Spartina alterniflora invasion alters soil bacterial communities and enhances soil N2O emissions by stimulating soil denitrification in mangrove wetland

Chinese mangrove, an important ecosystem in coastal wetlands, is sensitive to the invasive alien species Spartina alterniflora. However, the effects of the S. alterniflora invasion on mangrove soil N₂O emissions and the underlying mechanisms by which emissions are affected have not been well studied. In this study, the N₂O emitted from soils dominated by two typical native mangroves (i.e. Kandelia obovata: KO; Avicennia marina: AM), one invaded by S. alterniflora (SA), and one bare mudflat (Mud) were monitored at Zhangjiang Mangrove Estuary (where S. alterniflora is exotic). Together with soil biogeochemical properties, the potential denitrification rate and the composition of soil bacterial communities were determined simultaneously by ¹⁵NO₃^- tracer and high-throughput sequencing techniques, respectively. Our results showed that S. alterniflora invasion significantly (p < 0.05) increases soil N₂O emissions by 15-28-fold. In addition, isotope results revealed that the soil potential denitrification rate was significantly (p < 0.05) enhanced after S. alterniflora invasion. Moreover, the S. alterniflora invasion significantly (p < 0.05) decreased soil bacterial α-diversity and strongly modified soil bacterial communities. Indicator groups strongly associated with S. alterniflora were Chloroflexia, Alphaproteobacteria, and Bacilli, each of which was abundant and acts as connector in the co-occurrence network. FAPROTAX analysis implied that the S. alterniflora invasion stimulated soil denitrification and nitrification while depressing anaerobic ammonium oxidation (anammox) and dissimilatory nitrate reduction to ammonium (DNRA). Redundancy analysis (RDA) found that soil organic matter (SOM) and pH were the most important environmental factors in altering soil bacterial communities. Taken together, our results imply that the S. alterniflora invasion in mangrove wetlands significantly stimulates soil denitrification and N₂O emissions, thereby contributing N₂O to the atmosphere and contributing to global climate change.

Overdose fertilization induced ammonia-oxidizing archaea producing nitrous oxide in intensive vegetable fields

Little is known about the effects of nitrogen (N) fertilization rates on ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB) and their differential contribution to nitrous oxide (N₂O) production, particularly in greenhouse based high N input vegetable soils. Six N treatments (N1, N2, N3, N4, N5 and N6 representing 0, 293, 587, 880, 1173 and 1760 kg N ha^-1 yr^-1, respectively) were continuously managed for three years in a typically intensified vegetable field in China. The aerobic incubation experiment involving these field-treated soils was designed to evaluate the relative contributions of AOA and AOB to N₂O production by using acetylene or 1-octyne as inhibitors. The results showed that the soil pH and net nitrification rate gradually declined with increasing the fertilizer N application rates. The AOA were responsible for 44-71% of the N₂O production with negligible N₂O from AOB in urea unamended control soils. With urea amendment, the AOA were responsible for 48-53% of the N₂O production in the excessively fertilized soils, namely the N5-N6 soils, while the AOB were responsible for 42-55% in the conventionally fertilized soils, namely the N1-N4 soils. Results indicated that overdose fertilization induced higher AOA-dependent N₂O production than AOB, whereas urea supply led to higher AOB-dependent N₂O production than AOA in conventionally fertilized soils. Additionally, a positive relationship existed between N₂O production and NO₂^- accumulation during the incubation. Further mechanisms for NO₂^--dependent N₂O production in intensive vegetable soils therefore deserve urgent attention.

Nitrification inhibitor DMPSA mitigated N2O emission and promoted NO sink in rainfed wheat

Fertilized cropping systems are important sources of nitrous oxide (N₂O) and nitric oxide (NO) to the atmosphere, and biotic and abiotic processes control the production and consumption of these gases in the soil. In fact, the inhibition of nitrification after application of urea or an ammonium-based fertilizer to agricultural soils has resulted in an efficient strategy to mitigate both N₂O and NO in aerated agricultural soils. Therefore, the NO and N₂O mitigation capacity of a novel nitrification inhibitor (NI), 2-(3,4-dimethyl-1H-pyrazol-1-yl) succinic acid isomeric mixture (DMPSA), has been studied in a winter wheat crop. A high temporal resolution of fluxes of NO and NO₂, obtained by using automatic chambers for urea (U) and urea with DMPSA, allowed a better understanding of the temporal net emissions of these gases under field conditions. Seventy-five days after fertilization, the effective reduction of nitrification by DMPSA significantly decreased the production of NO with respect to the treatment without it, giving net consumption of NO in the soil (-61.72 g-N ha^-1) for U + DMPSA in comparison to net production (227.44 g-N ha^-1) for U. The explanation of NO deposition after NI application, due to biotic and abiotic processes in the soil-plant system, supposes a challenge that needs to be studied in the future. In the case of N₂O, the addition of DMPSA significantly mitigated the emissions of this gas by 71%, though the total N₂O emissions in both fertilized treatments were significantly greater than those of the control (43.69 g-N ha^-1). Regarding the fertilized treatments, no significant effect of DMPSA in comparison to urea alone was observed on grain yield nor bread-making wheat quality. To sum up, we got a significant reduction of N₂O and NO with the addition of DMPSA, without a loss in yield and quality parameters in wheat.

Nitrous oxide effluxes from plants as a potentially important source to the atmosphere

The global budget for nitrous oxide (N₂ O), an important greenhouse gas and probably dominant ozone-depleting substance emitted in the 21^st century, is far from being fully understood. Cycling of N₂ O in terrestrial ecosystems has traditionally exclusively focused on gas exchange between the soil surface (nitrification-denitrification processes) and the atmosphere. Terrestrial vegetation has not been considered in the global budget so far, even though plants are known to release N₂ O. Here, we report the N₂ O emission rates of 32 plant species from 22 different families measured under controlled laboratory conditions. Furthermore, the first isotopocule values (δ¹⁵ N, δ¹⁸ O and δ¹⁵ N^sp ) of N₂ O emitted from plants were determined. A robust relationship established between N₂ O emission and CO₂ respiration rates, which did not alter significantly over a broad range of changing environmental conditions, was used to quantify plant-derived emissions on an ecosystem scale. Stable isotope measurements (δ¹⁵ N, δ¹⁸ O and δ¹⁵ N^sp ) of N₂ O emitted by plants clearly show that the dual isotopocule fingerprint of plant-derived N₂ O differs from that of currently known microbial or chemical processes. Our work suggests that vegetation is a natural source of N₂ O in the environment with a large fraction released by a hitherto unrecognized process.

Biochar amendment and water stress alter rhizosphere carbon and nitrogen budgets in bauxite-processing residue sand under rehabilitation

Nitrogen (N) bioavailability is one of the main limiting factors for microbial activity and vegetation establishment in bauxite-processing residue sand (BRS). Although beneficial effects of biochar on reducing N loss in the early stages of BRS rehabilitation have been observed previously, the underlying mechanisms of this complicated process, particularly the interactions between applied biochar and the plant rhizosphere is largely unknown. This glasshouse study (116 days), investigated the coupled effects of biochar and water stress on N bioavailability in the rhizosphere of ryegrass (Lolium rigidum) grown in BRS amended with di-ammonium phosphate (DAP) fertiliser (at rates of 0 or 2.7 t ha^-1) with and without biochar amendment. The applied biochar was characterised as either aged acidic (AC) or alkaline pine (PC) and was mixed with BRS at a rate of 5% v/v under four moisture regimes (50%, 40%, 20% and 7.5% water holding capacity). Amending BRS with AC and PC biochars increased NH₄⁺ retention and decreased cumulative NH₃ volatilization within both the rhizosphere and root-free zones compared with fertiliser only treatment. These effects were more pronounced for the AC than PC biochar, suggesting that aged acidic biochar has the great potential for use in rapid establishment of vegetation in BRS disposal areas. The biochar amendment increased cumulative nitrous oxide emissions compared with DAP only treatment, with no significant differences among different moisture regimes. The Control and 20% water holding capacity (WHC) treatment showed the highest dissolved organic carbon (DOC) concentrations compared with other treatments and moisture regimes in the ryegrass rhizosphere, while the highest dissolved organic N concentration were observed in the DAP + AC treatment. Reducing moisture levels below 20% WHC generally decreased microbial biomass carbon (MBC) concentrations and activity in both the rhizosphere and root-free zones of all treatments, while total N generally decreased as moisture levels decreased from 50% to 7.5% WHC. Plant took up more N in the DAP + AC treatment compared with DAP + PC and DAP only treatments, while increasing water stress generally resulted in decreased aboveground biomass.

Producing more grain yield of rice with less ammonia volatilization and greenhouse gases emission using slow/controlled-release urea

Ammonia (NH₃) volatilization and greenhouse gas (GHG) emission from rice (Oryza sativa L.) fields contaminate the atmospheric environment and lead to global warming. Field trials (2013-2015) were conducted to estimate the influences of different types of fertilization practices on grain yield, NH₃ volatilization, and methane (CH₄) and nitrous oxide (N₂O) emissions in a double rice cropping system in Central China. Results showed that grain yields of rice were improved significantly by using slow/controlled-release urea (S/C-RU). Compared with farmers' fertilizer practice (FFP) treatment, average annual grain yield with application of polymer-coated urea (CRU), nitrapyrin-treated urea (CP), and urea with effective microorganism (EM) treatments was increased by 18.0%, 16.2%, and 15.4%, respectively. However, the effects on NH₃ volatilization and CH₄ and N₂O emissions differed in diverse S/C-RU. Compared with that of the FFP treatment, the annual NH₃ volatilization, CH₄ emission, and N₂O emissions of the CRU treatment were decreased by 64.8%, 19.7%, and 35.2%, respectively; the annual CH₄ and N₂O emissions of the CP treatment were reduced by 33.7% and 40.3%, respectively, while the NH₃ volatilization was increased by 18.5%; the annual NH₃ and N₂O emissions of the EM treatment were reduced by 6.3% and 28.7%, while the CH₄ emission was improved by 4.3%. Overall, CP showed the best emission reduction with a decrement of 34.3% in global warming potential (GWP) and 44.4% in the greenhouse gas intensity (GHGI), followed by CRU treatment with a decrement of 21.1% in GWP and 31.7% in GHGI, compared with that of the FFP treatment. Hence, it is suggested that polymer-coated urea can be a feasible way of mitigating NH₃ volatilization and CH₄ and N₂O emission from rice fields while maintaining or increasing the grain yield in Chinese, the double rice cropping system.

Nitritation, nitrous oxide emission pathways and in situ microbial community in a modified University of Cape Town process

Achieving nitritation is a prerequisite to promote nutrients removal and save energy, but emission of nitrous oxide as a greenhouse gas cannot be ignored. This study established the nitritation in a continuous-flow MUCT process and investigated the mechanism of N₂O generation. The nitrite accumulation ratio (NAR) reached 95% by controlling the low DO of 0.3-0.5 mg/L and short HRT of 8 h. The ¹⁵N-isotope tracer experiment indicated that the percentage of nitrifier-denitrification (ND) pathway increased by 12.7% under the limited-aeration mode, improving the stable operating of nitritation. Meanwhile, the autotrophic anammox pathway increased with the contribution ratio of 14.7% to N₂ emission under the nitritation mode. The ¹⁵N-DNA-SIP revealed that the Nitrosomonas executed the ND pathway and the Planctomycetes conducted the anammox process, respectively. The integration of autotrophic and heterotrophic process based on nitritation technique has potential to solve the carbon-limited issue for total nitrogen removal in mainstream WWTPs.

Hyperthermophilic composting significantly decreases N2O emissions by regulating N2O-related functional genes

This study reported for the first time that hyperthermophilic composting (HTC) could mitigate 90% of the cumulative amount of N₂O emissions compared to traditional composting (TC) in a full-scale experiment. The concentrations of NO₂^--N and NO₃^--N in HTC were significantly lower than those in TC, which may be the main reason for the reduced N₂O emissions. Furthermore, this study found that the decrease in N₂O emissions in HTC compared to TC was mainly due to the inhibition of the abundance of the bacterial amoA and norB genes, which could decrease the nitrification rate and control N₂O formation, respectively. Partial least squares path modeling revealed that a high temperature was the key factor in lowering N₂O emissions in HTC, while physicochemical properties were the dominant factor in enhancing N₂O emissions in TC. These results suggested that HTC is a promising technique for reducing N₂O emissions in manure composting.

Tree stem bases are sources of CH4 and N2 O in a tropical forest on upland soil during the dry to wet season transition

Tropical forests on upland soils are assumed to be a methane (CH₄ ) sink and a weak source of nitrous oxide (N₂ O), but studies of wetland forests have demonstrated that tree stems can be a substantial source of CH₄ , and recent evidence from temperate woodlands suggests that tree stems can also emit N₂ O. Here, we measured CH₄ and N₂ O fluxes from the soil and from tree stems in a semi-evergreen tropical forest on upland soil. To examine the influence of seasonality, soil abiotic conditions and substrate availability (litter inputs) on trace greenhouse gas (GHG) fluxes, we conducted our study during the transition from the dry to the wet season in a long-term litter manipulation experiment in Panama, Central America. Trace GHG fluxes were measured from individual stem bases of two common tree species and from soils beneath the same trees. Soil CH₄ fluxes varied from uptake in the dry season to minor emissions in the wet season. Soil N₂ O fluxes were negligible during the dry season but increased markedly after the start of the wet season. By contrast, tree stem bases emitted CH₄ and N₂ O throughout the study. Although we observed no clear effect of litter manipulation on trace GHG fluxes, tree species and litter treatments interacted to influence CH₄ fluxes from stems and N₂ O fluxes from stems and soil, indicating complex relationships between tree species traits and decomposition processes that can influence trace GHG dynamics. Collectively, our results show that tropical trees can act as conduits for trace GHGs that most likely originate from deeper soil horizons, even when they are growing on upland soils. Coupled with the finding that the soils may be a weaker sink for CH₄ than previously thought, our research highlights the need to reappraise trace gas budgets in tropical forests.

Arbuscular mycorrhizal fungi reduce nitrous oxide emissions from N2 O hotspots

Nitrous oxide (N2 O) is a potent, globally important, greenhouse gas, predominantly released from agricultural soils during nitrogen (N) cycling. Arbuscular mycorrhizal fungi (AMF) form a mutualistic symbiosis with two-thirds of land plants, providing phosphorus and/or N in exchange for carbon. As AMF acquire N, it was hypothesized that AMF hyphae may reduce N2 O production. AMF hyphae were either allowed (AMF) or prevented (nonAMF) access to a compartment containing an organic matter and soil patch in two independent microcosm experiments. Compartment and patch N2 O production was measured both before and after addition of ammonium and nitrate. In both experiments, N2 O production decreased when AMF hyphae were present before inorganic N addition. In the presence of AMF hyphae, N2 O production remained low following ammonium application, but increased in the nonAMF controls. By contrast, negligible N2 O was produced following nitrate application to either AMF treatment. Thus, the main N2 O source in this system appeared to be via nitrification, and the production of N2 O was reduced in the presence of AMF hyphae. It is hypothesized that AMF hyphae may be outcompeting slow-growing nitrifiers for ammonium. This has significant global implications for our understanding of soil N cycling pathways and N2 O production.

Linking abundance and community of microbial N2O-producers and N2O-reducers with enzymatic N2O production potential in a riparian zone

As aquatic-terrestrial ecotones, riparian zones are hotspots not only for denitrification but also for nitrous oxide (N₂O) emission. Due to the potential role of nosZ II in N₂O mitigation, emerging studies in terrestrial ecosystems have taken this newly reported N₂O-reducer into account. However, our knowledge about the interactions between denitrification activities and both N₂O-producers and reducers (especially for nosZ II) in aquatic ecosystems remains limited. In this study, we investigated spatiotemporal distributions of in situ N₂O flux, potential N₂O production rate, and potential denitrification rate, as well as of the related genes in a riparian zone of Baiyangdian Lake. Real-time quantitative PCR (qPCR) and high-throughput sequencing targeted functional genes were used to analyze the denitrifier communities. Results showed that great differences in microbial activities and abundances were observed between sites and seasons. Waterward sediments (constantly flooded area) had the lowest N₂O production potential in both seasons. Not only the environmental factors (moisture content, NH₄⁺ content and TOM) but also the community structure of N₂O-producers and N₂O-reducers (nirK/nirS and nosZ II/nosZ I ratios) could affect the potential N₂O production rate. The abundance of the four functional genes in the winter was higher than in the summer, and the values all peaked at the occasionally flooded area in the winter. The dissimilarity in community composition was mainly driven by moisture content. Altogether, we propose that the N₂O production potential was largely regulated by the community structure of N₂O-producers and N₂O-reducers in riparian zones. Increasing the constantly flooded area and reducing the occasionally flooded area of lake ecosystems may help reduce the level of denitrifier-produced N₂O.

A bet-hedging strategy for denitrifying bacteria curtails their release of N2O

When oxygen becomes limiting, denitrifying bacteria must prepare for anaerobic respiration by synthesizing the reductases NAR (NO3- → NO2-), NIR (NO2- → NO), NOR (2NO → N2O), and NOS (N2O → N2), either en bloc or sequentially, to avoid entrapment in anoxia without energy. Minimizing the metabolic burden of this precaution is a plausible fitness trait, and we show that the model denitrifier Paracoccus denitrificans achieves this by synthesizing NOS in all cells, while only a minority synthesize NIR. Phenotypic diversification with regards to NIR is ascribed to stochastic initiation of gene transcription, which becomes autocatalytic via NO production. Observed gas kinetics suggest that such bet hedging is widespread among denitrifying bacteria. Moreover, in response to oxygenation, P. denitrificans preserves NIR in the poles of nongrowing persister cells, ready to switch to anaerobic respiration in response to sudden anoxia. Our findings add dimensions to the regulatory biology of denitrification and identify regulatory traits that decrease N2O emissions.

Nitrite accumulation inside sludge flocs significantly influencing nitrous oxide production by ammonium-oxidizing bacteria

This work aims to clarify the role of potential nitrite (NO₂^-) accumulation inside sludge flocs in N₂O production by ammonium-oxidizing bacteria (AOB) at different dissolved oxygen (DO) levels with focus on the conditions of no significant bulk NO₂^- accumulation (<0.2 mg N/L). To this end, an augmented nitrifying sludge with much higher abundance of nitrite-oxidizing bacteria (NOB) than AOB was enriched and then used for systematically designed batch tests, which targeted a range of DO levels from 0 to 3.0 mg O₂/L at a fixed ammonium concentration of 10 mg N/L. A two-pathway N₂O model was applied to facilitate the interpretation of batch experimental data, thus shedding light on the relationships between N₂O production pathways and key process parameters (i.e., DO and NO₂^- accumulation inside sludge flocs). The results demonstrated (i) the biomass specific N₂O production rate firstly increased and then decreased with DO, with the maximum value of 3.03 ± 0.05 mg N/h/g VSS obtained at DO level of 0.75 mg O₂/L, (ii) the AOB denitrification pathway for N₂O production was dominant (98.0%) at all DO levels tested even without significant bulk NO₂^- accumulation (<0.2 mg N/L) observed in the system, but its contribution decreased with DO, (iii) DO had a positive impact on the hydroxylamine pathway for N₂O production which therefore increased with DO, and (iv) the nitrite accumulation existed inside the sludge flocs and induced significant N₂O production from the AOB denitrification pathway.

Carnosine protects pancreatic beta cells and islets against oxidative stress damage

Islet transplantation is a valid therapeutic option for type 1 diabetes treatment. However, in this procedure one of the major problems is the oxidative stress produced during pancreatic islet isolation. The aim of our study was to evaluate potential protective effects of L-carnosine and its isomer D-carnosine against oxidative stress. We evaluated the carnosine effect on cell growth, cell death, insulin production, and the main markers of oxidative stress in rat and murine stressed beta cell lines as well as in human pancreatic islets. Both isomers clearly inhibited hydrogen peroxide induced cytotoxicity, with a decrease in intracellular reactive oxygen and nitrogen species, prevented hydrogen peroxide induced apoptosis/necrosis, nitrite production, and reduced glucose-induced insulin secretion. In addition, NF-κB expression/translocation and nitrated protein induced in stressed cells was significantly reduced. Furthermore, both isomers improved survival and function, and decreased reactive oxygen and nitrogen species, and nitrite and nitrotyrosine production in human islets cultured for 1, 3, and 7 days. These results seem to indicate that both L and D-carnosine have a significant cytoprotective effect by reducing oxidative stress in beta cell lines and human islets, suggesting their potential use to improve islet survival during the islet transplantation procedure.

Modeling nitrous oxide production by a denitrifying-enhanced biologically phosphorus removing (EBPR) activated sludge in the presence of different carbon sources and electron acceptors

In this study, the IWA Activated Sludge Model No. 2d (ASM2d) was expanded to identify the most important mechanisms leading to the anoxic nitrous oxide (N₂O) production in the combined nitrogen (N) and phosphorus (P) removal activated sludge systems. The new model adopted a three-stage denitrification concept and was evaluated against the measured data from one/two-phase batch experiments carried out with activated sludge withdrawn from a local, large-scale biological nutrient removal wastewater treatment plant. The experiments were focused on investigating the effects of different external carbon sources (acetate, ethanol) and electron acceptors (nitrite, nitrate) on the mechanisms of N₂O production in enhanced biological P removal by polyphosphate accumulating organisms (PAOs) and external carbon-based denitrification by ordinary heterotrophic organisms (OHOs). The experimental results explicitly showed that N₂O production was predominantly governed by the presence of nitrite in the reactor regardless of the examined carbon source and the ratio COD/N in the reactor. The model was capable of accurately predicting (with R² > 0.9) the behavior of not only N₂O-N, but also NO₃-N, NO₂-N, soluble COD, and PO₄-P. The simulation results revealed that only OHOs were responsible for N₂O production, whereas the present denitrifying PAOs reduced only nitrate to nitrite.

Metagenomic insights into the effect of oxytetracycline on microbial structures, functions and functional genes in sediment denitrification

Denitrification is an indispensable pathway of nitrogen removal in aquatic ecosystems, and plays an important role in decreasing eutrophication induced by excessive reactive nitrogen pollution. Aquatic environments also suffer from antibiotic pollution due to runoff from farms and sewage systems. The aim of this study was to investigate the effect of oxytetracycline stress on denitrifying functional genes, the microbial community and metabolic pathways in sediments using high-throughput sequencing and metagenomic analysis. The oxytetracycline was observed to significantly inhibit the abundance of nirK and nosZ genes (P < 0.001). KEGG pathway annotation indicated that oxytetracycline treatment decreased the abundance of nitrate reductase, nitrite reductase and N₂O reductase. Functional annotations revealed that oxytetracycline exposure decreased the abundance of the protein metabolism subsystem in the bacterial community. Metagenomic sequencing demonstrated that the abundance of Proteobacteria and Firmicutes increased with oxytetracycline exposure while the Actinobacteria decreased. In sediments, Pseudomonas and Bradyrhizobium were major contributors to denitrification and oxytetracycline exposure resulted in a decreased abundance of Bradyrhizobium. These results indicated that oxytetracycline residues influences the denitrifier community and may heighten occurrence of reactive nitrogen in aquatic ecosystems.

Anammox granular sludge in low-ammonium sewage treatment: Not bigger size driving better performance

An integrated investigation to document high anammox abundance, activity and diversity in upflow anaerobic sludge blanket (UASB) reactor treating low-strength ammonium loading sewage was performed and showed that the optimal anammox granular sludge sizes could mitigate undesirable N₂O emission. The enhanced anammox bacterial abundance, activity and specific anammox rate were achieved with optimal granules sludge sizes of 0.5-0.9 mm with multiple "Jettenia", "Brocadia", and "Anammoxoglobus" species. The tightly-bound extracellular polymeric substance (TB-EPS) was the main EPS layer found in anammox granular sludge, in which polysaccharides play an important structural role. Over this granular sludge sizes, the anammox bacterial abundance and activity did not significantly decrease, but N₂O emission significantly increased. High throughput sequencing and ecological networks demonstrated the patterns of anammox and their co-occurring bacteria, with availability N₂O-producer and N₂O-reducer functional genes. Incomplete denitrification and insufficient carbon source mainly contributed to N₂O production in granular sludge, as supported by results of stratification analysis.

Improving leachate quality and optimizing CH4 and N2O emissions from a pre-aerated semi-aerobic bioreactor landfill using different pre-aeration strategies

Landfill aeration efficiently accelerates municipal solid waste (MSW) stabilization. This method also impacts methane (CH₄) and nitrous oxide (N₂O) emissions during aeration. In this study, the effects of three pre-aeration strategies on leachate quality variations and CH₄ and N₂O emissions from three lab-scale pre-aerated semi-aerobic bioreactor landfills, which were filled with MSW, were investigated: low frequency and high frequency intermittent aeration (LIA and HIA) and continuous micro-aeration (CMA). Experimental results showed that these three strategies effectively reduced organic and N-based pollutants concentration in leachate. Compared with intermittent aeration (IA), CMA increased cumulative CH₄ emissions (9234.3 mg) and resulted in a longer emission period (95 days). HIA generated the least cumulative CH₄ emissions (4297.6 mg) and shortest emission period (65 days) due to organic matter loss during aeration. N₂O emissions were present at low levels in early stages for each bioreactor, and then, increased by 1-3 orders of magnitude in the later stages due to low influent carbon-nitrogen ratio. HIA resulted in maximum cumulative N₂O emissions (2884.6 mg) and experienced a longer emission period (179 days) compared to CMA (2281.6 mg; 151 days). LIA had the longest N₂O emission period (209 days), but had the lowest cumulative N₂O emissions (1486.3 mg). CH₄ and N₂O emissions mainly occurred in the early and later stages of landfill stabilization, respectively. Therefore, the study proposes an optimized pre-aeration strategy for practical landfill aeration management: early CMA may promote rapid organic matter removal and effective CH₄ recovery; and late LIA may reduce N₂O emissions.

Systemic Effects of Segmental Vibration in an Animal Model of Hand-Arm Vibration Syndrome

OBJECTIVE

Epidemiology suggests that occupational exposure to hand-transmitted (segmental) vibration has local and systemic effects. This study used an animal model of segmental vibration to characterize the systemic effects of vibration.

METHODS

Male Sprague Dawley rats were exposed to tail vibration for 10 days. Genes indicative of inflammation, oxidative stress, and cell cycle, along were measured in the heart, kidney, prostate, and liver.

RESULTS

Vibration increased oxidative stress and pro-inflammatory gene expression, and decreased anti-oxidant enzymes in heart tissue. In the prostate and liver, vibration resulted in changes in the expression of pro-inflammatory factors and genes involved in cell cycle regulation.

CONCLUSIONS

These changes are consistent with epidemiological studies suggesting that segmental vibration has systemic effects. These effects may be mediated by changes in autonomic nervous system function, and/or inflammation and oxidative stress.

Is there a nitrogen fertilizer threshold emitting less N2O with the prerequisite of high wheat production?

Excessive use of synthetic nitrogen (N) fertilizer and lower nitrogen use efficiency (NUE) are threatening the wheat production in the middle and lower reaches of Yangtze River. Excess input of N fertilizers also results in severe environmental pollution, climate change and biodiversity loss. However, the study on reasonable nitrogen application and NUE improvement with the prerequisite of stable and high yield remains unexplored. In our study, the four different levels of nitrogen were applied to find out the nitrogen threshold which could be both friendly to environment and promise the stable and high yield. The experiment was carried out in Yangzhou University (Yangzhou, China). The wheat cultivar Yangmai 23 was selected as the research material. The four nitrogen levels were as follows: 0, 189, 229.5, and 270 kg ha^-1. The results showed that the grain yield under the application of 229.5 kg ha^-1 N was as high as that under 270 kg ha^-1 N level, with the observation of 20.3% increase in agronomic efficiency. The N₂O emission of 229.5 kg ha^-1 N application was as low as that of 189 kg ha^-1 N, but the grain yield and agronomic efficiency were significantly higher (11.9%) under 229.5 kg ha^-1 treatment than the lower one. Taken together, this indicated the nitrogen level at 229.5 kg ha^-1 could be identified as the fertilizer threshold, which will be beneficial for the future fieldwork.

Influence of nitrate and nitrite concentration on N2 O production via dissimilatory nitrate/nitrite reduction to ammonium in Bacillus paralicheniformis LMG 6934

Until now, the exact mechanisms for N2 O production in dissimilatory nitrate/nitrite reduction to ammonium (DNRA) remain underexplored. Previously, we investigated this mechanism in Bacillus licheniformis and Bacillus paralicheniformis, ubiquitous gram-positive bacteria with many industrial applications, and observed significant strain dependency and media dependency in N2 O production which was thought to correlate with high residual NO2- . Here, we further studied the influence of several physicochemical factors on NO3- (or NO2- ) partitioning and N2 O production in DNRA to shed light on the possible mechanisms of N2 O production. The effects of NO3- concentrations under variable or fixed C/N-NO3- ratios, NO2- concentrations under variable or fixed C/N-NO2- ratios, and NH4+ concentrations under fixed C/N-NO3- ratios were tested during anaerobic incubation of soil bacterium B. paralicheniformis LMG 6934 (previously known as B. licheniformis), a strain with a high nitrite reduction capacity. Monitoring of growth, NO3- , NO2- , NH4+ concentration, and N2 O production in physiological tests revealed that NO3- as well as NO2- concentration showed a linear correlation with N2 O production. Increased NO3- concentration under fixed C/N-NO3- ratios, NO2- concentration, and NH4+ concentration had a significant positive effect on NO3- (or NO2- ) partitioning ([N-NH4+ ]/[N-N2 O]) toward N2 O, which may be a consequence of the (transient) accumulation and subsequent detoxification of NO2- . These findings extend the information on several physiological parameters affecting DNRA and provide a basis for further study on N2 O production during this process.

Nitrogen removal and N2O emission by shunt distributing wastewater in aerated or non-aerated subsurface wastewater infiltration systems under different shunt ratios

This study investigated matrix oxidation-reduction potential (ORP), nitrogen removal, N₂O emission and nitrogen removal functional gene abundance in three subsurface wastewater infiltration systems (SWISs), named SWIS A (without aeration or shunt distributing wastewater), SWIS B (with shunt distributing wastewater) and SWIS C (with intermittent aeration and shunt distributing wastewater) under different shunt ratios. Aerobic conditions were produced at a depth of 50 cm and anoxic or anaerobic conditions were not changed at depths of 80 and 110 cm by aeration in SWIS C. High average removal rates of chemical oxygen demand (COD) (83.1% for SWIS B, 90.9% for SWIS C), NH₃-N (74.3% for SWIS B, 90.8% for SWIS C) and total nitrogen (TN) (61.1% for SWIS B, 87.9% for SWIS C) were obtained under shunt ratios of 1:3 and 1:2 for SWIS B and C, respectively. The lowest N₂O emission rate (28.4 mg/(m² d)) and highest nitrogen removal functional gene abundances were achieved in SWIS C under a 1:2 shunt ratio. The results suggested intermittent aeration and shunt distributing wastewater combined strategy would enhance nitrogen removal and reduce N₂O emission for SWISs.

Does influent COD/N ratio affect nitrogen removal and N2O emission in a novel biochar-sludge amended soil wastewater infiltration system (SWIS)?

Nitrogen removal and N₂O emission of a biochar-sludge amended soil wastewater infiltration system (SWIS) with/without intermittent aeration under different influent COD/N ratios was investigated. Nitrogen removal and N₂O emission were affected by influent COD/N ratio. Under a COD/N ratio between 1:1 and 15:1, average chemical oxygen demand (COD), NH₄ ⁺-N and total nitrogen (TN) removal rates decreased with COD/N ratio increase in non-aerated SWISs amended with/without biochar-sludge; an increasing COD/N ratio hardly affected COD and NH₄ ⁺-N removal in a biochar-sludge amended SWIS with intermittent aeration; the N₂O emission rate decreased with COD/N ratio increase in the studied SWISs. The biochar-sludge amended SWIS with intermittent aeration achieved high COD (92.2%), NH₄ ⁺-N (96.8%), and TN (92.7%) removal rates and a low N₂O emission rate (10.6 mg/(m² d)) under a COD/N ratio of 15:1, which was higher than those in non-aerated SWISs amended with/without biochar-sludge. Combining the biochar-sludge amended SWIS with intermittent aeration enhanced the number of nitrifying bacteria, denitrifying bacteria, nitrate reductase activities, nitrite reductase activities, and improved the abundance of nitrogen removal functional genes under a high influent COD/N ratio. The results suggested that the joint use of intermittent aeration and biochar-sludge in a SWIS could be an effective and appropriate strategy for improving nitrogen removal and reducing N₂O emissions in treating high COD/N ratio wastewater.

Gene cloning, expression, and reducing property enhancement of nitrous oxide reductase from Alcaligenes denitrificans strain TB

Nitrous oxide (N₂O) is a potent greenhouse gas and tends to accumulate as an intermediate in the process of bacteria denitrification. To achieve complete reduction of nitrogen oxide (NO_x) in bacteria denitrification, the structural gene nosZ encoding nitrous oxide reductase (N₂OR) was cloned from Alcaligenes denitrificans strain TB (GenBank JQ044686). The recombinant plasmid containing the nosZ gene was built, and the expression of nosZ gene in Escherichia coli was determined. Results show that the nosZ gene consisting of 1917 nucleotides achieves heterologous expression successfully by codon optimization strategy under optimal conditions (pre-induction inoculum OD₆₀₀ of 0.67, final IPTG concentration of 0.5 mM, inducing time of 6 h, and inducing temperature of 28 °C). Determination result of gas chromatography confirms that N₂O degradation efficiency of recombinant E. coli is strengthened by at least 1.92 times compared with that of original strain TB when treated with N₂O as substrate. Moreover, N₂OR activity in recombinant strain is 2.09 times higher than that in wild strain TB, which validates the aforementioned result and implies that the recombinant E. coli BL21 (DE3)-pET28b-nosZ is a potential candidate to control N₂O accumulation and alleviate greenhouse effect. In addition, the N₂OR structure and the possible N₂O binding site in Alcaligenes sp. TB are predicted, which open an avenue for further research on the relationship between N₂OR activity and its structure.

Absence of the Nitrous Oxide Reductase Gene Cluster in Commercial Alfalfa Inoculants Is Probably Due to the Extensive Loss of Genes During Rhizobial Domestication

As other legume crops, alfalfa cultivation increases the emission of the greenhouse gas nitrous oxide (N₂O). Since legume-symbiotic nitrogen-fixing bacteria play a crucial role in this emission, it is important to understand the possible impacts of rhizobial domestication on the evolution of denitrification genes. In comparison with the genomes of non-commercial strains, those of commercial alfalfa inoculants exhibit low total genome size, low number of ORFs and high numbers of both frameshifted genes and pseudogenes, suggesting a dramatic loss of genes during bacterial domestication. Genomic analysis focused on denitrification genes revealed that commercial strains have perfectly conserved the nitrate (NAP), nitrite (NIR) and nitric (NOR) reductase clusters related to the production of N₂O from nitrate but completely lost the nitrous oxide (NOS) reductase cluster (nosRZDFYLX genes) associated with the reduction of N₂O to gas nitrogen. Based on these results, we propose future screenings for alfalfa-nodulating isolates containing both nitrogen fixation and N₂O reductase genes for environmental sustainability of alfalfa production.