N2O Consumption Research Articles

Potential benefits of dietary nitrate ingestion in healthy and clinical populations: A brief review

This article provides an overview of the current literature relating to the efficacy of dietary nitrate (NO3−) ingestion in altering aspects of cardiovascular and metabolic health and exercise capacity in healthy and diseased individuals. The consumption of NO3−-rich vegetables, such as spinach and beetroot, have been variously shown to promote nitric oxide bioavailability, reduce systemic blood pressure, enhance tissue blood flow, modulate muscle O2 utilisation and improve exercise tolerance both in normoxia and in hypoxia, as is commonly observed in a number of disease states. NO3− ingestion may, therefore, act as a natural means for augmenting performance and attenuating complications associated with limited O2 availability or transport, hypertension and the metabolic syndrome. Recent studies indicate that dietary NO3− might also augment intrinsic skeletal muscle contractility and improve the speed and power of muscle contraction. Moreover, several investigations suggest that NO3− supplementation may improve aspects of cognitive performance both at rest and during exercise. Collectively, these observations position NO3− as more than a putative ergogenic aid and suggest that increasing natural dietary NO3− intake may act as a prophylactic in countering the predations of senescence and certain cardiovascular-metabolic diseases.

Land-use and abandonment alters methane and nitrous oxide fluxes in mountain grasslands

Grasslands cover more than one fifth of total land area in Europe and contribute significantly to the total greenhouse gas budget. The impact of management and land use on the carbon cycle and carbon sequestration in grasslands has been well-studied, however effects on emissions of N2O and CH4 remain uncertain. Additionally, the majority of studies have focussed on management differences between intensively managed grasslands, with few results available for lightly managed grasslands and in particular grassland abandonment. We present N2O and CH4 flux measurements for an abandonment trajectory at low land-use intensity, comparing meadow (fertilized and cut), pasture (grazed) and abandoned (unmanaged since 1983) grassland sites located in the Austrian Alps.Mean growing season N2O fluxes were 0.07, 0.07 and − 0.13 nmol m−2 s−1 and CH4 fluxes were − 1.0, − 0.5 and − 1.6 nmol m−2 s−1 for the meadow, pasture and abandoned sites respectively. Variability for both gases at the abandoned site was dominated by ‘hot moments', while ’hot spots' dominated at the managed meadow and pasture sites. Consideration of the diurnal cycle observed at the abandoned site, linear correlations within all data sets, and principal components analyses of the full data set revealed increased consumption of both N2O and CH4 with increasing temperature, but hardly any relationship between fluxes and soil moisture. Upscaled over a year, the observed fluxes correspond to enhanced non-CO2 greenhouse gas uptake of 172 g CO2-equiv. m−2 y−1 following abandonment. These results show that non-CO2 greenhouse gases form an important part of the total climate impact of land use change and grassland abandonment, such that abandoned grassland is a net sink for both CH4 and N2O.

Nitrification and coupled nitrification-denitrification at shallow depths are responsible for early season N2O emissions under alternate wetting and drying management in an Italian rice paddy system

There is increasing pressure to reduce water use in irrigated rice production to save water, reduce methane emissions and reduce grain arsenic uptake arising from anaerobic conditions. However, under such practices emissions of nitrous oxide (N2O) often increase. Rice systems generally exhibit strong stratification of environmental conditions that drive the balance between N2O production and consumption, and ultimately the emissions of N2O. We investigated how the introduction of alternate wetting and drying (AWD) relative to conventional flood (FLD) irrigation modifies the depth distribution of environmental conditions and nutrients (NO3−, NH4+, dissolved organic carbon, soil redox (Eh) and water filled pore space, (WFPS)). We then examined how these variables related to N2O production and consumption via the measurement of δ15N-N2Oemitted/poreair, δ15N-NO3−, N2Oturnover and subsurface N2O fluxes at five depths (5, 12.5, 25, 50 and 80 cm). These measurements, together with N2O surface emissions were taken on six days surrounding a broadcast urea fertilizer application and for six days surrounding the onset of final drainage. The highest emissions were observed in the AWD treatment at the onset of measurements. These emissions were driven by high NH4+ availability and could mainly be attributed to nitrification directly or indirectly via coupled nitrification-denitrification in the upper depths. In both irrigation treatments, an increase in NO3− and dissolved N2O concentrations and a drop in δ15N-NO3− values indicated rapid and ephemeral nitrification following the fertilization, but without significant effects on N2O surface emissions. At 50 and 80 cm, δ15N-N2Oporeair was enriched relative to upper depths, pointing to N2O reduction at these depths in both treatments. We conclude that the increased N2O emissions under AWD compared to FLD management were associated with enhanced nitrification in the upper soil layers during plant establishment and thus related to basal N fertilization and mineralization of native soil N rather than in-season fertilization.

Depth-dependent greenhouse gas production and consumption in an upland cropping system in northern China

Vertical profiles of greenhouse gas (GHG) production and consumption within soils have not been carefully quantified. The objective of this study was to quantify the depth-dependent contributions of CO2, CH4 and N2O fluxes in the soil profile to soil surface gas exchange. We simultaneously measured the soil surface GHG emissions and the subsurface fluxes (0–115cm) in situ by using a static chamber-based method (CM) and a concentration gradient-based method (GM) respectively, over two-year period in a maize-based upland cropping system in northern China.We found that unfertilized maize-based farmland acted as CO2 sources and CH4 and N2O sinks. Soil surface respiration was mostly contributed by the 0–15cm horizon; while CH4 and N2O consumption originated from the 0–40 and 0–15cm soil horizons, respectively. Specifically, we revealed that the soil surface respiration was contributed by the 0–5 and 5–15cm horizons, accounting for 70.9 and 27.3% of the surface exchange, respectively. The CH4 consumption at 0–5, 5–15 and 15–40cm depths accounted for 54.1, 32.3 and 12.1% of the surface exchange, respectively. And the N2O consumption at 0–5 and 5–15cm depths accounted for 80.4 and 6.6% of the surface exchange, respectively. The subsoil below 15cm acted largely as a CO2 buffer; the production/consumption potentials of CH4 and N2O were very weak below 40 and 15cm depths, respectively. In conclusion, our results highlight that the topsoil (0–40cm) plays a critical role in CO2 production and CH4 and N2O consumption in an unfertilized maize-based farmland in Taihang mountain areas of northern China. However, the mechanisms responsible for changes in stored greenhouse gas within soil pore space are not clear, and further observational and experimental research is required to understand those processes.

Enzymatic nitrous oxide emissions from wastewater treatment

Nitrous oxide (N2O), a potent greenhouse gas, is emitted during nitrogen removal in wastewater treatment, significantly contributing to greenhouse effect. Nitrogen removal generally involves nitrification and denitrification catalyzed by specific enzymes. N2O production and consumption vary considerably in response to specific enzyme-catalyzed nitrogen imbalances, but the mechanisms are not yet completely understood. Studying the regulation of related enzymes’ activity is essential to minimize N2O emissions during wastewater treatment. This paper aims to review the poorly understood related enzymes that most commonly involved in producing and consuming N2O in terms of their nature, structure and catalytic mechanisms. The pathways of N2O emission during wastewater treatment are briefly introduced. The key environmental factors influencing N2O emission through regulatory enzymes are summarized and the enzyme-based mechanisms are revealed. Several enzymebased techniques for mitigating N2O emissions directly or indirectly are proposed. Finally, areas for further research on N2O release during wastewater treatment are discussed.

The denitrification paradox: The role of O2 in sediment N2O production

We designed a novel laboratory sediment flux chamber in which we maintained the headspace O2 partial pressure at preselected values, allowing us to experimentally regulate “in-situ” O2 to evaluate its role in net N2O production by an intertidal estuarine sediment (Tyne, UK). In short-term (30 h) incubations with 10 L of overlying estuarine water (∼3 cm depth) and headspace O2 regulation (headspace: sediment/water ratio ∼9:1), net N2O production was highest at 1.2% O2 (sub-oxic; 32.3 nmol N2O m−2 d−1), an order of magnitude higher than at either 0.0% (anoxic; 2.5 N2O nmol m−2 d−1) or 20.85% (ambient; 2.3 nmol N2O m−2 d−1) O2. In a longer-term sealed incubation (∼490 h) without O2 control, time-dependent behaviour of N2O in the tank headspace was highly non-linear with time, showing distinct phases: (i) an initial period of no or little change in O2 or N2O up to ∼ 100 h; (ii) a quasi-linear, inverse correlation between O2 and N2O to ∼360 h, in which O2 declined to ∼2.1% and N2O rose to ∼7800 natm; (iii) over the following 50 h a slower O2 decline, to ∼1.1%, and a more rapid N2O increase, to ∼12000 natm; (iv) over the next 24 h a slowed O2 decline towards undetectable levels and a sharp fall in N2O to ∼4600 natm; (iv) a continued N2O decrease at zero O2, to ∼3000 natm by ∼ 490 h. These results show clearly that rapid N2O consumption (∼115 nmol m−2 d−1), presumably via heterotrophic denitrification (HD), occurs under fully anoxic conditions and therefore that N2O production, which was optimal for sub-oxic O2, results from other nitrogen transformation processes. In experiments in which we amended sediment overlying water to either 1 mM NH4+ or 1 mM NO3−, N2O production rates were 2–134 nmol N2O m−2 d−1 (NH4+ addition) and 0.4–2.2 nmol N2O m−2 d−1 (NO3− addition). We conclude that processes involving NH4+ oxidation (nitrifier nitrification; nitrifier denitrification; nitrification-coupled denitrification) are principally responsible for N2O production in Tyne sediments. Highest N2O production occurred under sub-oxic headspace (∼1.2 O2%) incubations. Anoxic sediments where HD was isolated acted as periodic N2O sinks or relatively small sources of N2O. Our experimental approach thus gives valuable insight into the O2 effect on N2O fluxes from intertidal sediments.

Cryptogamic stem covers may contribute to nitrous oxide consumption by mature beech trees

Naturally produced by microbial processes in soil, nitrous oxide (N2O) is an important greenhouse gas contributing to climate change. Accordingly, there is a need to accurately quantify the capability of forest ecosystems to exchange N2O with the atmosphere. While N2O emissions from soils have been well studied, trees have so far been overlooked in N2O inventories. Here, we show that stems of mature beech trees (Fagus sylvatica) may act as a substantial sink of N2O from the atmosphere under conditions of soils consuming N2O. Consistent consumption of N2O by all stems investigated (ranging between −2.4 and −3.8 µg m−2 h−1) is a novel finding in contrast to current studies presenting trees as N2O emitters. To understand these fluxes, N2O exchange of photoautotrophic organisms associated with beech bark (lichens, mosses and algae) was quantified under laboratory conditions. All these organisms were net N2O sinks at full rehydration and temperature of 25 °C. The consumption rates were comparable to stem consumption rates measured under field conditions. Cryptogamic stem covers could be a relevant sink of N2O in European beech forests.

The role of maize plants in regulating soil profile dynamics and surface emissions of nitrous oxide in a semiarid environment

To study the effects of maize plants on nitrous oxide (N2O) fluxes from a dryland experimental farm, in situ soil profiles of N2O concentrations and surface emissions were investigated in a field experiment from 2014 to 2015 in the semiarid areas of northwestern China. The experiment included four treatments: unplanted and N-unfertilized soil (C0), unplanted soil fertilized with 225 kg N ha−1 (CN), maize-planted and N-unfertilized soil (P0), and maize-planted soil fertilized with 225 kg N ha−1 (PN). Surface N2O emissions and soil N2O concentrations at depths of 0, 10, 20, 30, 40, and 50 cm were measured weekly. Nitrogen fertilization significantly increased the soil N2O concentrations and surface emissions. Compared to the unplanted soil, the presence of maize plants significantly decreased the N2O concentrations at depths of 10–40 cm during the maize growing season. The modeled N2O fluxes at a depth of 10 cm presented a similar pattern to the chamber measurements. However, there was a discrepancy between the concentration gradient and chamber methods when the fluxes were high, mainly because the gradient method could not detect N2O production and consumption process above the uppermost gas sampler (0–10 cm). Soil moisture and temperature were critical factors affecting the N2O concentrations and surface emissions. The respective cumulative surface emissions and effluxes at a depth of 10 cm during the maize growing season (PN treatment) were decreased by 8.9 and 17.9% in 2014 and by 14.7 and 17.5% in 2015 compared to values of the CN treatment. This was mainly due to the decrease in soil moisture caused by the growth of the maize plants, which resulted in a soil condition less suitable for N2O production.

Nitrogen and oxygen isotope measurements of nitrate along the US GEOTRACES Eastern Pacific Zonal Transect (GP16) yield insights into nitrate supply, remineralization, and water mass transport

In this study, stable isotope measurements of nitrate (δ15NNO3 and δ18ONO3) and suspended particulate nitrogen (δ15NSPN, > 51 μm) were made on the 2013 US GEOTRACES Eastern Pacific Zonal Transect (GP16). This cruise extended from the nutrient-rich coastal waters of the Peru upwelling system to the oligotrophic waters of the south Pacific subtropical gyre, where low surface nitrate (NO3−) concentrations (< 4 μM) and high δ15NNO3 and δ18ONO3 values (up to 28‰ and 25‰, respectively) were generated from consumption of NO3− by phytoplankton. Analysis of δ15NNO3 and δ18ONO3 patterns in surface waters relative to δ15NSPN provided insight into supply of nutrients to surface waters along the transect and remineralization at depth. In the subsurface oxygen deficient zone (ODZ), dissimilatory NO3− reduction to nitrite (NO2−) generated distinct elevations in δ15NNO3 and δ18ONO3 (up to ~ 37‰ and 31‰, respectively). Below the oxygen deficient zone, δ15NNO3 values (6 to 10‰) in the intermediate waters (σ0 = 27.0 to 27.5 kg m− 3; 500 m to 1500 m) were elevated relative to deep waters across the entire length of GP16, while intermediate δ18ONO3 values (2 to 3‰) were more similar to deep water values. These observations suggested that both NO3− regeneration and the transport of NO3− from the ODZ influence the intermediate waters of the central and eastern south Pacific. In order to explore this further, results from the GP16 water mass analysis and a NO3− isotope mixing model were employed to quantify the importance of those processes in the thermocline and intermediate waters across GP16. The contribution of NO3− from the ODZ contributed up to 10 μM across the thermocline, but generally < 5 μM in the intermediate waters. In contrast, it was estimated that remineralization contributed 5 to 15 μM of NO3− across the thermocline and intermediate waters. Overall, these results from GP16 confirm that NO3− reduction in the ODZ and organic matter remineralization play important roles in maintaining the widespread elevation in δ15NNO3 observed across the thermocline and intermediate waters of the eastern South Pacific.

Effect of COD/N ratio on N2O production during nitrogen removal by aerobic granular sludge.

N2O-production was investigated during nitrogen removal using aerobic granular sludge (AGS) technology. A pilot sequencing batch reactor (SBR) with AGS achieved an effluent in accordance with national discharge limits, although presented a nitrite accumulation rate of 95.79% with no simultaneous nitrification-denitrification. N2O production was 2.06 mg L-1 during the anoxic phase, with N2O emission during air pulses and the aeration phase of 1.6% of the nitrogen loading rate. Batch tests with AGS from the pilot reactor verified that at the greatest COD/N ratio (1.55), the N2O production (1.08 mgN2O-N L-1) and consumption (up to 0.05 mgN2O-N L-1), resulted in the lowest remaining dissolved N2O (0.03 mgN2O-N L-1), stripping the minimum N2O gas (0.018 mgN2O-N L-1). Conversely, the carbon supply shortage, under low C/N ratios, increased N2O emission (0.040 mgN2O-N L-1), due to incomplete denitrification. High abundance of ammonia-oxidizing and low abundance of nitrite-oxidizing bacteria were found, corroborating the fact of partial nitrification. A denitrifying heterotrophic community, represented mainly by Pseudoxanthomonas, was predominant in the AGS. Overall, the AGS showed stable partial nitrification ability representing capital and operating cost savings. The SBR operation flexibility could be advantageous for controlling N2O emissions, and extending the anoxic phase would benefit complete denitrification in cases of low C/N influents.

PH Control Enables Simultaneous Enhancement of Nitrogen Retention and N2O Reduction in Shewanella loihica Strain PV-4.

pH has been recognized as one of the key environmental parameters with significant impacts on the nitrogen cycle in the environment. In this study, the effects of pH on NO3–/NO2– fate and N2O emission were examined with Shewanella loihica strain PV-4, an organism with complete denitrification and respiratory ammonification pathways. Strain PV-4 was incubated at varying pH with lactate as the electron donor and NO3–/NO2– and N2O as the electron acceptors. When incubated with NO3– and N2O at pH 6.0, transient accumulation of N2O was observed and no significant NH4+ production was observed. At pH 7.0 and 8.0, strain PV-4 served as a N2O sink, as N2O concentration decreased consistently without accumulation. Respiratory ammonification was upregulated in the experiments performed at these higher pH values. When NO2– was used in place of NO3–, neither growth nor NO2– reduction was observed at pH 6.0. NH4+ was the exclusive product from NO2– reduction at both pH 7.0 and 8.0 and neither production nor consumption of N2O was observed, suggesting that NO2– regulation superseded pH effects on the nitrogen-oxide dissimilation reactions. When NO3– was the electron acceptor, nirK transcription was significantly upregulated upon cultivation at pH 6.0, while nrfA transcription was significantly upregulated at pH 8.0. The highest level of nosZ transcription was observed at pH 6.0 and the lowest at pH 8.0. With NO2– as the electron acceptor, transcription profiles of nirK, nrfA, and nosZ were statistically indistinguishable between pH 7.0 and 8.0. The transcriptions of nirK and nosZ were severely downregulated regardless of pH. These observations suggested that the kinetic imbalance between N2O production and consumption, but neither decrease in expression nor activity of NosZ, was the major cause of N2O accumulation at pH 6.0. The findings also suggest that simultaneous enhancement of nitrogen retention and N2O emission reduction may be feasible through pH modulation, but only in environments where C:N or NO2–:NO3– ratio does not exhibit overarching control over the NO3–/NO2– reduction pathways.

A multi‐year time‐series of N2O dynamics in a seasonally anoxic fjord: Saanich Inlet, British Columbia

AbstractWe present an 8‐yr time‐series of monthly water column N2O measurements from Saanich Inlet, a seasonally anoxic fjord in southern British Columbia. We document seasonal and inter‐annual variability in N2O concentrations driven by physical and biological forcing, and examine the relationship between N2O and O2 concentrations across the redoxcline in this system. Near‐surface N2O concentrations were typically supersaturated, and increased with depth to a maximum near the oxic‐anoxic transition. Genes associated with both the nitrification and denitrification pathways were widely distributed throughout the water column, suggesting the potential for simultaneous N2O production from nitrification and incomplete denitrification. Maximum N2O concentrations in Saanich Inlet were similar to other anoxic basins, but lower than those previously documented in coastal upwelling and open ocean oxygen minimum zones. N2O and O2 were inversely correlated throughout the water column, except below 10 μmol O2 L−1 where net N2O consumption appears to occur. In surface waters, maximum N2O concentrations and sea–air fluxes occurred during late summer/early fall, when O2 levels in the oxycline were low. Annual deep basin renewals often persisted over multiple months, supplying O2, , and N2O to the deep basin, with N2O subsequently declining to undetectable levels (&lt; 0.4 nmol N2O L−1) by late spring. An unusually weak renewal was observed during 2009, possibly linked to reduced upwelling off the coast of British Columbia during El Niño. Our results can be used as a baseline to identify the effect of longer‐term changes in Saanich Inlet, such as deoxygenation and its effect on N2O concentrations.

Spatial and phyloecological analyses of nosZ genes underscore niche differentiation amongst terrestrial N2O reducing communities

The only known biological sink for the greenhouse gas nitrous oxide (N2O) is its reduction to nitrogen gas (N2) by bacteria and archaea that possess the nosZ gene conferring this trait. Phylogenetic and genomic evidence indicates that N2O reducing communities can be divided into two genetically distinct groups, termed clade I and II. Differing abundance or diversity of each clade may have significant consequences for N2O emissions from soil. However, whether the two groups respond similarly to gradients of environmental or biotic factors in soils remains unclear. Here, we explore spatial patterns of nosZ clade I and II abundance, phylogenetic diversity and community structure across a 44-ha farm, and compare with edaphic factors and abundances of ammonia oxidizing and denitrifying communities, which are the main N2O producers in soil. Contrasting spatial distributions of the total abundance and phylogenetic diversity of each clade, as well as disparate associations with various edaphic and biotic factors indicated potential niche differentiation between the two clades. This is supported by the greater occurrence of significant phylogenetic clustering or overdispersion in clade I communities compared to clade II, indicating differences in the underlying mechanisms of community assembly for each group. Variance partitioning of phylogenetic community structure further showed that biotic factors, particularly the abundance of denitrifiers, played a more substantial role in explaining clade II community structure compared to that of clade I. Finally, identification of nosZ lineages driving differences in community structure and spatial distribution patterns of gene abundances suggests a shift in the genetic potential for N2O production and consumption dynamics across the farm.

Identification of hotspots for NO and N2O production and consumption in counter- and co-diffusion biofilms for simultaneous nitrification and denitrification

A membrane-aerated biofilm reactor (MABR) provides a counter-current substrate diffusion geometry in which oxygen is supplied from a gas-permeable membrane on which a biofilm is grown. This study hypothesized that an MABR would mitigate NO and N2O emissions compared with those from a conventional biofilm reactor (CBR). Two laboratory-scale reactors, representing an MABR and CBR, were operated by feeding synthetic industrial wastewater. The surficial nitrogen removal rate for the MABR [4.51±0.52g-N/(m2day)] was higher than that for the CBR [3.56±0.81g-N/(m2day)] (p<0.05). The abundance of β-proteobacterial ammonia-oxidizing bacteria in the MABR biofilm aerobic zone was high. The NO and N2O concentrations at the biofilm–liquid interface in the MABR were 0.0066±0.0014 and 0.01±0.0009mg-N/L, respectively, two and 28 times lower than those in the CBR. The NO and N2O production hotspots were closely located in the MABR aerobic zone.

Nitrous oxide (N2O)-reducing denitrifier-inoculated organic fertilizer mitigates N2O emissions from agricultural soils

The only known sink for nitrous oxide (N2O) is biochemical reduction to dinitrogen (N2) by N2O reductase (N2OR). We hypothesized that the application of N2O-reducing denitrifier-inoculated organic fertilizer could enhance soil N2O consumption while the disruption of nosZ genes could result in inactivation of N2O consumption. To test such hypotheses, a denitrifier-inoculated granular organic fertilizer was applied to both soil microcosms and fields. Of 41 denitrifier strains, 38 generated 30N2 in the end products of denitrification (30N2 and 46N2O) after the addition of Na15NO3 in culture condition, indicating their high N2O reductase activities. Of these 41 strains, 18 were screened in soil microcosms after their inoculation into the organic fertilizer, most of which were affiliated with Azospirillum and Herbaspirillum. These 18 strains were nutritionally starved to improve their survival in soil, and 14 starved and/or non-starved strains significantly decreased N2O emissions in soil microcosms. However, the N2O emission had not been decreased in soil microcosms after inoculating with a nosZ gene-disruptive strain, suggesting that N2O reductase activity might be essential for N2O consumption. Although the decrease of N2O was not significant at field scales, the application of organic fertilizer inoculated with Azospirillum sp. TSH100 and Herbaspirillum sp. UKPF54 had decreased the N2O emissions by 36.7% in Fluvisol and 23.4% in Andosol in 2014, but by 21.6% in Andosol in 2015 (H. sp. UKPF54 only). These results suggest that the application of N2O-reducing denitrifier-inoculated organic fertilizer may enhance N2O consumption or decrease N2O emissions in agricultural soils.

2D profiles of CO2, CH4, N2O and gas diffusivity in a well aerated soil: measurement and Finite Element Modeling

Soil gas fluxes depend on soil gas concentrations and physical properties of a soil. Taking soil samples for physical analysis into the laboratory strongly modifies soil gas concentrations and also cuts roots that sustain the activity in the rhizosphere. Since microbial processes interact with gas concentrations in soil, we need to study gas transport and production in situ.We developed a method to monitor the transport and production and consumption of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in soils in situ in a two dimensional (2D) profile using tetrafluoromethane (CF4) and sulfur hexafluoride (SF6) as tracer gases and Finite Element Modeling of soil gas transport. Continuous injection of the inert tracer gases and 2D gas sampling in a soil profile allowed for inverse modeling of the 2D profile of soil gas diffusivity. In a second step, the 2D profiles of the production and consumption of CO2, CH4, and N2O were inversely determined.Soil gas concentrations were monitored in a Scots pine stand in South-West Germany during a rain-free week in the fall. The 2D relative (so as to be independent of gas species) soil gas diffusivity profile showed large horizontal variability. Relative soil gas diffusivity was found to be anisotropic with the vertical direction greater by a factor of 1.26. Topsoil moisture decreased slowly over time resulting in an increase in relative soil gas diffusivity. The soil was found to be a source of CO2, and a net sink of CH4 and N2O, with the highest production (CO2) and consumption (CH4, N2O) occurring in the topsoil. The gas concentration and production profiles of CO2 were nearly horizontally homogenous, while those for CH4 showed larger horizontal differences. Net consumption of CH4 and net production of CO2 both increased as the soil dried. This occurred despite reverse trends for these variables in the topsoil (0–8cm depth) which were more than offset by the underlying soil becoming more active. Sensitivity tests showed that the determination of 2D profiles of soil gas diffusivity and production and consumption of CO2 and CH4 were more reliable than the estimates for N2O because the magnitudes of these for N2O were very low. Our method represents a useful tool for the analyses of soil gas flux heterogeneities and associated microbial processes within soil profiles.

Counter-diffusion biofilms have lower N2O emissions than co-diffusion biofilms during simultaneous nitrification and denitrification: Insights from depth-profile analysis

The goal of this study was to investigate the effectiveness of a membrane-aerated biofilm reactor (MABR), a representative of counter-current substrate diffusion geometry, in mitigating nitrous oxide (N2O) emission. Two laboratory-scale reactors with the same dimensions but distinct biofilm geometries, i.e., a MABR and a conventional biofilm reactor (CBR) employing co-current substrate diffusion geometry, were operated to determine depth profiles of dissolved oxygen (DO), nitrous oxide (N2O), functional gene abundance and microbial community structure. Surficial nitrogen removal rate was slightly higher in the MABR (11.0 ± 0.80 g-N/(m2 day) than in the CBR (9.71 ± 0.94 g-N/(m2 day), while total organic carbon removal efficiencies were comparable (96.9 ± 1.0% for MABR and 98.0 ± 0.8% for CBR). In stark contrast, the dissolved N2O concentration in the MABR was two orders of magnitude lower (0.011 ± 0.001 mg N2O-N/L) than that in the CBR (1.38 ± 0.25 mg N2O-N/L), resulting in distinct N2O emission factors (0.0058 ± 0.0005% in the MABR vs. 0.72 ± 0.13% in the CBR). Analysis on local net N2O production and consumption rates unveiled that zones for N2O production and consumption were adjacent in the MABR biofilm. Real-time quantitative PCR indicated higher abundance of denitrifying genes, especially nitrous oxide reductase (nosZ) genes, in the MABR versus the CBR. Analyses of the microbial community composition via 16S rRNA gene amplicon sequencing revealed the abundant presence of the genera Thauera (31.2 ± 11%), Rhizobium (10.9 ± 6.6%), Stenotrophomonas (6.8 ± 2.7%), Sphingobacteria (3.2 ± 1.1%) and Brevundimonas (2.5 ± 1.0%) as potential N2O-reducing bacteria in the MABR.

Community Composition of Nitrous Oxide Consuming Bacteria in the Oxygen Minimum Zone of the Eastern Tropical South Pacific.

The ozone-depleting and greenhouse gas, nitrous oxide (N2O), is mainly consumed by the microbially mediated anaerobic process, denitrification. N2O consumption is the last step in canonical denitrification, and is also the least O2 tolerant step. Community composition of total and active N2O consuming bacteria was analyzed based on total (DNA) and transcriptionally active (RNA) nitrous oxide reductase (nosZ) genes using a functional gene microarray. The total and active nosZ communities were dominated by a limited number of nosZ archetypes, affiliated with bacteria from marine, soil and marsh environments. In addition to nosZ genes related to those of known marine denitrifiers, atypical nosZ genes, related to those of soil bacteria that do not possess a complete denitrification pathway, were also detected, especially in surface waters. The community composition of the total nosZ assemblage was significantly different from the active assemblage. The community composition of the total nosZ assemblage was significantly different between coastal and off-shore stations. The low oxygen assemblages from both stations were similar to each other, while the higher oxygen assemblages were more variable. Community composition of the active nosZ assemblage was also significantly different between stations, and varied with N2O concentration but not O2. Notably, nosZ assemblages were not only present but also active in oxygenated seawater: the abundance of total and active nosZ bacteria from oxygenated surface water (indicated by nosZ gene copy number) was similar to or even larger than in anoxic waters, implying the potential for N2O consumption even in the oxygenated surface water.

Nitrous oxide, methane and carbon dioxide patterns and dynamics from an experimental pig mass grave

The objective of the three-year study was to examine spatial and temporal patterns of fluxes and soil pore air concentrations of methane (CH4), carbon dioxide (CO2) and nitrous oxide (N2O) from an experimental mass grave located in a temperate environment. The mass grave (5×10m) contained twenty pig carcasses at a depth of approximately 1m was compared to a plot of the same dimensions containing only disturbed soil, as well as an undisturbed plot. Soil pore air CH4 concentrations were sub-ambient (<1.8ppm) except at 75 and 100cm depths at the mass grave in years 1 and 2 but decreased in year 3. The consumption of CH4 within the aerobic soil resulted in small negative fluxes at the soil surface. Soil pore air CO2 concentration showed an increase with depth in all three plots, with the largest increase (>100,000ppm at 1m) in the mass grave, though there was a marked decrease from years 1 to 3. Surface fluxes of CO2 showed strong seasonal variations, peaking in summer. Soil pore air N2O concentration showed major increases in the mass grave, compared to the other two plots with the pattern maintained over the three years, resulting in larger surface fluxes of N2O. To establish the role of the carcasses in N2O dynamics, we incubated a soil sample containing carcass material which resulted in fast rates of N2O production and consumption. The maintenance of elevated pore air concentration and surface flux of N2O throughout the 3 years suggests that this is a long-term pattern and likely the best of the three gases to use to detect graves. Thus, we suggest that measurement of soil pore air concentrations, especially of N2O, could be a simple and effective approach to help determine the location of clandestine graves.

Shifting nitrous oxide source/sink behaviour in a subtropical estuary revealed by automated time series observations

The oceans are a major source of the potent greenhouse gas nitrous oxide (N2O) to the atmosphere. However, little information is available on how estuaries and the coastal ocean may contribute to N2O budgets, and on the drivers of N2O in aquatic environments. This study utilised five time series stations along the freshwater to marine continuum in a sub-tropical estuary in Australia (Coffs Creek, Australia). Each time series station captured N2O, radon (222Rn, a natural submarine groundwater discharge tracer), dissolved nitrogen, and dissolved organic carbon (DOC) concentrations for a minimum of 25 h. The use of automated time series observations enabled spatial and tidal-scale variability of N2O to be captured. Groundwater was highly enriched in N2O (up to 306 nM) compared to the receiving surface water. Dissolved N2O supersaturation as high as 386% (27.4 nM) was observed in the upstream freshwater and brackish water areas which represented only a small (∼13%) proportion of the total estuary area. A large area of N2O undersaturation (as low as 53% or 3.9 nM) was observed in the mangrove-dominated lower estuary. This undersaturated area likely resulted from N2O consumption due to nitrate/nitrite (NOx) limitation in mangrove sediments subject to shallow porewater exchange. Overall, the estuary was a minor source of N2O to the atmosphere as the lower mangrove-dominated estuary sink of N2O counteracted groundwater-dominated source of N2O in the upper estuary. Average area-weighted N2O fluxes at the water-air interface approached zero (0.2–0.7 μmol m−2 d−1, depending on piston velocity model used), and were much lower than nitrogen-rich Northern Hemisphere estuaries that are considered large sources of N2O to the atmosphere. This study revealed a temporally and spatially diverse estuary, with areas of N2O production and consumption related to oxygen and total dissolved nitrogen availability, submarine groundwater discharge, and uptake within mangroves.