Isotopic fractionation by a fungal P450 nitric oxide reductase during the production of N2O

Microplastics removal mechanisms in constructed wetlands and their impacts on nutrient (nitrogen, phosphorus and carbon) removal: A critical review

Microplastics (MPs) are now prevalent in aquatic ecosystems, prompting the use of constructed wetlands (CWs) for remediation. However, the interaction between MPs and CWs, including removal efficiency, mechanisms, and impacts, remains a subject requiring significant investigation. This review investigates the removal of MPs in CWs and assesses their impact on the removal of carbon, nitrogen, and phosphorus. The analysis identifies crucial factors influencing the removal of MPs, with substrate particle size and CWs structure playing key roles. The review highlights substrate retention as the primary mechanism for MP removal. MPs hinder plant nitrogen uptake, microbial growth, community composition, and nitrogen-related enzymes, reducing nitrogen removal in CWs. For phosphorus and carbon removal, adverse effects of MPs on phosphorus elimination are observed, while their impact on carbon removal is minimal. Further research is needed to understand their influence fully. In summary, CWs are a promising option for treating MPs-contaminated wastewater, but the intricate relationship between MPs and CWs necessitates ongoing research to comprehend their dynamics and potential consequences.

Acidification Offset Warming-Induced Increase in N2O Production in Estuarine and Coastal Sediments

Global warming and acidification, induced by a substantial increase in anthropogenic CO2 emissions, are expected to have profound impacts on biogeochemical cycles. However, underlying mechanisms of nitrous oxide (N2O) production in estuarine and coastal sediments remain rarely constrained under warming and acidification. Here, the responses of sediment N2O production pathways to warming and acidification were examined using a series of anoxic incubation experiments. Denitrification and N2O production were largely stimulated by the warming, while N2O production decreased under the acidification as well as the denitrification rate and electron transfer efficiency. Compared to warming alone, the combination of warming and acidification decreased N2O production by 26 ± 4%, which was mainly attributed to the decline of the N2O yield by fungal denitrification. Fungal denitrification was mainly responsible for N2O production under the warming condition, while bacterial denitrification predominated N2O production under the acidification condition. The reduced site preference of N2O under acidification reflects that the dominant pathways of N2O production were likely shifted from fungal to bacterial denitrification. In addition, acidification decreased the diversity and abundance of nirS-type denitrifiers, which were the keystone taxa mediating the low N2O production. Collectively, acidification can decrease sediment N2O yield through shifting the responsible production pathways, partly counteracting the warming-induced increase in N2O emissions, further reducing the positive climate warming feedback loop.

Increased Nitrogen Loading Facilitates Nitrous Oxide Production through Fungal and Chemodenitrification in Estuarine and Coastal Sediments

Estuarine and coastal environments are assumed to contribute to nitrous oxide (N₂O) emissions under increasing nitrogen loading. However, isotopic and molecular mechanisms underlying N₂O production pathways under elevated nitrogen concentration remain poorly understood. Here we used microbial inhibition, isotope mass balance, and molecular approaches to investigate N₂O production mechanisms in estuarine and coastal sediments through a series of anoxic incubations. Site preference of the N₂O molecule increased due to increasing nitrate concentration, suggesting the changes in N₂O production pathways. Enhanced N₂O production under high nitrate concentration was not mediated by bacterial denitrification, but instead was mainly regulated by fungal denitrification. Elevated nitrate concentration increased the contribution of fungal denitrification to N₂O production by 11-25%, whereas it decreased bacterial N₂O production by 16-33%. Chemodenitrification was also enhanced by high nitrate concentration, contributing to 13-28% of N₂O production. Elevated nitrate concentration significantly mediated nirK-type denitrifiers structure and abundance, which are the keystone taxa driving N₂O production. Collectively, these results suggest that increasing nitrate concentration can shift N₂O production pathways from bacterial to fungal and chemodenitrification, which are mainly responsible for the enhanced N₂O production and have widespread implications for N₂O projections under ongoing nitrogen pollution in estuarine and coastal ecosystems.

Microplastics reduce nitrogen uptake in peanut plants by damaging root cells and impairing soil nitrogen cycling

Microplastic (MP) pollution severely impairs the sustainable development of modern agriculture. However, the mechanisms underlying the effects of MP contaminants on nutrient cycles in agroecosystems are poorly understood. In this study, we examined the impacts of two types of MPs, polypropylene (PP) and rubber crumb (RC), on nitrogen (N) transformation and N cycling in soil-peanut system. High concentrations of PP (1% w/w) and RC (1% w/w) inhibited vegetative growth and N uptake in peanut plants by damaging root cells and disturbing soil N cycling. These MPs damaged the plasma membranes of root cells and caused oxidative stress, as evidenced by the decreased number of xylem vessels, which in turn inhibited N uptake by roots. Integrated metagenomic and metabolomic analyses revealed that the differential soil metabolite levels in response to MP treatment affected the microbial community structure in the rhizosphere and the expression of key N cycling-related genes, resulting in altered N transformation and the decreased availability of N in rhizosphere soil. These findings provide the first evidence of the effects of MPs on N uptake in peanut plants and shed light on the importance of rational management of MPs for crop growth and yield in agroecosystems.

Distinct oxygen isotope fractionations driven by different electron donors during microbial nitrate reduction in lake sediments

Microbial nitrate reduction can be driven by organic carbon oxidation, as well as by inorganic electron donors, such as reduced forms of sulfur and iron. An apparent inverse oxygen isotope fractionation effect was observed during nitrate reduction in sediment incubations from five sampling sites of a freshwater lake, Hongze Lake, China. Incubations with organic and inorganic electron donor additions were performed. Especially, the inverse oxygen isotope effect was intensified after glucose addition, whereas the incubations with sulfide and Fe²⁺ showed normal fractionation factors. Nitrate reductase encoding genes, napA and narG, were analysed with metagenomics. Higher napA/narG ratios were associated with higher oxygen fractionation factors. The most abundant clade (59%) of NapA in the incubation with glucose was affiliated with Rhodocyclales. In contrast, it only accounted for 8%-9% of NapA in the incubations with sulfide and Fe²⁺ . Differences in nitrate reductases might explain different oxygen isotope effects. Our findings also suggested that large variance of O-nitrate isotope fractionations might have to be considered in the interpretation of natural isotope records.

Recent advances in impacts of microplastics on nitrogen cycling in the environment: A review

Nitrogen cycling plays a decisive role in biogeochemistry, and largely depends on microbial driven nitrogen transformation. The environmental problems caused by microplastics are becoming more serious, and the analysis and control of its pollution in the environment have become a research hotspot in the field. The nitrogen transformation and nitrogen cycling in the environment are mainly driven by microorganisms in the environment, and the existence of microplastics can affect the microbial population, abundance and type, thus affecting the transformation of nitrogen. The effect of microplastics on microorganisms involved in nitrogen transformation is briefly described. This paper mainly reviews the research progress on the impacts of microplastics on nitrogen transformation and nitrogen cycling in water, soil, sediment and sewage sludge. Microplastic type, size and concentration can cause obvious difference in the impacts of microplastics on nitrogen transformation. Then, response and mechanism of microplastics to microorganism mediated nitrogen transformation and nitrogen cycling are introduced. Processes of nitrogen transformation are affected by interfering with microorganism diversity and structure, enzyme activities and related coding genes and oxygen flux. Additionally, additives released from microplastics can also affect the microbial activity. However, mechanisms of microplastics on environmental nitrogen transformation and nitrogen cycling are not fully understood due to the lack of relevant research. There are effective strategies to evaluate complex environmental systems, prolong action time, strengthen multi factor and multi-level research, and assist molecular biology and stable isotope technology. This review article can provide valuable insights into the impact of microplastics on microorganisms mediated nitrogen transformation processes and evaluate the impact on ecological and environmental health.

N2O emission dynamics along an intertidal elevation gradient in a subtropical estuary: Importance of N2O consumption

Studying nitrous oxide (N₂O) production and consumption processes along an intertidal elevation gradient can improve the understanding of N₂O dynamics among coastal wetlands. A natural-abundance isotope technique was applied to characterize the processes responsible for N₂O emission in high, middle and low intertidal zones in the Yangtze Estuary. The results showed that N₂O emission rates in high tidal zones (0.84 ± 0.35 nmol g^-1 h^-1) were significantly higher than those in middle (0.21 ± 0.04 nmol g^-1 h^-1) and low tidal zones (0.26 ± 0.05 nmol g^-1 h^-1). Gross N₂O production and consumption rates were greater in high and low tidal zones than in middle tidal zones, whereas N₂O consumption proportions generally increased from high to low tidal zones. N₂O consumption was quite pronounced, implying that N₂O emission in estuarine wetlands accounts for only a small fraction of the total production. Higher degrees of N₂O consumption were the pivotal driver of less N₂O emission in low tidal zones. Bacterial denitrification (>84%) was the dominant pathway, although hydroxylamine (NH₂OH) oxidation/fungal denitrification contributed substantially to N₂O production in high tidal flats. The contribution to N₂O production exhibited a decrease in NH₂OH oxidation/fungal denitrification and an increase in bacterial denitrification with decreasing elevation. Changes in N₂O dynamics along the elevation gradient were affected by carbon and nitrogen substrate availabilities as well as the redox environments. Overall, our findings highlight the importance of N₂O consumption in controlling N₂O emission in intertidal wetlands, especially with higher inundation frequencies and durations.

In-depth analysis of N2O fluxes in tropical forest soils of the Congo Basin combining isotope and functional gene analysis

Primary tropical forests generally exhibit large gaseous nitrogen (N) losses, occurring as nitric oxide (NO), nitrous oxide (N₂O) or elemental nitrogen (N₂). The release of N₂O is of particular concern due to its high global warming potential and destruction of stratospheric ozone. Tropical forest soils are predicted to be among the largest natural sources of N₂O; however, despite being the world's second-largest rainforest, measurements of gaseous N-losses from forest soils of the Congo Basin are scarce. In addition, long-term studies investigating N₂O fluxes from different forest ecosystem types (lowland and montane forests) are scarce. In this study we show that fluxes measured in the Congo Basin were lower than fluxes measured in the Neotropics, and in the tropical forests of Australia and South East Asia. In addition, we show that despite different climatic conditions, average annual N₂O fluxes in the Congo Basin's lowland forests (0.97 ± 0.53 kg N ha^-1 year^-1) were comparable to those in its montane forest (0.88 ± 0.97 kg N ha^-1 year^-1). Measurements of soil pore air N₂O isotope data at multiple depths suggests that a microbial reduction of N₂O to N₂ within the soil may account for the observed low surface N₂O fluxes and low soil pore N₂O concentrations. The potential for microbial reduction is corroborated by a significant abundance and expression of the gene nosZ in soil samples from both study sites. Although isotopic and functional gene analyses indicate an enzymatic potential for complete denitrification, combined gaseous N-losses (N₂O, N₂) are unlikely to account for the missing N-sink in these forests. Other N-losses such as NO, N₂ via Feammox or hydrological particulate organic nitrogen export could play an important role in soils of the Congo Basin and should be the focus of future research.

Response of N2O production rate to ocean acidification in the western North Pacific

Ocean acidification induced by the increase of anthropogenic CO₂ emissions has a profound impact on marine organisms and biogeochemical processes.¹ The response of marine microbial activities to ocean acidification might play a crucial role in the future evolution of air-sea fluxes of biogenic gases such as nitrous oxide (N₂O), a strong greenhouse gas and the dominant stratospheric ozone-depleting substance.² Here, we examine the response of N₂O production from nitrification to acidification in a series of incubation experiments conducted in subtropical and subarctic western North Pacific. The experiments show that, when pH was reduced, the N₂O production rate during nitrification measured at subarctic stations increased significantly whereas nitrification rates remained stable or decreased. Contrary to what was previously thought, these results suggest that the effect of ocean acidification on N₂O production during nitrification and nitrification rates are likely uncoupled. Collectively these results suggest that, if seawater pH continues to decline at the same rate, ocean acidification could increase the marine N₂O production during nitrification in subarctic North Pacific by 185 to 491% by the end of the century.

In Situ Quantification of Biological N2 Production Using Naturally Occurring 15N15N

We describe an approach for determining biological N₂ production in soils based on the proportions of naturally occurring ¹⁵N¹⁵N in N₂. Laboratory incubation experiments reveal that biological N₂ production, whether by denitrification or anaerobic ammonia oxidation, yields proportions of ¹⁵N¹⁵N in N₂ that are within 1‰ of that predicted for a random distribution of ¹⁵N and ¹⁴N atoms. This relatively invariant isotopic signature contrasts with that of the atmosphere, which has ¹⁵N¹⁵N proportions in excess of the random distribution by 19.1 ± 0.1‰. Depth profiles of gases in agricultural soils from the Kellogg Biological Station Long-Term Ecological Research site show biological N₂ accumulation that accounts for up to 1.6% of the soil N₂. One-dimensional reaction-diffusion modeling of these soil profiles suggests that subsurface N₂ pulses leading to surface emission rates as low as 0.3 mmol N₂ m^-2 d^-1 can be detected with current analytical precision, decoupled from N₂O production.

Constraining N cycling in the ecosystem model LandscapeDNDC with the stable isotope model SIMONE

The isotopic composition (ic) of soil nitrogen (N) and, more recently, the intramolecular distribution of ¹⁵ N in the N₂ O molecule (site preference, SP) are powerful instruments to identify dominant N turnover processes, and to attribute N₂ O emissions to their source processes. Despite the process information contained in the ic of N species and the associated potential for model validation, the implementation of isotopes in ecosystem models has lagged behind. To foster the validation of ecosystem models based on the ic of N species, we developed the stable isotope model for nutrient cycles (SIMONE). SIMONE uses fluxes between ecosystem N pools (soil organic N, mineral N, plants, microbes) calculated by biogeochemical models, and literature isotope effects for these processes to calculate the ic of N species. Here, we present the concept of SIMONE, apply it to simulations of the biogeochemical model LandscapeDNDC, and assess the capability of ¹⁵ N-N₂ O and, to our knowledge for the first time, SP, to constrain simulated N fluxes by LandscapeDNDC. LandscapeDNDC successfully simulated N₂ O emission, soil nitrate, and ammonium, as well as soil environmental conditions of an intensively managed grassland site in Switzerland. Accordingly, the dynamics of ¹⁵ N-N₂ O and SP of soil N₂ O fluxes as simulated by SIMONE agreed well with measurements, though ¹⁵ N-N₂ O was on average underestimated and SP overestimated (root-mean-square error [RMSE] of 8.4‰ and 7.3‰, respectively). Although ¹⁵ N-N₂ O could not constrain the N cycling process descriptions of LandscapeDNDC, the overestimation of SP indicated an overestimation of simulated nitrification rates by 10-59% at low water content, suggesting the revision of the corresponding model parameterization. Our findings show that N isotope modeling in combination with only recently available high- frequency measurements of the N₂ O ic are promising tools to identify and address weaknesses in N cycling of ecosystem models. This will finally contribute to augmenting the development of model-based strategies for mitigating N pollution.

Phylogenomics Reveal the Dynamic Evolution of Fungal Nitric Oxide Reductases and Their Relationship to Secondary Metabolism

Fungi expressing P450nor, an unconventional nitric oxide (NO) reducing cytochrome P450, are considered significant contributors to environmental nitrous oxide (N₂O) emissions. Despite extensive efforts, fungal contributions to N₂O emissions remain uncertain. For example, the majority of N₂O emitted from antibiotic-amended soil microcosms is attributed to fungal activity, yet axenic fungal cultures do not couple N-oxyanion respiration to growth and these fungi produce only minor quantities of N₂O. To assist in reconciling these conflicting observations and produce a benchmark genomic analysis of fungal denitrifiers, genes underlying denitrification were examined in >700 fungal genomes. Of 167 p450nor—containing genomes identified, 0, 30, and 48 also harbored the denitrification genes narG, napA, or nirK, respectively. Compared with napA and nirK, p450nor was twice as abundant and exhibited 2–5-fold more gene duplications, losses, and transfers, indicating a disconnect between p450nor presence and denitrification potential. Furthermore, cooccurrence of p450nor with genes encoding NO-detoxifying flavohemoglobins (Spearman r = 0.87, p = 1.6e⁻¹⁰) confounds hypotheses regarding P450nor’s primary role in NO detoxification. Instead, ancestral state reconstruction united P450nor with actinobacterial cytochrome P450s (CYP105) involved in secondary metabolism (SM) and 19 (11%) p450nor-containing genomic regions were predicted to be SM clusters. Another 40 (24%) genomes harbored genes nearby p450nor predicted to encode hallmark SM functions, providing additional contextual evidence linking p450nor to SM. These findings underscore the potential physiological implications of widespread p450nor gene transfer, support the undiscovered affiliation of p450nor with fungal SM, and challenge the hypothesis of p450nor’s primary role in denitrification.

Bonding and spectroscopic analyses of N2O-CS2 and N2O-OCS heterodimer complexes and their atmospheric consequences

Different isomers of the valence isoelectronic pairs of the heterodimers N₂O-SCS and N₂O-OCS were investigated using MP2 and CCSD(T) methods with the aug-cc-pVXZ (X = D, T) basis set with anharmonic frequency calculations. Interaction energies of both the heterodimers were estimated at the CBS limit and for CCSD(T)/aug-cc-pVTZ. One isomer was located for the N₂O-SCS heterodimer. In the case of the N₂O-OCS heterodimer, three planar slipped-parallel isomers were characterized. There was no significant change in electron density at the bond critical points (BCPs) of various intra-bonds but the presence of inter-bonds with a weak electron density at the BCPs in the complexes was found as per the results from the quantum theory of atoms in molecules (QTAIM) analysis. The interorbital interactions of the monomer in the heterodimers are discussed herein in terms of the natural bond orbital (NBO) charge transfer. Symmetry-adapted perturbation analysis was performed for all the isomers. In the case of the stable complex of N₂O-SCS, induction dominated over the dispersion energy. Among the N₂O-OCS isomers, the dispersion effect predominates for the most stable isomer, while for the other two isomers, the induction effect is dominant. Frequency calculations for these complexes showed a number of interactions induced by the low frequency modes in the far IR region. The atmospheric implications are also discussed for these complexes.

Evidence for fungal and chemodenitrification based N2O flux from nitrogen impacted coastal sediments

Although increasing atmospheric nitrous oxide (N₂O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N₂O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N₂O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N₂O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N₂O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N₂O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N₂O fluxes, especially in redox-dynamic sediments of coastal ecosystems.

Isotopocule analysis of biologically produced nitrous oxide in various environments

Natural abundance ratios of isotopocules, molecules that have the same chemical constitution and configuration, but that only differ in isotope substitution, retain a record of a compound's origin and reactions. A method to measure isotopocule ratios of nitrous oxide (N₂ O) has been established by using mass analysis of molecular ions and fragment ions. The method has been applied widely to environmental samples from the atmosphere, ocean, fresh water, soils, and laboratory-simulation experiments. Results show that isotopocule ratios, particularly the ¹⁵ N-site preference (difference between isotopocule ratios ¹⁴ N¹⁵ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O and ¹⁵ N¹⁴ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O), have a wide range that depends on their production and consumption processes. Observational and laboratory studies of N₂ O related to biological processes are reviewed and discussed to elucidate complex material cycles of this trace gas, which causes global warming and stratospheric ozone depletion. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:135-160, 2017.

Acidification Enhances Hybrid N2O Production Associated with Aquatic Ammonia-Oxidizing Microorganisms

Ammonia-oxidizing microorganisms are an important source of the greenhouse gas nitrous oxide (N₂O) in aquatic environments. Identifying the impact of pH on N₂O production by ammonia oxidizers is key to understanding how aquatic greenhouse gas fluxes will respond to naturally occurring pH changes, as well as acidification driven by anthropogenic CO₂. We assessed N₂O production rates and formation mechanisms by communities of ammonia-oxidizing bacteria (AOB) and archaea (AOA) in a lake and a marine environment, using incubation-based nitrogen (N) stable isotope tracer methods with ¹⁵N-labeled ammonium (¹⁵NH4+) and nitrite (¹⁵NO2−), and also measurements of the natural abundance N and O isotopic composition of dissolved N₂O. N₂O production during incubations of water from the shallow hypolimnion of Lake Lugano (Switzerland) was significantly higher when the pH was reduced from 7.54 (untreated pH) to 7.20 (reduced pH), while ammonia oxidation rates were similar between treatments. In all incubations, added NH4+ was the source of most of the N incorporated into N₂O, suggesting that the main N₂O production pathway involved hydroxylamine (NH₂OH) and/or NO2− produced by ammonia oxidation during the incubation period. A small but significant amount of N derived from exogenous/added ¹⁵NO2− was also incorporated into N₂O, but only during the reduced-pH incubations. Mass spectra of this N₂O revealed that NH4+ and ¹⁵NO2− each contributed N equally to N₂O by a “hybrid-N₂O” mechanism consistent with a reaction between NH₂OH and NO2−, or compounds derived from these two molecules. Nitrifier denitrification was not an important source of N₂O. Isotopomeric N₂O analyses in Lake Lugano were consistent with incubation results, as ¹⁵N enrichment of the internal N vs. external N atoms produced site preferences (25.0–34.4‰) consistent with NH₂OH-dependent hybrid-N₂O production. Hybrid-N₂O formation was also observed during incubations of seawater from coastal Namibia with ¹⁵NH4+ and NO2−. However, the site preference of dissolved N₂O here was low (4.9‰), indicating that another mechanism, not captured during the incubations, was important. Multiplex sequencing of 16S rRNA revealed distinct ammonia oxidizer communities: AOB dominated numerically in Lake Lugano, and AOA dominated in the seawater. Potential for hybrid N₂O formation exists among both communities, and at least in AOB-dominated environments, acidification may accelerate this mechanism.

Chemical formation of hybrid di-nitrogen calls fungal codenitrification into question

Removal of excess nitrogen (N) can best be achieved through denitrification processes that transform N in water and terrestrial ecosystems to di-nitrogen (N₂) gas. The greenhouse gas nitrous oxide (N₂O) is considered an intermediate or end-product in denitrification pathways. Both abiotic and biotic denitrification processes use a single N source to form N₂O. However, N₂ can be formed from two distinct N sources (known as hybrid N₂) through biologically mediated processes of anammox and codenitrification. We questioned if hybrid N₂ produced during fungal incubation at neutral pH could be attributed to abiotic nitrosation and if N₂O was consumed during N₂ formation. Experiments with gas chromatography indicated N₂ was formed in the presence of live and dead fungi and in the absence of fungi, while N₂O steadily increased. We used isotope pairing techniques and confirmed abiotic production of hybrid N₂ under both anoxic and 20% O₂ atmosphere conditions. Our findings question the assumptions that (1) N₂O is an intermediate required for N₂ formation, (2) production of N₂ and N₂O requires anaerobiosis, and (3) hybrid N₂ is evidence of codenitrification and/or anammox. The N cycle framework should include abiotic production of N₂.

Valence tautomerism in synthetic models of cytochrome P450

CytP450s have a cysteine-bound heme cofactor that, in its as-isolated resting (oxidized) form, can be conclusively described as a ferric thiolate species. Unlike the native enzyme, most synthetic thiolate-bound ferric porphyrins are unstable in air unless the axial thiolate ligand is sterically protected. Spectroscopic investigations on a series of synthetic mimics of cytP450 indicate that a thiolate-bound ferric porphyrin coexists in organic solutions at room temperature (RT) with a thiyl-radical bound ferrous porphyrin, i.e., its valence tautomer. The ferric thiolate state is favored by greater enthalpy and is air stable. The ferrous thiyl state is favored by entropy, populates at RT, and degrades in air. These ground states can be reversibly interchanged at RT by the addition or removal of water to the apolar medium. It is concluded that hydrogen bonding and local electrostatics protect the resting oxidized cytP450 active site from degradation in air by stabilizing the ferric thiolate ground state in contrast to its synthetic analogs.

Isotopic evidence for nitrous oxide production pathways in a partial nitritation-anammox reactor

Nitrous oxide (N2O) production pathways in a single stage, continuously fed partial nitritation-anammox reactor were investigated using online isotopic analysis of offgas N2O with quantum cascade laser absorption spectroscopy (QCLAS). N2O emissions increased when reactor operating conditions were not optimal, for example, high dissolved oxygen concentration. SP measurements indicated that the increase in N2O was due to enhanced nitrifier denitrification, generally related to nitrite build-up in the reactor. The results of this study confirm that process control via online N2O monitoring is an ideal method to detect imbalances in reactor operation and regulate aeration, to ensure optimal reactor conditions and minimise N2O emissions. Under normal operating conditions, the N2O isotopic site preference (SP) was much higher than expected - up to 40‰ - which could not be explained within the current understanding of N2O production pathways. Various targeted experiments were conducted to investigate the characteristics of N2O formation in the reactor. The high SP measurements during both normal operating and experimental conditions could potentially be explained by a number of hypotheses: i) unexpectedly strong heterotrophic N2O reduction, ii) unknown inorganic or anammox-associated N2O production pathway, iii) previous underestimation of SP fractionation during N2O production from NH2OH, or strong variations in SP from this pathway depending on reactor conditions. The second hypothesis - an unknown or incompletely characterised production pathway - was most consistent with results, however the other possibilities cannot be discounted. Further experiments are needed to distinguish between these hypotheses and fully resolve N2O production pathways in PN-anammox systems.

From the ground up: global nitrous oxide sources are constrained by stable isotope values

Rising concentrations of nitrous oxide (N₂O) in the atmosphere are causing widespread concern because this trace gas plays a key role in the destruction of stratospheric ozone and it is a strong greenhouse gas. The successful mitigation of N₂O emissions requires a solid understanding of the relative importance of all N₂O sources and sinks. Stable isotope ratio measurements (δ¹⁵N-N₂O and δ¹⁸O-N₂O), including the intramolecular distribution of ¹⁵N (site preference), are one way to track different sources if they are isotopically distinct. ‘Top-down’ isotope mass-balance studies have had limited success balancing the global N₂O budget thus far because the isotopic signatures of soil, freshwater, and marine sources are poorly constrained and a comprehensive analysis of global N₂O stable isotope measurements has not been done. Here we used a robust analysis of all available in situ measurements to define key global N₂O sources. We showed that the marine source is isotopically distinct from soil and freshwater N₂O (the continental source). Further, the global average source (sum of all natural and anthropogenic sources) is largely controlled by soils and freshwaters. These findings substantiate past modelling studies that relied on several assumptions about the global N₂O cycle. Finally, a two-box-model and a Bayesian isotope mixing model revealed marine and continental N₂O sources have relative contributions of 24–26% and 74–76% to the total, respectively. Further, the Bayesian modeling exercise indicated the N₂O flux from freshwaters may be much larger than currently thought.