Pathways of N2O production by marine ammonia-oxidizing archaea determined from dual-isotope labeling

The ocean is a net source of the greenhouse gas and ozone-depleting substance, nitrous oxide (N2O), to the atmosphere. Most of that N2O is produced as a trace side product during ammonia oxidation, primarily by ammonia-oxidizing archaea (AOA), which numerically dominate the ammonia-oxidizing community in most marine environments. The pathways to N2O production and their kinetics, however, are not completely understood. Here, we use 15N and 18O isotopes to determine the kinetics of N2O production and trace the source of nitrogen (N) and oxygen (O) atoms in N2O produced by a model marine AOA species, Nitrosopumilus maritimus. We find that during ammonia oxidation, the apparent half saturation constants of nitrite and N2O production are comparable, suggesting that both processes are enzymatically controlled and tightly coupled at low ammonia concentrations. The constituent atoms in N2O are derived from ammonia, nitrite, O2, and H2O via multiple pathways. Ammonia is the primary source of N atoms in N2O, but its contribution varies with ammonia to nitrite ratio. The ratio of 45N2O to 46N2O (i.e., single or double labeled N) varies with substrate ratio, leading to widely varying isotopic signatures in the N2O pool. O2 is the primary source for O atoms. In addition to the previously demonstrated hybrid formation pathway, we found a substantial contribution by hydroxylamine oxidation, while nitrite reduction is an insignificant source of N2O. Our study highlights the power of dual 15N-18O isotope labeling to disentangle N2O production pathways in microbes, with implications for interpretation of pathways and regulation of marine N2O sources.

Mycorrhiza-mediated recruitment of complete denitrifying Pseudomonas reduces N2O emissions from soil

BACKGROUND

Arbuscular mycorrhizal fungi (AMF) are key soil organisms and their extensive hyphae create a unique hyphosphere associated with microbes actively involved in N cycling. However, the underlying mechanisms how AMF and hyphae-associated microbes may cooperate to influence N₂O emissions from "hot spot" residue patches remain unclear. Here we explored the key microbes in the hyphosphere involved in N₂O production and consumption using amplicon and shotgun metagenomic sequencing. Chemotaxis, growth and N₂O emissions of isolated N₂O-reducing bacteria in response to hyphal exudates were tested using in vitro cultures and inoculation experiments.

RESULTS

AMF hyphae reduced denitrification-derived N₂O emission (max. 63%) in C- and N-rich residue patches. AMF consistently enhanced the abundance and expression of clade I nosZ gene, and inconsistently increased that of nirS and nirK genes. The reduction of N₂O emissions in the hyphosphere was linked to N₂O-reducing Pseudomonas specifically enriched by AMF, concurring with the increase in the relative abundance of the key genes involved in bacterial citrate cycle. Phenotypic characterization of the isolated complete denitrifying P. fluorescens strain JL1 (possessing clade I nosZ) indicated that the decline of net N₂O emission was a result of upregulated nosZ expression in P. fluorescens following hyphal exudation (e.g. carboxylates). These findings were further validated by re-inoculating sterilized residue patches with P. fluorescens and by an 11-year-long field experiment showing significant positive correlation between hyphal length density with the abundance of clade I nosZ gene.

CONCLUSIONS

The cooperation between AMF and the N₂O-reducing Pseudomonas residing on hyphae significantly reduce N₂O emissions in the microsites. Carboxylates exuded by hyphae act as attractants in recruiting P. fluorescens and also as stimulants triggering nosZ gene expression. Our discovery indicates that reinforcing synergies between AMF and hyphosphere microbiome may provide unexplored opportunities to stimulate N₂O consumption in nutrient-enriched microsites, and consequently reduce N₂O emissions from soils. This knowledge opens novel avenues to exploit cross-kingdom microbial interactions for sustainable agriculture and for climate change mitigation. Video Abstract.

Nitric oxide and Nitrous oxide accumulation, oxygen production during nitrite denitrification in an anaerobic/anoxic sequencing batch reactor: exploring characteristics and mechanism

Nitrite denitrification has received increasing attention due to its high efficiency, low energy consumption, and sludge yield. However, the nitric oxide (NO) and nitrous oxide (N₂O) which are harmful to the environment, microorganisms, and humans are produced in this process. In order to mitigate NO and N₂O production, the biological mechanisms of NO and N₂O accumulation, as well as NO detoxification during nitrite denitrification in a sequencing batch reactor were studied. Results showed that the peak of NO accumulation increased from 0.29 [Formula: see text] 0.01 to 3.12 [Formula: see text] 0.34 mg L^-1 with the increase of carbon to nitrogen ratio (COD/N), which is caused by the sufficient electron donor supply for NO₂^--N reduction process at high COD/N. Furthermore, the result suggested that NO accumulation with no pH adjustment was 12 times higher than that with pH adjustment. It is related to the inhibition on NO reductase caused by the high free nitrous acid (FNA) and NO concentration with no pH adjustment. The pathways of NO detoxification included NO emission, reduction, and dismutation, and the more NO produced, the high proportion of NO dismutation pathway. Result showed that the maximum of oxygen production during NO dismutation reached to 1.39 mg L^-1. N₂O accumulation was mainly associated with FNA and NO inhibition, COD/N. The peak of N₂O accumulation presented a completely opposite trend at pH adjustment and no pH adjustment, it is because that the higher FNA and NO concentration at high COD/N without pH adjustment will inhibit the N₂O reductase activity, resulting in the N₂O reduction was hindered during nitrite denitrification.

Significant production of nitric oxide by aerobic nitrite reduction at acidic pH

The acidic (i.e., pH ∼5) activated sludge process is attracting attention because it enables stable nitrite accumulation and enhances sludge reduction and stabilization, compared to the conventional process at neutral pH. Here, this study examined the production and potential pathways of nitric oxide (NO) and nitrous oxide (N₂O) during acidic sludge digestion. With continuous operation of a laboratory-scale aerobic digester at high dissolved oxygen concentration (DO>4 mg O₂ L^-1) and low pH (4.7±0.6), a significant amount of total nitrogen (TN) loss (i.e., 18.6±1.5% of TN in feed sludge) was detected. Notably, ∼40% of the removed TN was emitted as NO, with ∼8% as N₂O. A series of batch assays were then designed to explain the observed TN loss under aerobic conditions. All assays were conducted with a low concentration of volatile solids (VS), i.e., VS<4.5 g L^-1. This VS concentration is commensurate with the values commonly found in the aeration tanks of full-scale wastewater treatment systems, and thus no significant nitrogen loss should be expected when DO is controlled above 4 mg O₂ L^-1. However, nitrite disappeared at a significant rate (with the chemical decomposition of nitrite excluded), leading to NO production in the batch assays at pH 5. The nitrite reduction could be associated with endogenous microbial activities, e.g., nitrite detoxification. The significant NO production illustrates the importance of aerobic nitrite reduction during acidic aerobic sludge digestion, suggesting this process cannot be neglected in developing acidic activated sludge technology.

Towards efficient heterotrophic recovery of N2O via Fe(II)EDTA-NO: A modeling study

Efficient recovery of nitrous oxide (N₂O) through heterotrophic denitrification with the help of Fe(II)EDTA-NO as a chelating agent has been regarded as an ideal technology to treat nitric oxide (NO)-rich flue gas. In this study, an integrated NO-based biological denitrification model was developed to describe the sequential reduction of the NO fixed in Fe(II)EDTA-NO with organic carbon as the electron donor. With the inclusion of only the key pathways contributing to nitrogen transformation, the model was firstly developed and then calibrated/validated and evaluated using the data of batch tests mediated by the identified functional heterotrophic bacteria at various substrates concentrations and then used to explore the possibility of enhancing N₂O recovery by altering the substrates condition and reactor setup. The results demonstrated that the optimal COD/N ratio decreased consistently from 1.5 g-COD/g-N at the initial NO concentration of 40 g-N/m³ to 1.0 g-COD/g-N at the initial NO concentration of 420 g-N/m³. Furthermore, sufficiently increasing the headspace volume of the reactor was considered an ideal strategy to obtain ideal N₂O production of 86.6 % under the studied conditions. The production of high-purity N₂O (98 %) confirmed the practical application potential of this integrated treatment technology to recover a valuable energy resource from NO-rich flue gas.

Effects of Different Guests on Pyrolysis Mechanism of α-CL-20/Guest at High Temperatures by Reactive Molecular Dynamics Simulations at High Temperatures

The host-guest inclusion strategy has the potential to surpass the limitations of energy density and suboptimal performances of single explosives. The guest molecules can not only enhance the detonation performance of host explosives but also can enhance their stability. Therefore, a deep analysis of the role of guest influence on the pyrolysis decomposition of the host-guest explosive is necessary. The whole decomposition reaction stage of CL-20/H₂O, CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH was calculated by ReaxFF-MD. The incorporation of CO₂, N₂O and NH₂OH significantly increase the energy levels of CL-20. However, different guests have little influence on the initial decomposition paths of CL-20. The E_a1 and E_a2 values of CL-20/CO₂, CL-20/N₂O, CL-20/NH₂OH systems are higher than the CL-20/H₂O system. Clearly, incorporation of CO₂, N₂O, NH₂OH can inhibit the initial decomposition and intermediate decomposition stage of CL-20/H₂O. Guest molecules become heavily involved in the reaction and influence on the reaction rates. k₁ of CL-20/N₂O and CL-20/NH₂OH systems are significantly larger than that of CL-20/H₂O at high temperatures. k₁ of CL-20/CO₂ system is very complex, which can be affected deeply by temperatures. k₂ of the CL-20/CO₂, CL-20/N₂O systems is significantly smaller than that of CL-20/H₂O at high temperatures. k₂ of CL-20/NH₂OH system shows little difference at high temperatures. For the CL-20/CO₂ system, the k₃ value of CO₂ is slightly higher than that for CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems, while the k₃ values of N₂ and H₂O are slightly smaller than that for the CL-20/H₂O, CL-20/N₂O, CL-20/NH₂OH systems. For the CL-20/N₂O system, the k₃ value of CO₂ is slightly smaller than that for CL-20/H₂O, CL-20/CO₂, CL-20/NH₂OH systems. For the CL-20/NH₂OH system, the k₃ value of H₂O is slightly larger than that for CL-20/H₂O, CL-20/CO₂, CL-20/N₂O systems. These mechanisms revealed that CO₂, N₂O and NH₂OH molecules inhibit the early stages of the initial decomposition of CL-20 and play an important role for the decomposition subsequently.

Chronic high-dose silver nanoparticle exposure stimulates N2O emissions by constructing anaerobic micro-environment

Silver nanoparticles (Ag-NPs) were found to be responsible for nitrous oxide (N₂O) generation; however, the mechanism of Ag-NP induced N₂O production remains controversial and needs to be elucidated. In this study, chronic Ag-NP exposure experiments were conducted in five independent sequencing batch biofilm reactors to systematically assess the effects of Ag-NPs on N₂O emission. The results indicated that a low dose of Ag-NPs (< 1 mg/L) slightly suppressed N₂O generation by less than 22.99% compared with the no-Ag-NP control method. In contrast, a high dose (5 mg/L) of Ag-NPs stimulated N₂O emission by 67.54%. ICP-MS and SEM-EDS together revealed that high Ag-NP content accumulated on the biofilm surface when exposed to 5 mg/L Ag-NPs. N₂O and DO microelectrodes, as well as N₂O isotopic composition analyses, further demonstrated that the accumulated Ag-NPs construct the anaerobic zone in the biofilm, which is the primary factor for the stimulation of the nitrite reduction pathway to release N₂O. A metagenomic analysis further attributed the higher N₂O emissions under exposure to a high dose of Ag-NPs to the higher relative abundance of narB and nirK genes (i.e. 1.52- and 1.29-fold higher, respectively). These findings collectively suggest that chronic exposure to high doses of Ag-NPs could enhance N₂O emissions by forming anaerobic micro-environments in biofilms.

Insights into N2O turnovers under polyethylene terephthalate microplastics stress in mainstream biological nitrogen removal process

The ubiquitous microplastics in wastewater have raised growing concerns due to their unintended effects on microbial activities. However, whether and how microplastics affect nitrous oxide (N₂O) (a potent greenhouse gas) turnovers in mainstream biological nitrogen removal (BNR) process remain unclear. This work therefore aimed to fill such knowledge gap by conducting both long-term and batch tests. After over 100 days of feeding with wastewater containing polyethylene terephthalate (PET) microplastics (0-500 μg/L), the long-term results showed that both production and reduction of N₂O during denitrification were reduced, as well as the N₂O production during nitrification. Accordingly, 60% reduction in N₂O accumulation and 70% reduction in N₂O production were observed in the denitrification and nitrification batch tests, respectively. Nevertheless, the long-term N₂O emission factors under PET microplastics stress were comparable to that in the control reactor, mainly because PET microplastics led to more nitrite accumulation in anoxic period. With the aid of online N₂O sensors and site-preference analysis, it was demonstrated that the heterotrophic bacteria pathway and ammonia oxidizing bacteria denitrification pathway for N₂O production were negatively affected by PET microplastics, whereas a clear increase in the contribution of hydroxylamine pathway (+ 22.9%) was observed. Further investigation revealed that PET microplastics even at environmental level (i.e. 10 μg/L) significantly reshaped the BNR sludge characteristics (e.g. much larger particle size) and microbial communities (e.g. Thauera, Rhodobacte and Nitrospira) as well as the nitrogen metabolism pathways, which were chiefly responsible for the changes of N₂O turnovers and N₂O production pathways under the PET microplastics stress.

Thermodynamic Analysis of Intermediary Metabolic Steps and Nitrous Oxide Production by Ammonium-Oxidizing Bacteria

Nitrous oxide (N₂O) is a greenhouse gas emitted from wastewater treatment, soils, and agriculture largely by ammonium-oxidizing bacteria (AOB). While AOB are characterized by being aerobes that oxidize ammonium (NH₄⁺) to nitrite (NO₂^-), fundamental studies in microbiology are revealing the importance of metabolic intermediates and reactions that can lead to the production of N₂O. These findings about the metabolic pathways for AOB were integrated with thermodynamic electron-equivalents modeling (TEEM) to estimate kinetic and stoichiometric parameters for each of the AOB's nitrogen (N)-oxidation and -reduction reactions. The TEEM analysis shows that hydroxylamine (NH₂OH) oxidation to nitroxyl (HNO) is the most energetically efficient means for the AOB to provide electrons for ammonium monooxygenation, while oxidations of HNO to nitric oxide (NO) and NO to NO₂^- are energetically favorable for respiration and biomass synthesis. The respiratory electron acceptor can be O₂ or NO, and both have similar energetics. The TEEM-predicted value for biomass yield, maximum-specific rate of NH₄⁺ utilization, and maximum specific growth rate are consistent with empirical observations. NO reduction to N₂O is thermodynamically favorable for respiration and biomass synthesis, but the need for O₂ as a reactant in ammonium monooxygenation likely precludes NO reduction to N₂O from becoming the major pathway for respiration.

Antibiotics sulfamethoxazole alter nitrous oxide production and pathways in estuarine sediments: Evidenced by the N15-O18 isotopes tracing

Estuarine antibiotic residues are profoundly impacting microbial nitrogen (N) cycling and associated N₂O production, but the response of N₂O production pathways to antibiotics remains poorly understood. Here, ¹⁵N-¹⁸O labeling technique combined with molecular methods were used to investigate the impacts of sulfamethoxazole on the contribution of ammonia oxidation (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N₂O production in estuarine sediments. Results showed that environmental concentration of sulfamethoxazole (4 ng/g) promoted the total N₂O production by 17.1% through nitrifier denitrification. Environmentally relevant (40-4000 ng/g) and irrelevant (40,000 ng/g) concentration of sulfamethoxazole drove nitrification denitrification to gradually lose the dominant role in total N₂O production and ammonia oxidation-derived N₂O, replaced by HD and nitrifier nitrification, while total N₂O production were inhibited. Furthermore, when HD dominated the total N₂O production, the HD-derived N₂O increased by 63.6% with sulfamethoxazole concentration reaching 40,000 ng/g. The mechanistic investigation further showed that nitrifying bacteria were more susceptible to sulfamethoxazole than nitrifying archaea and denitrifiers. The increased expression of nirS gene carried by non-dominant denitrifiers improved the ratio of nirS:nosZ and hence increased HD-derived N₂O under high sulfamethoxazole stresses. Overall, our results provide a comprehensive view into how antibiotics regulate N₂O production and its pathways in estuarine sediments.

Growth, hemato-biochemical, immune-antioxidant response, and gene expression in Nile tilapia (Oreochromis niloticus) received nano iron oxide-incorporated diets

Nanotechnology has recently played a key role in tackling many aquacultures issues. Hence, the present study targets the evaluation of dietary inclusion of nano iron oxide (nFe₂O₃) on growth performance, hematology, immune-antioxidant responses, ionic regulation and expression of related genes in Nile tilapia (Oreochromis niloticus). Fish were fed supplementary nFe₂O₃ at rates of zero (control), 0.5, and 1 g/kg diet for 30 days. Obtained data demonstrated that nFe₂O₃ significantly (P < 0.05) augmented growth performance (final weight and length, body mass gain, specific growth rate, feed conversion ratio, and length gain rate). Hematological picture {RBCs, Hb, MCV, MCH and MCHC, and leukocytes interpretations (WBCs and monocytes)}; and biochemical indexes including (AST and ALT; total protein; and glucose, and cortisol) were significantly (P < 0.05) improved in nFe₂O₃ supplemented groups. Plasma ionic concentration was also altered with nFe₂O₃ supplementation, and 1g nFe₂O₃ revealed the most marked increase in plasma (Na⁺) potassium (K⁺) levels. Similarly, IgM, nitrous oxide (NO), and lysozyme activity, plus superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activities showed a remarkable improvement in 1g nFe₂O₃ group compared to the control. Expression of Insulin-Like Growth Factor-1 (IGF-1) and interleukin 1-β (IL-1β) genes were significantly up-regulated in nFe₂O₃ supplemented groups. Briefly, dietary nFe₂O₃ inclusion had enhanced properties on growth; hemato-biochemical; immune, antioxidative profiles; and related genes expression of O. niloticus, with a recommended concentration of 1g nFe₂O₃.

Microplastics alter nitrous oxide production and pathways through affecting microbiome in estuarine sediments

Increasing microplastics (MPs) pollution in estuaries profoundly impacts microbial ecosystems and biogeochemical processes. Nitrous oxide (N₂O), a powerful greenhouse gas, is an important intermediate product of microbial nitrogen cycling. However, how MPs regulate N₂O production and its pathways remain poorly understood. Here, impacts of traditional petroleum-based and emerging biodegradable MPs on microbial N₂O production and its pathways were studied through dual-isotope (¹⁵N-¹⁸O) labeling technique and molecular methods. Results indicated that both traditional petroleum-based and emerging biodegradable MPs promoted sedimentary N₂O production, whereas pathways varied. Biodegradable polylactic acid (PLA) MPs displayed greater promotion of N₂O production than petroleum-based MPs, polyvinyl chloride (PVC) and polyethylene (PE), of which PLA promoted through nitrifier nitrification (NN) and heterotrophic denitrification (HD), PE through nitrifier denitrification and HD, and PVC through NN. By combining the analysis of N₂O production rates with sediment chemical and microbiological properties, we demonstrated that the enrichment of nitrifying and denitrifying bacteria, as well as related functional genes directly and/or indirectly increased N₂O production primarily by interacting with carbon and nitrogen substrates. Different response of nitrogen cycling microbes to MPs led to the difference in N₂O increase pathways, of which nitrifying bacteria significantly enriched in all MPs treatments due to the niches provided by MPs. However, part of denitrifying bacteria significantly enriched in treatments containing PLA and PE MPs, which may serve as organic carbon substrates. This work highlights that the presence of MPs can promote sedimentary N₂O production, and the emerging biodegradable MPs represented by PLA may have a greater potential to enhance estuarine N₂O emissions and accelerate global climate change.

Warming and redistribution of nitrogen inputs drive an increase in terrestrial nitrous oxide emission factor

Anthropogenic nitrogen inputs cause major negative environmental impacts, including emissions of the important greenhouse gas N2O. Despite their importance, shifts in terrestrial N loss pathways driven by global change are highly uncertain. Here we present a coupled soil-atmosphere isotope model (IsoTONE) to quantify terrestrial N losses and N2O emission factors from 1850-2020. We find that N inputs from atmospheric deposition caused 51% of anthropogenic N2O emissions from soils in 2020. The mean effective global emission factor for N2O was 4.3 ± 0.3% in 2020 (weighted by N inputs), much higher than the surface area-weighted mean (1.1 ± 0.1%). Climate change and spatial redistribution of fertilisation N inputs have driven an increase in global emission factor over the past century, which accounts for 18% of the anthropogenic soil flux in 2020. Predicted increases in fertilisation in emerging economies will accelerate N2O-driven climate warming in coming decades, unless targeted mitigation measures are introduced.

Nitrite removal by Acinetobacter sp.TX: a candidate of curbing N2O emission

The nitrite removal pathway in Acinetobacter sp. TX5 was explored through the key gene identification and the corresponding enzyme purification, after which the capability to reduce nitrite by immobilized beads was investigated in a fixed-bed reactor. Results revealed that a nosZ gene encoding nitrous oxide reductase (N₂OR) exists in TX5 cells, and a N₂OR responsible for the reduction of N₂O to N₂ was purified successfully with a molecular weight of 70.05 kDa, a purification fold of 16.30 and a recovery rate of 5.17%. For TX5 immobilization, the optimal values of polyvinyl alcohol (PVA), spent mushroom substrate (SMS) and Aci (TX5) obtained by response surface methodology (RSM) were 6.32%, 2.92% and 4.57%, respectively. In a fixed-bed reactor packed with immobilized TX5, the removal efficiency (RE) achieved 90% (at 50 h) for NO2--N and 85% (at 96 h) for total nitrogen (TN). On the basis of these results, a nitrite removal pathway in TX5 was proposed. Overall, Acinetobacter sp. TX5 might be a promising candidate for nitrite removal with an ability to suppress N₂O accumulation.

Microbial regulation of nitrous oxide emissions from chloropicrin-fumigated soil amended with biochar

The microbial mechanism underpinning biochar's ability to reduce emissions of the potent greenhouse gas nitrous oxide (N₂O) is little understood. We combined high-throughput gene sequencing with a dual-label ¹⁵N-¹⁸O isotope to examine microbial mechanisms operative in biochar made from Crofton Weed (BC1) or pine wood pellets (BC2) and the N₂O emissions from those biochar materials when present in chloropicrin (CP)-fumigated soil. Both BC1 and BC2 reduced N₂O total emissions by 62.9-71.9% and 48.8-52.0% in CP-fumigated soil, respectively. During the 7-day fumigation phase, however, both BC1 and BC2 increased N₂O production by significantly promoting nirKS and norBC gene abundance, which indicated that the N₂O emission pathway had switched from heterotrophic denitrification to nitrifier denitrification. During the post-fumigation phase, BC1 and BC2 significantly decreased N₂O production as insufficient nitrogen was available to support rapid population increases of nitrifying or denitrifying bacteria. BC1 and BC2 significantly reduced CP's inhibition of nitrifying archaeal bacteria (AOA, AOB) and the denitrifying bacterial genes (nirS, nirK, nosZ), which promoted those bacterial populations in fumigated soil to similar levels observed in unfumigated soil. Our study provided insight on the impact of biochar and microbes on N₂O emissions.

Synergistic Effects of Aquatic Plants and Cyanobacterial Blooms on the Nitrous Oxide Emission from Wetlands

Wetlands provide a habitat for the symbiosis of multiple plants and play a significant role in global N₂O emissions. The metabolic traits and effects on microorganisms, which regulate the conversion of nitrogen to N₂O, varies with plant species. The frequent occurrences of cyanobacterial blooms in wetlands can also have a positive or negative effect on denitrification, entangling N₂O emissions. In situ observations of the Dongting Lake reveal that the fluxes in N₂O emissions vary with the vegetation. Maximum emissions occurred in the mud flat, while the zone with the minimum emissions was populated with carex. In 210-day batch cultures, the addition of cyanobacteria synergistically enhanced N₂O production during the degredation of phalaris and reed. The abundance of the nirS and nirK genes decreased over time except in the phalaris-algae group. To mitigate the N₂O emissions from wetlands, the macrophyte communities need to be protected, and the cyanobacterial blooms need to be avoided by reducing the nitrogen pollution.

Regulation of the Emissions of the Greenhouse Gas Nitrous Oxide by the Soybean Endosymbiont Bradyrhizobium diazoefficiens

The greenhouse gas nitrous oxide (N2O) has strong potential to drive climate change. Soils are a major source of N2O, with microbial nitrification and denitrification being the primary processes involved in such emissions. The soybean endosymbiont Bradyrhizobium diazoefficiens is a model microorganism to study denitrification, a process that depends on a set of reductases, encoded by the napEDABC, nirK, norCBQD, and nosRZDYFLX genes, which sequentially reduce nitrate (NO3-) to nitrite (NO2-), nitric oxide (NO), N2O, and dinitrogen (N2). In this bacterium, the regulatory network and environmental cues governing the expression of denitrification genes rely on the FixK2 and NnrR transcriptional regulators. To understand the role of FixK2 and NnrR proteins in N2O turnover, we monitored real-time kinetics of NO3-, NO2-, NO, N2O, N2, and oxygen (O2) in a fixK2 and nnrR mutant using a robotized incubation system. We confirmed that FixK2 and NnrR are regulatory determinants essential for NO3- respiration and N2O reduction. Furthermore, we demonstrated that N2O reduction by B. diazoefficiens is independent of canonical inducers of denitrification, such as the nitrogen oxide NO3-, and it is negatively affected by acidic and alkaline conditions. These findings advance the understanding of how specific environmental conditions and two single regulators modulate N2O turnover in B. diazoefficiens.

Response of area- and yield-scaled N2 O emissions from croplands to deep fertilization: a meta-analysis of soil, climate, and management factors

BACKGROUND

Nitrous oxide (N₂ O) is an important and persistent greenhouse gas making a significant contribution to global climate change. Deep fertilization has been demonstrated to increase crop yield and nutrient use efficiency by decreasing losses of volatilization and surface runoff. However, N₂ O emissions from croplands induced by deep fertilization are variable and mitigation strategies remain uncertain. This study aimed to (i) quantify the response of area-scaled (N₂ O emissions) and yield-scaled N₂ O emissions (N₂ O intensity) from croplands to deep fertilization, and (ii) identify the soil, climate, and management factors that mitigate N₂ O emissions and N₂ O intensity under deep fertilization.

RESULTS

Compared with the control, deep fertilization increased N₂ O emissions by 18.6% (P < 0.001) but decreased N₂ O intensity by 20.1% (P = 0.018). By adopting deep fertilization, N₂ O emissions could be significantly mitigated in rice-paddies soils (-48.8%), with fertilizer depth > 10 cm (-33.0%), and with fertilizer N amount > 200 kg N ha^-1 (-8.2%). N₂ O intensity following deep fertilization significantly decreased in soils with pH ≤6 (-22.5%), at sites with precipitation of 500-1000 mm (-25.5%), in rice-paddies soils (-53.0%), with the method of mixed fertilizer in the control (-21.2%), and with fertilizer depth > 10 cm (-33.6%).

CONCLUSION

This study provides a basis for assessing the effect of deep fertilization on N₂ O emissions and provides potential measures to mitigate N₂ O emissions associated with deep fertilization practices.

Effect of pH on the denitrification proteome of the soil bacterium Paracoccus denitrificans PD1222

Denitrification is a respiratory process by which nitrate is reduced to dinitrogen. Incomplete denitrification results in the emission of the greenhouse gas nitrous oxide and this is potentiated in acidic soils, which display reduced denitrification rates and high N2O/N2 ratios compared to alkaline soils. In this work, impact of pH on the proteome of the soil denitrifying bacterium Paracoccus denitrificans PD1222 was analysed with nitrate as sole energy and nitrogen source under anaerobic conditions at pH ranging from 6.5 to 7.5. Quantitative proteomic analysis revealed that the highest difference in protein representation was observed when the proteome at pH 6.5 was compared to the reference proteome at pH 7.2. However, this difference in the extracellular pH was not enough to produce modification of intracellular pH, which was maintained at 6.5 ± 0.1. The biosynthetic pathways of several cofactors relevant for denitrification and nitrogen assimilation like cobalamin, riboflavin, molybdopterin and nicotinamide were negatively affected at pH 6.5. In addition, peptide representation of reductases involved in nitrate assimilation and denitrification were reduced at pH 6.5. Data highlight the strong negative impact of pH on NosZ synthesis and intracellular copper content, thus impairing active NosZ assembly and, in turn, leading to elevated nitrous oxide emissions.

Influence of rice varieties, organic manure and water management on greenhouse gas emissions from paddy rice soils

The study is focused on impact of manure application, rice varieties and water management on greenhouse gas (GHG) emissions from paddy rice soil in pot experiment. The objectives of this study were a) to assess the effect of different types of manure amendments and rice varieties on greenhouse gas emissions and b) to determine the optimum manure application rate to increase rice yield while mitigating GHG emissions under alternate wetting and drying irrigation in paddy rice production. The first pot experiment was conducted at the Department of Agronomy, Yezin Agricultural University, Myanmar, in the wet season from June to October 2016. Two different organic manures (compost and cow dung) and control (no manure), and two rice varieties; Manawthukha (135 days) and IR-50 (115 days), were tested. The results showed that cumulative CH₄ emission from Manawthukha (1.084 g CH₄ kg^-1 soil) was significantly higher than that from IR-50 (0.683 g CH₄ kg^-1 soil) (P<0.0046) with yield increase (P<0.0164) because of the longer growth duration of the former. In contrast, higher cumulative nitrous oxide emissions were found for IR-50 (2.644 mg N₂O kg^-1 soil) than for Manawthukha (2.585 mg N₂O kg^-1 soil). However, IR-50 showed less global warming potential (GWP) than Manawthukha (P<0.0050). Although not significant, the numerically lowest CH₄ and N₂O emissions were observed in the cow dung manure treatment (0.808 g CH₄ kg^-1 soil, 2.135 mg N₂O kg^-1 soil) compared to those of the control and compost. To determine the effect of water management and organic manures on greenhouse gas emissions, second pot experiments were conducted in Madaya township during the dry and wet seasons from February to October 2017. Two water management practices {continuous flooding (CF) and alternate wetting and drying (AWD)} and four cow dung manure rates {(1) 0 (2) 2.5 t ha^-1 (3) 5 t ha^-1 (4) 7.5 t ha^-1} were tested. The different cow dung manure rates did not significantly affect grain yield or greenhouse gas emissions in this experiment. Across the manure treatments, AWD irrigation significantly reduced CH₄ emissions by 70% during the dry season and 66% during the wet season. Although a relative increase in N₂O emissions under AWD was observed in both rice seasons, the global warming potential was significantly reduced in AWD compared to CF in both seasons (P<0.0002, P<0.0000) according to reduced emission in CH₄. Therefore, AWD is the effective mitigation practice for reducing GWP without compromising rice yield while manure amendment had no significant effect on GHG emission from paddy rice field. Besides, AWD saved water about 10% in dry season and 19% in wet season.

Efficient Nitrification and Low-Level N2O Emission in a Weakly Acidic Bioreactor at Low Dissolved-Oxygen Levels Are Due to Comammox

Nitrification is an essential process for nutrient removal from wastewater and an important emission source of nitrous oxide (N2O), which is a powerful greenhouse gas and a dominant ozone-depleting substance. In this study, nitrification and N2O emissions were tested in two weakly acidic (pH 6.3 to 6.8) reactors: one with dissolved oxygen (DO) at over 2.0 mg/liter and the other with DO at approximately 0.5 mg/liter. Efficient nitrification was achieved in both reactors. Compared to that in the high-DO reactor, N2O emission in the low-DO reactor decreased slightly, by 20%, and had insignificant correlation with the fluctuations of DO (P = 0.935) and nitrite (P = 0.713), indicating that N2O might not be produced mainly via nitrifier denitrification. Based on quantitative PCR (qPCR), quantitative fluorescent in situ hybridization (qFISH), and functional gene amplicon and metagenome sequencing, it was found that complete ammonia oxidizers (comammox), i.e., Nitrospira organisms, significantly outnumbered canonical ammonia-oxidizing bacteria (AOB) in both weakly acidic reactors, especially in the low-DO reactor with the comammox/AOB amoA gene ratio increasing from 6.6 to 17.1. Therefore, it was speculated that the enriched comammox was the primary cause for the slightly decreased N2O emission under long-term low DO in the weakly acidic reactor. This study demonstrated that the comammox Nitrospira can survive well under the weakly acidic and low-DO conditions, implying that achieving efficient nitrification with low N2O emission as well as low energy and alkalinity consumption is feasible for wastewater treatment.IMPORTANCE Nitrification in wastewater treatment is an important process for eutrophication control and an emission source for the greenhouse gas N2O. The nitrifying process is usually operated at a slightly alkaline pH and high DO (>2 mg/liter) to ensure efficient nitrification. However, it consumes a large amount of energy and chemicals, especially for wastewater without sufficient alkalinity. This paper demonstrates that comammox can adapt well to the weakly acidic and low-DO bioreactors, with a result of efficient nitrification and low N2O emission. These findings indicate that comammox organisms are significant for sustainable wastewater treatment, which provides an opportunity to achieve efficient nitrification with low N2O production as well as low energy and chemical consumption simultaneously.

Cardamonin Attenuates Experimental Colitis and Associated Colorectal Cancer

Cardamonin is a naturally occurring chalcone, majorly from the Zingiberaceae family, which includes a wide range of spices from India. Herein, we investigated the anti-inflammatory property of cardamonin using different in vitro and in vivo systems. In RAW 264.7 cells, treatment with cardamonin showed a reduced nitrous oxide production without affecting the cell viability and decreased the expression of iNOS, TNF-α, and IL-6, and inhibited NF-kB signaling which emphasizes the role of cardamonin as an anti-inflammatory molecule. In a mouse model of dextran sodium sulfate (DSS)-induced colitis, cardamonin treatment protected the mice from colitis. Subsequently, we evaluated the therapeutic potential of this chalcone in a colitis-associated colon cancer model. We performed microRNA profiling in the different groups and observed that cardamonin modulates miRNA expression, thereby inhibiting tumor formation. Together, our findings indicate that cardamonin has the potential to be considered for future therapy against colorectal cancer.

Response of soil N2O emission and nitrogen utilization to organic matter in the wheat and maize rotation system

The appropriate nitrogen (N) fertilizer regulator could increase N utilization of crops and reduce N losses in the North China Plain. We investigated the effects of reduced inorganic-N rate combined with an organic fertilizer on nitrous oxide (N2O) emissions in winter wheat and summer maize rotation system. Simultaneously studied the effect of different treatments on N use efficiency (NUE), N balance and net income. After reducing the amount of nitrogen fertilizer in the wheat-corn rotation system, the results showed that the cumulative emission of soil N2O from the RN40% + HOM [40% of RN (recommended inorganic-N rate) with homemade organic matter] treatment was 41.0% lower than that of the RN treatment. In addition, the N production efficiency, agronomic efficiency, and apparent utilization were significantly increased by 50.2%, 72.4% and 19.5% than RN, respectively. The use of RN40% + HOM resulted in 22.0 and 30.1% lower soil N residual and N losses as compared with RN. After adding organic substances, soil N2O cumulative emission of RN40% + HOM treatment decreased by 20.9% than that of the HAN (zinc and humic acid urea at the same inorganic-N rate of RN) treatment. The N production efficiency, N agronomic efficiency and NUE of RN40% + HOM treatment were 36.6%, 40.9% and 15.3% higher than HAN's. Moreover, soil residual and apparent loss N were 23.3% and 18.0% less than HAN's. The RN40% + HOM treatment appears to be the most effective as a fertilizer control method where it reduced N fertilizer input and its loss to the environment and provided the highest grain yield.

Non-denitrifier nitrous oxide reductases dominate marine biomes

Microbial enzymes often occur as distinct variants that share the same substrate but differ in substrate affinity, sensitivity to environmental conditions, or phylogenetic ancestry. Determining where variants occur in the environment helps identify thresholds that constrain microbial cycling of key chemicals, including the greenhouse gas nitrous oxide (N₂O). To understand the enzymatic basis of N₂O cycling in the ocean, we mined metagenomes to characterize genes encoding bacterial nitrous oxide reductase (NosZ) catalyzing N₂O reduction to N₂. We examined data sets from diverse biomes but focused primarily on those from oxygen minimum zones where N₂O levels are often elevated. With few exceptions, marine nosZ data sets were dominated by 'atypical' clade II gene variants. Atypical nosZ has been associated with low oxygen, enhanced N₂O affinity, and organisms lacking enzymes for complete denitrification, i.e., non-denitrifiers. Atypical nosZ often occurred in metagenome-assembled genomes (MAGs) with nitrate or nitrite respiration genes, although MAGs with genes for complete denitrification were rare. We identified atypical nosZ in several taxa not previously associated with N₂O consumption, in addition to known N₂O-associated groups. The data suggest that marine environments generally select for high N₂O-scavenging ability across diverse taxa and have implications for how N₂O concentration may affect N₂O removal rates.