic and have NO3− concentrations that range from < 0.1 to 4.2 mg N-NO3−/L and DOC concentrations that range from < 0.1 to 50 mg C/L. Maximum spring water N2O concentrations are 3.85 µg N-N2O/L or ~ 12 times greater than water equilibrated with atmospheric N2O. The highest N2O concentrations correspond with the lowest NO3− concentrations. Where recharge water has residence times of a few days, partial denitrification to N2O occurs, while complete denitrification to N2 is more prominent in springs with longer subsurface residence times. Springs with short residence times have groundwater emission factors greater than the global average of 0.0060, reflecting N2O production, whereas springs with residence times of months to years have groundwater emission factors less than the global average. These findings imply that N2O cycling in karst aquifers depends on DOC and DO concentrations in recharged surface water and subsequent time available for N processing in the subsurface.