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Nitrous oxide (N2O) is a potent greenhouse gas and a strong ozone-depleting substance. Ocean is an important natural source of atmospheric N2O, which has attracted increasing attention recently as to understand the biogeochemical cycles of nitrogen in marine systems. Nitrification is one of the major N2O production pathways during which N2O is produced as a side product and hydroxylamine (NH2OH) is produced as an obligate intermediate. However, few studies have reported the NH2OH variability in natural seawater and its relationship between N2O remains ambiguous. In summer 2022, spatial distribution and influencing factors of N2O and NH2OH in the northern South China Sea (SCS) were investigated. Dissolved N2O varied from 5.79 to 34.80 nmol L-1 in water column and generally increased with water depth above 800 m. N2O was correlated with dissolved oxygen and nitrate, inferring that N2O was mainly produced via nitrification. NH2OH varied from below detection limit to 3.91 nmol L-1. The adoption of multiple standard additions can largely reduce the bias in NH2OH measurement. Structural equation modeling showed that NH2OH has a potential effect favoring excessive N2O. Surface N2O saturation ranged between 102% and 278%, indicating that the northern SCS was a net source of atmospheric N2O in summer. Hydroxylamine (NH2OH) is an important intermediate product and a precursor of N2O in nitrification, but debates persist regarding NH2OH variability and N2O production. We investigated the distribution, variation and regulatory mechanism of dissolved N2O and NH2OH in the northern SCS, analyzed their potential relationship during nitrification, and provided a new perspective for the greenhouse gas research in the future. We have also proved that a shift in recovery factor can cause large error and thus necessitate the introduction of multiple calibration curves in NH2OH measurement. We have found a weak but positive correlation between NH2OH and triangle N2O, indicating the poten

Anthropogenic input of excess nutrients stimulates massive nitrous oxide (N2O) production in estuaries with distinct seasonal variations. Here, nitrogen isotopic and isotopomeric signatures were utilized to investigate the seasonal dynamics of N2O production and nitrification at the middle reach of the eutrophic Pearl River Estuary in the south of China. Elevated N2O production primarily via ammonia oxidation (> 1 nM-N d-1) occurred from April to November, along with increased temperature and decreased dissolved oxygen concentration. This consistently oxygenated water column showed active denitrification, contributing 20-40 % to N2O production. The water column microbial N2O production generally constituted a minor fraction (10-15 %) of the estuarine water-air interface efflux, suggesting that upstream transport and tidal dilution regulated the dissolved N2O inventory in the middle reach of the estuary. Nitrification (up to 3000 nM-N d-1) played a critical role in bioavailable nitrogen conversion and N2O production, albeit with N2O yields below 0.05 %.

Nitrous oxide (N2O) is a strong greenhouse gas with ozone layer destruction ability, and its atmospheric concentration has been increasing rapidly due to anthropogenic activities. N2O reduction to dinitrogen (N2), the last step of denitrification, was recognized as the only biological N2O sink. Recently, diazotrophic N2O assimilation to organic nitrogen in biomass by nitrogenase has been discovered in the eastern South Pacific Ocean and cultured diazotroph Crocosphaera and Trichodesmium. N2O assimilation to organic nitrogen is thermodynamically more favored than N2 fixation in higher N2O concentration and cooler environments, but the distribution and detailed mechanism of this new N2O sink are still unclear. We applied isotopic tracing experiments to validate and measure N2O assimilation and built an enzymatic kinetics model for a mechanistic explanation. Cultured diazotroph Crocosphaera (WH8501) and Trichodesmium (IMS101) both showed evident N2O assimilation rates of 0.751 nM N h-1 for Crocosphaera at [N2O]/[N2] = 0.0075, 0.690 nM N h-1 for Trichodesmium at [N2O]/[N2] = 0.01, and 0.481 nM N h-1 for Trichodesmium at [N2O]/[N2] = 0.0005. Although N2O assimilation was assumed to be carried out by nitrogenase, it was asynchronous with the diel rhythmicity of N2 fixation. Field samples from the Pearl River Estuary did not demonstrate the presence of N2O assimilation. Since N2 fixation was absent as well, the isotopic tracer 46N2O barely introduced influences on nitrogen isotopic composition compared to photosynthesis and remineralization, indicating that N2O assimilation is an insignificant N2O sink in eutrophic estuarine waters. Our enzymatic kinetic model revealed that N2 rather than N2O dominated the overall growth rates of cultured diazotrophs. The model indicated the [N2O]/[N2] required for the presence of N2O assimilation in isotopic tracing experiments and explained the absence of this process under natural N2 concentration environments. The insights from this st

Salt marsh sediments are known hotspots for nitrogen cycling, including the production and consumption of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting agent. Coastal eutrophication, particularly elevated nitrogen loading from the application of fertilizers, is accelerating nitrogen cycling processes in salt marsh sediments. Here, we examine the impact of long-term fertilization on nitrogen cycling processes with a focus on N2O dynamics in a New England salt marsh. By combining 15N-tracer experiments with numerical modeling, we found that both nitrification and denitrification contribute to net N2O production in fertilized sediments. Long-term fertilization increased the relative importance of nitrification to N2O production, likely a result of increased oxygen penetration from nutrient-induced increases in marsh elevation. Substrate utilization rates of key nitrogen cycling processes revealed links between functions and the corresponding microbial communities. Higher specific substrate utilization rates leading to N2O production from nitrification in fertilized sediments indicate a shift in the community composition of ammonia oxidizers, whereas the lack of change in specific substrate utilization of N2O production from denitrification under long-term fertilization suggests resilience of the denitrifying communities. Both are consistent with previous studies on the functional gene community composition in these experimental plots.

The open ocean is a major source of nitrous oxide (N2O), an atmospheric trace gas attributable to global warming and ozone depletion. Intense sea-to-air N2O fluxes occur in major oceanic upwelling regions such as the eastern tropical South Pacific (ETSP). The ETSP is influenced by the El Niño–Southern Oscillation that leads to inter-annual variations in physical, chemical, and biological properties in the water column. In October 2015, a strong El Niño event was developing in the ETSP; we conduct field observations to investigate (1) the N2O production pathways and associated biogeochemical properties and (2) the effects of El Niño on water column N2O distributions and fluxes using data from previous non-El Niño years. Analysis of N2O natural abundance isotopomers suggested that nitrification and partial denitrification (nitrate and nitrite reduction to N2O) were occurring in the near-surface waters; indicating that both pathways contributed to N2O effluxes. Higher-than-normal sea surface temperatures were associated with a deepening of the oxycline and the oxygen minimum layer. Within the shelf region, surface N2O supersaturation was nearly an order of magnitude lower than that of non-El Niño years. Therefore, a significant reduction of N2O efflux (75 %–95 %) in the ETSP occurred during the 2015 El Niño. At both offshore and coastal stations, the N2O concentration profiles during El Niño showed moderate N2O concentration gradients, and the peak N2O concentrations occurred at deeper depths during El Niño years; this was likely the result of suppressed upwelling retaining N2O in subsurface waters. At multiple stations, water-column inventories of N2O within the top 1000 m were up to 160 % higher than those measured in non-El Niño years, indicating that subsurface N2O during El Niño could be a reservoir for intense N2O effluxes when normal upwelling is resumed after El Niño.

The Eastern Tropical North Pacific Ocean hosts one of the world's largest oceanic oxygen deficient zones (ODZs). Hot spots for reactive nitrogen (N-r) removal processes, ODZs generate conditions proposed to promote N-r inputs via dinitrogen (N-2) fixation. In this study, we quantified N-2 fixation rates by N-15 tracer bioassay across oxygen, nutrient, and light gradients within and adjacent to the ODZ. Within subeuphotic oxygen-deplete waters, N-2 fixation was largely undetectable; however, addition of dissolved organic carbon stimulated N-2 fixation in suboxic (<20 mu mol/kg O-2) waters, suggesting that diazotroph communities are likely energy limited or carbon limited and able to fix N-2 despite high ambient concentrations of dissolved inorganic nitrogen. Elevated rates (>9 nmol N center dot L-1 center dot day(-1)) were also observed in suboxic waters near volcanic islands where N-2 fixation was quantifiable to 3,000 m. Within the overlying euphotic waters, N-2 fixation rates were highest near the continent, exceeding 500 mu mol N center dot m(-2)center dot day(-1) at one third of inshore stations. These findings support the expansion of the known range of diazotrophs to deep, cold, and dissolved inorganic nitrogen-replete waters. Additionally, this work bolsters calls for the reconsideration of ocean margins as important sources of N-r. Despite high rates at some inshore stations, regional N-2 fixation appears insufficient to compensate for N-r loss locally as observed previously in the Eastern Tropical South Pacific ODZ.

Salt marshes provide many key ecosystem services that have tremendous ecological and economic value. One critical service is the removal of fixed nitrogen from coastal waters, which limits the negative effects of eutrophication resulting from increased nutrient supply. Nutrient enrichment of salt marsh sediments results in higher rates of nitrogen cycling and, commonly, a concurrent increase in the flux of nitrous oxide, an important greenhouse gas. Little is known, however, regarding controls on the microbial communities that contribute to nitrous oxide fluxes in marsh sediments. To address this disconnect, we generated profiles of microbial communities and communities of micro-organisms containing specific nitrogen cycling genes that encode several enzymes (amoA, norB, nosZ) related to nitrous oxide flux from salt marsh sediments. We hypothesized that communities of microbes responsible for nitrogen transformations will be structured by nitrogen availability. Taxa that respond positively to high nitrogen inputs may be responsible for the elevated rates of nitrogen cycling processes measured in fertilized sediments. Our data show that, with the exception of ammonia-oxidizing archaea, the community composition of organisms involved in the production and consumption of nitrous oxide was altered under nutrient enrichment. These results suggest that previously measured rates of nitrous oxide production and consumption are likely the result of changes in community structure, not simply changes in microbial activity.

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 with 15N 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.

Nitrous oxide (N2O) is a greenhouse gas and an ozone depletion agent. Estuaries that are subject to seasonal anoxia are generally regarded as N2O sources. However, insufficient understanding of the environmental controls on N2O production results in large uncertainty about the estuarine contribution to the global N2O budget. Incubation experiments with nitrogen stable isotope tracer were used to investigate the geochemical factors controlling N2O production from denitrification in the Chesapeake Bay, the largest estuary in North America. The highest potential rates of water column N2O production via denitrification (7.5 ± 1.2 nmolN L−1 h −1 ) were detected during summer anoxia, during which oxidized nitrogen species (nitrate and nitrite) were absent from the water column. At the top of the anoxic layer, N2O production from denitrification was stimulated by addition of nitrate and nitrite. The relative contribution of nitrate and nitrite to N2O production was positively correlated with the ratio of nitrate to nitrite concentrations. Increased oxygen availability, up to 7 µmol L−1 oxygen, inhibited both N2O production and the reduction of nitrate to nitrite. In spring, high oxygen and low abundance of denitrifying microbes resulted in undetectable N2O production from denitrification. Thus, decreasing the nitrogen input into the Chesapeake Bay has two potential impacts on the N2O production: a lower availability of nitrogen substrates may mitigate short-term N2O emissions during summer anoxia; and, in the long-run (timescale of years), eutrophication will be alleviated and subsequent reoxygenation of the bay will further inhibit N2O production.

The ocean is a major source of atmospheric nitrous oxide (N2O), an important greenhouse gas and ozone-depleting agent. The oceanic flux of N2O varies regionally, and in the midlatitude North Atlantic, the production of N2O is poorly constrained. Incubation experiments with 15N-ammonium and 15N-nitrite revealed active N2O production from ammonium oxidation and nitrite reduction in the surface ocean, suggesting the midlatitude North Atlantic could be a net source for N2O, with a flux density of 0.06 µmol-N2O m−2 d−1 in the top 120 m. The peak of N2O production was detected in the upper 100 m, shallower than the depth at which highest rates of ammonium oxidation to nitrite occurred. Oxygen was not depleted in the water column, but its concentration minimum corresponded to highest N2O oversaturation and low in situ N2O production. The apparent N2O yield, i.e., the molar ratio of N2O-N production over nitrite production was 1.7% at the peak N2O production depths in the surface layer and decreased to less than 0.1% at peak ammonium oxidation depths. The majority of N2O production was apparently through “hybrid formation,” in which ammonium and nitrite each contribute one nitrogen atom to N2O formation, a process that is proposed to be mediated by ammonia oxidizing archaea.

Salt marshes provide numerous valuable ecological services. In particular, nitrogen (N) removal in salt marsh sediments alleviates N loading to the coastal ocean. N removal reduces the threat of eutrophication caused by increased N inputs from anthropogenic sources. It is unclear, however, whether chronic nutrient over-enrichment alters the capacity of salt marshes to remove anthropogenic N. To assess the effect of nutrient enrichment on N cycling in salt marsh sediments, we examined important N cycle pathways in experimental fertilization plots in a New England salt marsh. We determined rates of nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) using sediment slurry incubations with 15 N labeled ammonium or nitrate tracers under oxic headspace (20% oxygen / 80% helium). Nitrification and denitrification rates were more than ten-fold higher in fertilized plots compared to control plots. By contrast, DNRA, which retains N in the system, was high in control plots but not detected in fertilized plots. The relative contribution of DNRA to total nitrate reduction largely depends on the carbon/nitrate ratio in the sediment. These results suggest that long-term fertilization shifts N cycling in salt marsh sediments from predominantly retention to removal. Long-term fertilization alters the relative importance of nitrate reduction pathways in salt marsh sediments: NO 3 - reduction in salt marsh sediments (PDF Download Available). Available from: https://www.researchgate.net/publication/305480944_Long-term_fertilization_alters_the_relative_importance_of_nitrate_reduction_pathways_in_salt_marsh_sediments_NO_3_-_reduction_in_salt_marsh_sediments [accessed Jun 6, 2017].

Nitrous oxide (N2O) is important to Earth's climate because it is a strong absorber of radiation and an important ozone depletion agent. Increasing anthropogenic nitrogen input into the marine environment, especially to coastal waters, has led to increasing N2O emissions. Identifying the nitrogen compounds that serve as substrates for N2O production in coastal waters reveals important pathways and helps us understand their control by environmental factors. In this study, sediments were collected from a long-term fertilization site in Great Sippewissett Marsh, Falmouth, Massachusetts. The 15N tracer incubation time course experiments were conducted and analyzed for potential N2O production and consumption rates. The two nitrogen substrates of N2O production, ammonium and nitrate, correspond to the two production pathways, nitrification and denitrification, respectively. When measurable nitrate was present, despite ambient high ammonium concentrations, denitrification was the major N2O production pathway. When nitrate was absent, ammonium became the dominant substrate for N2O production, via nitrification and coupled nitrification-denitrification. Net N2O consumption was enhanced under low oxygen and nitrate conditions. N2O production and consumption rates increased with increasing levels of nitrogen fertilization in long-term experimental plots. These results indicate that increasing anthropogenic nitrogen input to salt marshes can stimulate sedimentary N2O production via both nitrification and denitrification, whereas episodic oxygen depletion results in net N2O consumption.