N2O Reduction Rate Research Articles

Denitrifying communities enriched with mixed nitrogen oxides preferentially reduce N2O under conditions of electron competition in wastewater

The reduction rate of nitrous oxide (N2O) is affected by the electron competition among the four denitrifying steps, limiting the mitigation of N2O emissions during wastewater treatment. We foresee essential to understand how combinations of electron acceptors (EAs) affect the microbial composition and reduction mechanisms of denitrifying communities. We enriched three denitrifying communities from activated sludge biomass with equivalent loads of different EAs: NO3– (R1), N2O (R2), and NO3– + N2O (R3). The resulting enrichments were compared in terms of (1) reduction of nitrogen oxides in absence/presence of other EAs (NO3–, NO2–, N2O), (2) their denitrification gene composition and (3) their microbial community composition. Batch results showed the presence of NO3– and NO2– suppressed N2O reduction rates in all three reactors. The effect was lower in the mixed-substrate feed community than in the single-substrate feed under infinite sludge retention time and chemostat operation modes. N2O-reducers of type nosZ II were enriched when N2O serves as the sole EA, whereas nosZ I type N2O-reducers were more prone to enrichment with NO3– as EA. The EA composition rather than the sludge retention mode differentiated the microbial communities. The genus Flavobacterium seems to play a significant role in alleviating the suppression of the N2O reduction rate caused by electron competition. Limited conditions of electron supply are the norm independently of high C/N levels, and a community co-enriched with NO3– and N2O alleviates more the competition for electrons in the nitrous oxide reductase enzyme than communities enriched with NO3– or N2O individually.

NosZ II/nosZ I ratio regulates the N2O reduction rates in the eutrophic lake sediments

Nitrous oxide (N2O) is a more potent greenhouse gas with an atmospheric lifetime of 121 years, contributing significantly to climate change and stratospheric ozone depletion. Lakes are hotspots for N2O release due to the imbalance between N2O sources and sinks. N2O-reducing bacteria are the only biological means to mitigate N2O emission, yet their roles in lakes are not well studied. This study investigated the potential for N2O reduction, keystones of typical and atypical N2O-reducing bacterial communities, and their correlations with environmental factors in the sediments of Lake Taihu through microcosm experiments, high-throughput sequencing of the nosZ gene, and statistical modeling. The results showed that potential N2O reduction rates in sediments ranged from 13.71 to 76.95 μg N2O g−1 d−1, with lower rates in December compared to March and July. Correlation analysis indicated that the nosZ II/nosZ I ratio and the trophic lake index (TLI) were the primary factors influencing N2O reduction rates and N2O-reducing bacterial community structures. The genera Pseudogulbenkiania and Ardenticatena were identified as the most abundant typical and atypical N2O-reducing bacteria, respectively, and were also recognized as the keystone taxa. Quantitative real-time PCR (qPCR) results revealed that nosZ II was more abundant than nosZ I in the sediments. Partial least squares path modeling (PLS-PM) further demonstrated that atypical N2O-reducing bacteria had significant positive effects on N2O reduction process in the sediments (p < 0.05). Overall, this study highlights the crucial ecological roles of atypical N2O-reducing bacteria in the sediments of the eutrophic lake of Taihu, underscoring their potential in mitigating N2O emissions.

Determining how oxygen legacy affects trajectories of soil denitrifier community dynamics and N2O emissions

Denitrification – a key process in the global nitrogen cycle and main source of the greenhouse gas N2O – is intricately controlled by O2. While the transition from aerobic respiration to denitrification is well-studied, our understanding of denitrifier communities’ responses to cyclic oxic/anoxic shifts, prevalent in natural and engineered systems, is limited. Here, agricultural soil is exposed to repeated cycles of long or short anoxic spells (LA; SA) or constant oxic conditions (Ox). Surprisingly, denitrification and N2O reduction rates are three times greater in Ox than in LA and SA during a final anoxic incubation, despite comparable bacterial biomass and denitrification gene abundances. Metatranscriptomics indicate that LA favors canonical denitrifiers carrying nosZ clade I. Ox instead favors nosZ clade II-carrying partial- or non-denitrifiers, suggesting efficient partnering of the reduction steps among organisms. SA has the slowest denitrification progression and highest accumulation of intermediates, indicating less functional coordination. The findings demonstrate how adaptations of denitrifier communities to varying O2 conditions are tightly linked to the duration of anoxic episodes, emphasizing the importance of knowing an environment’s O2 legacy for accurately predicting N2O emissions originating from denitrification.

Impact of aerobic granular sludge sizes and dissolved oxygen concentration on greenhouse gas N2O emission

Aerobic granular sludge (AGS) at wastewater treatment plants (WWTPs) are known to produce nitrous oxide (N2O), a greenhouse gas which has a ∼300 times higher global warming potential than carbon dioxide. In this research, we studied N2O emissions from different sizes of AGS developed at a dissolved oxygen (DO) level of 2 mgO2/L while exposing them to disturbances at various DO concentrations ranging from 1 to 4 mgO2/L. Five different AGS size classes were studied: 212–600 µm, 600–1000 µm, 1000–1400 µm, 1400–2000 µm, and > 2000 µm. Metagenomic data showed N2O reductase genes (nosZ) were more abundant in the smaller AGS sizes which aligned with the observation of higher N2O reduction rates in small AGS under anaerobic conditions. However, when oxygen was present, the activity measurements of N2O emission showed an opposite trend compared to metagenomic data, smaller AGS (212 to 1000 µm) emitted significantly higher N2O (p < 0.05) than larger AGS (1000 µm to >2000 µm) at DO of 2, 3, and 4 mgO2/L. The N2O emission rate showed positive correlation with both oxygen levels and nitrification rate. This pattern indicates a connection between N2O emission and nitrification. In addition, the data suggested the penetration of oxygen into the anoxic zone of granules might have hindered nitrous oxide reduction, resulting in incomplete denitrification stopping at N2O and consequently contributing to an increase in N2O emissions. This work sets the stage to better understand the impacts of AGS size on N2O emissions in WWTPs under different disturbance of DO conditions, and thus ensure that wastewater treatment will comply with possible future regulations demanding lowering greenhouse gas emissions in an effort to combat climate change.

Effects of microplastics on N2O production and reduction potential in crop soils of northern China

Microplastics (MPs) pollution are found to be increasing in vegetable soils and potentially affecting N2O production and their associated pathways; however, its specific effects remain unclear. Here, we selected two common MPs, PE and PP at four different concentration levels of 0, 0.5, 1.5 and 3%, and conducted several incubation experiments aiming to explore soil bacterial and fungal N2O production. Results showed that the bacteria were the main contributors for the production of N2O, regardless of the absence or presence of MPs; and its contribution was decreased with increasing concentrations of PE and PP. The nosZ clade I and II genes were positively correlated with N2O reduction rates, indicating a combined regulation on soil N2O reduction. PE significantly inhibited the bacterial nitrification and denitrification, but did not affect the total N2O production rates; while PP significantly reduced both the bacterial and fungal N2O production rates. The resistance of fungal N2O production to MPs pollution was stronger than that of the bacterial N2O production. It highlights that the MPs pollution could reduce the potential of N2O production and reduction, and thus disturb soil nitrogen cycling system; while the inhibition on N2O production via bacteria and fungi varies with different types of MPs. This study is conducive to an improved and more comprehensive understanding of the ecological impacts of MPs within the agroecosystem.

The imbalance between N2O production and reduction by multi-microbial communities determines sedimentary N2O emission potential in the Pearl River Estuary

Denitrification is the dominant process of nitrogen removal and nitrous oxide (N2O) emissions in estuarine ecosystems. However, little is known regarding the microbial mechanism of the production and reduction of N2O in estuaries. We investigated in situ dissolved N2O as well as potential N2O production rate (NPR), reduction rate (NRR), and emission rate (NER), and key functional genes related to N2O transformation of denitrification in the Pearl River Estuary. Higher N2O emission potential was found in the upstream and midstream regions with higher NPR and lower NRR values. In contrast, higher NRR values were detected in downstream. Notably, nirS and nirK type N2O producers dominated the upstream zone, whereas abundant N2O reducers, especially nosZ II type N2O reducers, were observed in downstream. Most importantly, the gene abundance ratio (Rnir/nosZ) was significantly correlated with the N2O emission potential (Re). Niche differentiation between N2O producers and N2O reducers from upstream to downstream affected N2O emission potential. This study highlights the N2O emission potential in estuarine sediments is determined by an imbalance between N2O production and the reduction of multi-bacterial communities.

Effect and microbial mechanism of suspended sediments particle size on nitrous oxide emission in eutrophic lakes

Suspended sediment (SPS) is an important environmental factor in eutrophic lakes, where they may play a significant role in the microbial nitrogen cycle and thus affect the N2O source and sink function. This study investigated the correlation and corresponding microbial mechanisms between N2O emission fluxes and SPS particle sizes. N2O emission characteristics were investigated in four parallel operated lab-scale microcosmic systems, in which different sizes of SPS particles were inoculated (i.e., <75, 75–150, 150–300, and >300 μm). The results show that, N2O emission fluxes in the eutrophic lakes were exponentially correlated with the lake trophic level index (TLI) (R2 = 0.94, p < 0.01) and the specific surface area of the SPS (R2 = 0.38, p < 0.05). In the microcosmic systems, SPS with 75–150 μm particles had the highest N2O emission rate of 5.94 ± 0.007 μg N/L/d, which was 2.6 times that of the <75 μm particle size system. The microcosmic system with particle size >300 μm had the highest N2O reduction rate (Vmax) of 6.776 μmol/L/h, which was 16–50 times that of the other three groups. Larger particle size SPS have a smaller specific surface area, which could affect the microenvironment on SPS surface and thus affect the microbe functions. The microbial community structure results indicated that the dominant microorganisms on the SPS surface were denitrifying bacteria. The maximum (nirS + nirK)/nosZ ratio was 30.2 for the 75–150 μm system, which was nearly 2 times higher than the other systems. The >300 μm system had the highest nosZ abundance, indicating a strong ability to reduce N2O. The co-occurrence networks analysis indicated that the cooperation and competition among nitrifiers and denitrifiers determined N2O emissions. These results provide fundamental insights into the influence of SPS size on N2O emissions in eutrophic lakes.

Mitigation of N2O emissions via enhanced denitrification in a biological landfill leachate treatment using external carbon from fermented sludge

The effects of an external carbon source (C-source) on the mitigation of N2O gas (N2O(g)) emissions from landfill leachate were investigated via enhanced denitrification using anaerobically fermented sewage sludge. Anaerobic fermentation of sewage sludge was conducted under thermophilic conditions with progressively increasing organic loading rates (OLR). Optimal conditions for fermentation were determined based on the efficiency of hydrolysis and the concentrations of sCOD and volatile fatty acids (VFAs) as follows: at an OLR of 40.48 ± 0.77 g COD/L·d with 1.5 days of solid retention time (SRT), 14.68 ± 0.59% of efficiency of hydrolysis, 14.42 ± 0.30 g sCOD/L and 7.85 ± 0.18 g COD/L of VFAs. Analysis of the microbial community in the anaerobic fermentation reactor revealed that degradation of sewage sludge might be potentially affected by proteolytic microorganisms producing VFAs from proteinaceous materials. Sludge-fermentate (SF) retrieved from the anaerobic fermentation reactor was used as the external C-source for denitrification testing. The specific nitrate removal rate (KNR) of the SF-added condition was 7.54 mg NO3−N/g VSS·hr, which was 5.42 and 2.43 times higher than that of raw landfill leachate (LL) and a methanol-added condition, respectively. In the N2O(g) emission test, the liquid phase N2O (N2O–N(l)) of 20.15 mg N/L was emitted as N2O(g) of 19.64 ppmv under only LL-added condition. On the other hand, SF led to the specific N2O(l) reduction rate (KN2O) of 6.70 mg N/g VSS hr, resulting in mitigation of 1.72 times the N2O(g) emission compared to under the only-LL-added condition. The present study revealed that N2O(g) emissions from biological landfill leachate treatment plants can be attenuated by simultaneous reduction of NO3−N and N2O(l) during enhanced denitrification via a stable supply of an external C-source retrieved from anaerobically fermented organic waste.

NosZ–II–type N2O-reducing bacteria play dominant roles in determining the release potential of N2O from sediments in the Pearl River Estuary, China

The microbial reduction of N2O serves as a “gatekeeper” for N2O emissions, determining the flux of N2O release into the atmosphere. Estuaries are active regions for N2O emissions, but the microbial functions of N2O-reducing bacteria in estuarine ecosystems are not well understood. In this study, the 15N isotope tracer method, qPCR, and high-throughput sequencing were used to analyze N2O production, reduction, and emission processes in surface sediments of the Pearl River Estuary. The 15N isotope tracer experiment showed that the N2O production rates declined and the N2O reduction potential (Rr, the ratio of N2O reduction rates to N2O production rates) increased from upstream to downstream of the Pearl River Estuary, leading to a corresponding decrease of the N2O emission rates from upstream to downstream. The gene abundance ratio of nosZ/nir gradually increased from upstream to downstream and was negatively correlated with the water N2O saturation. The gene abundance of nosZ II was significantly higher than that of nosZ I in the estuary, and the nosZ II/nosZ I abundance ratio was positively correlated with N2O reduction potential. Furthermore, the community composition of NosZ-I- and NosZ–II–type N2O-reducing bacteria shifted from upstream to downstream. NosZ–II–type N2O-reducing bacteria, especially Myxococcales, Thiotrichales, and Gemmatimonadetes species, contributed to the high N2O reduction potential in the downstream. Our results suggest that NosZ–II–type N2O-reducing bacteria play a dominant role in determining the release potential of N2O from sediments in the Pearl River Estuary. This study provides a new insight into the function of microbial N2O reduction in estuarine ecosystems.

Effect of tillage state of paddy soils with heavy metal pollution on the nosZ gene of N2O reductase

Paddy soils are an important source of atmospheric nitrous oxide (N2O). However, numerous studies have focused on N2O production during the soil tillage period, neglecting the N2O production during the dry fallow period. In this study, we conducted an incubation experiment using the acetylene inhibition technique to investigate N2O emission and reduction rates of paddy soil profiles (0-1 m) from Guangdong Province and Jinlin Province in China, with different heavy-metal pollution levels. The abundance and community structures of denitrifying bacteria were determined via quantitative-PCR and Illumina MiSeq sequencing of nosZ, nirK, and nirS genes. Our results showed that the potential N2O emission rate, N2O production rate, and denitrification rate have decreased with increasing soil vertical depth and heavy-metal pollution. More importantly, we found that the functional gene type of N2O reductase switched with the tillage state of paddy soils, which clade Ⅱ nosZ genes were the dominant gene during the tillage period, while clade Ⅰ nosZ genes were the dominant gene during the dry fallow period. The heavy-metal pollution has less effect on the niche differentiation of the nosZ gene. The N2O emission rate was significantly regulated by the genus Bradyhizobium, which contains both N2O reductase and nitrite reductase genes. Our findings suggests that the nosZ gene of N2O reductase can significantly impact the N2O emission from paddy soils.

Effect of nitrous oxide (N2O) on the structure and function of nitrogen-oxide reducing microbial communities

Nitrous oxide (N2O) is a potent greenhouse gas that can be produced by nitrifying and denitrifying bacteria. Yet the effects of N2O on microbial communities is not well understood. We used batch tests to explore the effects of N2O on mixed denitrifying communities. Batch tests were carried out with acetate as the electron donor and with the following electron acceptors: nitrate (NO3−), nitrite (NO2−), N2O, NO3− + N2O, and NO2− + N2O. Activated sludge from a municipal wastewater treatment plant was used as the inoculum. The bacteria grew readily with N2O as the sole acceptor. When N2O was provided along with NO3− or NO2−, it was used concurrently and resulted in higher growth rates than the same acceptors without added N2O. The microbial communities resulting from N2O addition were significantly different at the genus level from those with just NO3− or NO2−. Tests with N2O as the sole added acceptor revealed a reduced diversity. Analysis of inferred gene content using PICRUSt2 indicated a greater abundance of genera with a complete denitrification pathway when growing on N2O or NO2−, relative to all other tests. This suggests that specific N2O reduction rates are high, and that N2O alone selects for a low-diversity, fully denitrifying community. When N2O is present with NO2− or NO3−, the microbial communities were more diverse and did not select exclusively for full denitrifiers. N2O alone appears to select for a “generalist” community with full denitrification pathways and lower diversity. In terms of denitrification genes, the combination of acceptors with N2O appeared to increase the number of microbes carrying nirK, while fully denitrifying bacteria appear more likely to carry nirS. Lastly, all the taxa in NO2− and N2O samples were predicted to harbor nosZ. This suggests the potential for reduced N2O emissions in denitrifying systems.

Inoculation effect of Pseudomonas sp. TF716 on N2O emissions during rhizoremediation of diesel-contaminated soil

The demand for rhizoremediation technology that can minimize greenhouse gas emissions while effectively removing pollutants in order to mitigate climate change has increased. The inoculation effect of N2O-reducing Pseudomonas sp. TF716 on N2O emissions and on remediation performance during the rhizoremediation of diesel-contaminated soil planted with tall fescue (Festuca arundinacea) or maize (Zea mays) was investigated. Pseudomonas sp. TF716 was isolated from the rhizosphere soil of tall fescue. The maximum N2O reduction rate of TF716 was 18.9 mmol N2O g dry cells−1 h−1, which is superior to the rates for previously reported Pseudomonas spp. When Pseudomonas sp. TF716 was added to diesel-contaminated soil planted with tall fescue, the soil N2O-reduction potential was 2.88 times higher than that of soil with no inoculation during the initial period (0–19 d), and 1.08–1.13 times higher thereafter. However, there was no enhancement in the N2O-reduction potential for the soil planted with maize following inoculation with strain TF716. In addition, TF716 inoculation did not significantly affect diesel degradation during rhizoremediation, suggesting that the activity of those microorganisms involved in diesel degradation was unaffected by TF716 treatment. Analysis of the dynamics of the bacterial genera associated with N2O reduction showed that Pseudomonas had the highest relative abundance during the rhizoremediation of diesel-contaminated soil planted with tall fescue and treated with strain TF716. Overall, these results suggest that N2O emissions during the rhizoremediation of diesel-contaminated soil using tall fescue can be reduced with the addition of Pseudomonas sp. TF716.

A Study on the N2O Reduction Rate According to Temperature and Residence Time in the Exhaust Gas Atmosphere Emitted on Combustion of Air and Oxygen.

N2O is a hazardous greenhouse gas. It should be reduced to solve global warming problems. In this study, experiments of N2O thermal decomposition were conducted by simulating the exhaust gas atmosphere emitted during the combustion of air and pure oxygen in an actual circulating fluidized bed system and incinerator system. As a result of comparing the N2O reduction rate in N2 and CO2 atmospheres, the N2O reduction rate in the CO2 atmosphere was 20% higher than that in the N2 atmosphere. It is judged that the N2O reduction rate is high in a CO2 atmosphere (exhaust gas from pure oxygen combustion) due to complex factors such as the reverse reaction, the diffusion coefficient, and static pressure-specific heat. Therefore, pure oxygen combustion increases the reduction rate of nitrous oxide. In addition, when operated with an appropriate residence time and temperature, a reduction effect of more than 95% can be expected, and the fuel consumption rate is also expected to improve.

NosZ gene cloning, reduction performance and structure of Pseudomonas citronellolis WXP-4 nitrous oxide reductase.

Nitrous oxide reductase (N2OR) is the only known enzyme that can reduce the powerful greenhouse gas nitrous oxide (N2O) to harmless nitrogen at the final step of bacterial denitrification. To alleviate the N2O emission, emerging approaches aim at microbiome biotechnology. In this study, the genome sequence of facultative anaerobic bacteria Pseudomonas citronellolis WXP-4, which efficiently degrades N2O, was obtained by de novo sequencing for the first time, and then, four key reductase structure coding genes related to complete denitrification were identified. The single structural encoding gene nosZ with a length of 1914 bp from strain WXP-4 was cloned in Escherichia coli BL21(DE3), and the N2OR protein (76 kDa) was relatively highly efficiently expressed under the optimal inducing conditions of 1.0 mM IPTG, 5 h, and 30 °C. Denitrification experiment results confirmed that recombinant E. coli had strong denitrification ability and reduced 10 mg L−1 of N2O to N2 within 15 h under the optimal conditions of pH 7.0 and 40 °C, its corresponding N2O reduction rate was almost 2.3 times that of Alcaligenes denitrificans strain TB, but only 80% of that of wild strain WXP-4, meaning that nos gene cluster auxiliary gene deletion decreased the activity of N2OR. The 3D structure of N2OR predicted on the basis of sequence homology found that electron transfer center CuA had only five amino acid ligands, and the S2 of the catalytically active center CuZ only bound one CuI atom. The unique 3D structure was different from previous reports and may be closely related to the strong N2O reduction ability of strain WXP-4 and recombinant E. coli. The findings show a potential application of recombinant E. coli in alleviating the greenhouse effect and provide a new perspective for researching the relationship between structure and function of N2OR.

N2O Reduction by Gemmatimonas aurantiaca and Potential Involvement of Gemmatimonadetes Bacteria in N2O Reduction in Agricultural Soils.

Agricultural soil is the primary N2O sink limiting the emission of N2O gas into the atmosphere. Although Gemmatimonadetes bacteria are abundant in agricultural soils, limited information is currently available on N2O reduction by Gemmatimonadetes bacteria. Therefore, the effects of pH and temperature on N2O reduction activities and affinity constants for N2O reduction were examined by performing batch experiments using an isolate of Gemmatimonadetes bacteria, Gemmatimonas aurantiaca (NBRC100505T). G. aurantiaca reduced N2O at pH 5–9 and 4–50°C, with the highest activity being observed at pH 7 and 30°C. The affinity constant of G. aurantiaca cells for N2O was 4.4‍ ‍μM. The abundance and diversity of the Gemmatimonadetes 16S rRNA gene and nosZ encoding nitrous oxide reductase in agricultural soil samples were also investigated by quantitative PCR (qPCR) and amplicon sequencing ana­lyses. Four N2O-reducing agricultural soil samples were assessed, and the copy numbers of the Gemmatimonadetes 16S rRNA gene (clades G1 and G3), nosZ DNA, and nosZ mRNA were 8.62–9.65×108, 5.35–7.15×108, and 2.23–4.31×109 copies (g dry soil)–1, respectively. The abundance of the nosZ mRNA of Gemmatimonadetes bacteria and OTU91, OUT332, and OTU122 correlated with the N2O reduction rates of the soil samples tested, suggesting N2O reduction by Gemmatimonadetes bacteria. Gemmatimonadetes 16S rRNA gene reads affiliated with OTU4572 and OTU3759 were predominant among the soil samples examined, and these Gemmatimonadetes OTUs have been identified in various types of soil samples.

High N2O reduction potential by denitrification in the nearshore site of a riparian zone

As a potent atmospheric greenhouse gas and a major source of ozone depletion, nitrous oxide (N2O) emission has been given increasing attention in aquatic systems, particularly at the aquatic-terrestrial interfaces, such as riparian zones. However, the microbial mechanisms regulating N2O emission in riparian zones remain unknown. Here, we measured the contributions of denitrification and ammonium oxidation to N2O emission along with the abundance and community structure of nirK-, nirS-, nosZ I- and nosZ II-harbouring bacteria in both surface sediments (0–10 cm) and overlying water along a lake riparian zone (including nearshore sites and offshore sites). Overall, the nearshore sites of the riparian zones emitted less N2O than the offshore sites. Nearshore N2O emission was dominated by denitrification with a high N2O reduction rate, whereas offshore N2O emission was driven by ammonium oxidation. Furthermore, N2O derived from ammonium oxidation was influenced by the NH4+-N content, and denitrification N2O was modulated by denitrifier communities. The N2O-producing community was dominated by nirS-harbouring bacteria, while the N2O-reducing community was dominated by nosZ I-harbouring bacteria. The relative abundance of Hydrogenophilales from nirS-denitrifiers and Chloroflexi unclassified from nosZ II-type communities influenced the N2O produced by denitrification, according to high-throughput sequencing analysis. Additionally, we also found lower levels of N2O production per unit volume in overlying water, which were 3–4 orders of magnitude less than in the surface sediment. Overall, we propose that using riparian zones can be an effective management tool for N2O mitigation by enhancing the N2O reduction process of denitrification and decreasing ammonium oxidation.

Roles of bulk and rhizosphere denitrifying bacteria in denitrification from paddy soils under straw return condition

Roles of bulk and rhizosphere denitrifying bacteria in paddy soil denitrification under straw return conditions are rarely discriminated, limiting our understanding on nitrogen biogeochemistry in soils amended with straw. The objective of this study was to explore the microbial mechanisms accounting for the altered rhizosphere and bulk soil denitrification with straw amendment. In this study, straw was added into two representative paddy soils (0.5% w/w) from Yixing (YX) and Taizhou (TZ) in Jiangsu city, China, for rice growth over a 37-day period. Denitrification potentials (net N2O emission, total denitrification, and N2O reduction rates) of rhizosphere and bulk soils at the end of rice growth were measured using an acetylene inhibition technique. In addition, denitrifying bacterial community compositions and functional gene abundances were analyzed using Illumina sequencing and quantitative real-time PCR. For control treatment without straw addition, total denitrification and N2O reduction rates were significantly higher in the rhizosphere than that in bulk soils; however, net N2O emission potentials were similar between rhizosphere and bulk for TZ (22.6–53.3 vs. 9.44–46.6 mg N kg−1) and YX (946 ± 126 vs. 699 ± 350 mg N kg−1). Under straw return condition, N2O emission potentials from the bulk of both soils were significantly elevated, corresponding to the increase in the relative abundance of some taxa of denitrifying bacteria (unnamed environmental samples and unclassified proteobacteria) and increase in nirK, nirS, and nosZ genes. This indicates a positive response of bulk soil denitrifying bacteria to straw addition. In contrast, straw addition produced inhibitive effects on the growth of denitrifying bacteria with decreased nirK, nirS, and nosZ gene abundances in the rhizosphere. This led to decreased N2O emission potentials at the end of incubation for rhizosphere soils, which were 98.2% and 25.2% lower for TZ and YX compared to those of bulk soils. These results suggested that the roles of bulk and rhizosphere denitrifying bacteria in paddy soil denitrification shifted with straw addition, which might inspire further studies to target the denitrification hotspots to effectively mitigate greenhouse emissions at the early period of rice growth.

Dependence of N2O/NO Decomposition and Formation on Temperature and Residence Time in Thermal Reactor

Nitrogen dioxide (N2O) is a greenhouse gas that is harmful to the ozone layer and contributes to global warming. Many other nitrogen oxide emissions are controlled using the selective non-catalytic reaction (SNCR) process, but N2O reduction methods are few. To avoid future air pollution problems, N2O reduction from industrial sources is essential. In this study, a N2O decomposition and NO formation under an argon atmospheric N2O gas mixture were observed in a lab-scale SNCR system. The reaction rate and mechanism of N2O were calculated using a reaction path analyzer (CHEMKIN-PRO). The residence time of the gas mixture and the temperature in the reactor were set as experimental variables. The results confirmed that most of the N2O was converted to N2 and NO. The change in the N2O reduction rate increased with the residence time at 1013 and 1113 K, but decreased at 1213 K due to the inverse reaction. NO concentration increased with the residence time at 1013 and 1113 K, but decreased at 1213 K owing to the conversion of NO back to N2O.

Characterization of nitrous oxide reduction by Azospira sp. HJ23 isolated from advanced wastewater treatment sludge

A new nitrous oxide (N2O)-reducing bacterium was isolated from a consortium that was enriched using advanced wastewater treatment sludge as an inoculum and N2O as the sole nitrogen source. The isolated facultative anaerobe was identified as Azospira sp. HJ23. Azospira sp. HJ23 exhibited optimum N2O-reducing activity with a C/N ratio of 62 at pH 6 in the temperature range of 37 °C to 40 °C. The optimum carbon source for N2O reduction was a mixture of glucose and acetate. The maximum rate of N2O reduction by Azospira sp. HJ23 was 4.8 mmol·g-dry cell−1·h−1, and its N2O-reducing activity was higher than other known N2O reducers. Azospira sp. HJ23 possessed several functional genes for denitrification. These included narG (NO3- reductase), nirK (NO2- reductase), norB (NO reductase), and nosZ (N2O reductase) genes. These results suggest that Azospira sp. HJ23 can be applied in the denitrification process to minimalize N2O emission.

Nitrous oxide dynamics during denitrification along a hydrological gradient of subtropical grasslands

AbstractNitrous oxide (N2O) dynamics during denitrification, including N2O production and reduction, particularly as related to soil depth, are poorly understood. The objective of this study was to investigate the rates of N2O production and reduction processes at various soil depths along a hydrological gradient in grazed subtropical grasslands. A batch incubation study was conducted on soils collected along a hydrological gradient representing isolated wetland (Center), transient edge (Edge) and pasture upland (Upland) in south‐central Florida. Significantly different N2O production and reduction rates between hydrological zones were observed for surface soils (0–10 cm) under ambient conditions, with average N2O production rates of 0.368, 0.178 and 0.003 N2O‐N kg−1 dry soil h−1 for Center, Edge and Upland, respectively, and average N2O reduction rates of 0.063, 0.132 and 0.002 N2O‐N kg−1 dry soil h−1. Nitrous oxide production and reduction in subsurface soils maintained low rates and showed small variations between depths and hydrological zones. Our results suggest that N2O dynamics were affected by depth, mainly through labile organic carbon (C) and microbial biomass C, being influenced by hydrological zone primarily through soil NO3‐ content. The spatial distribution of N2O fluxes from denitrification along the hydrological gradient is likely attributed to the differences in N2O production and reduction in surface soils.