CS2 increasing CH4-derived carbon emissions and active microbial diversity in lake sediments.

Lakes are important methane (CH4) sources to the atmosphere, especially eutrophic lakes with cyanobacterial blooms accompanied by volatile sulfur compound (VSC) emissions. CH4 oxidation is a key strategy to mitigate CH4 emission from lakes. In this study, we characterized the fate of CH4-derived carbon and active microbial communities in lake sediments with CS2 used as a typical VSC, based on the investigation of CH4 and VSC fluxes from Meiliang Bay in Lake Taihu. Stable isotope probing microcosm incubation showed that the efficiency of CH4-derived carbon incorporated into organic matter was 21.1% in the sediment with CS2 existence, which was lower than that without CS2 (27.3%). SO42--S was the main product of CS2 oxidation under aerobic condition, accounting for 59.3-62.7% of the input CS2-S. CS2 and CH4 coexistence led to a decrease of methanotroph and methylotroph abundances and stimulated the production of extracellular polymeric substances. CS2 and its metabolites including total sulfur, SO42- and acid volatile sulfur acted as the main drivers influencing the active microbial community structure in the sediments. Compared with α-proteobacteria methanotrophs, γ-proteobacteria methanotrophs Methylomicrobium, Methylomonas, Crenothrix and Methylosarcina were more dominant in the sediments. CH4-derived carbon mainly flowed into methylotrophs in the first stage. With CH4 consumption, more CH4-derived carbon flowed into non-methylotrophs. CS2 could prompt more CH4-derived carbon flowing into non-methanotrophs and non-methylotrophs, such as sulfur-metabolizing bacteria. These findings can help elucidate the influence of VSCs on microorganisms and provide insights to carbon fluxes from eutrophic lake systems.

Characterization of anaerobic oxidation of methane and microbial community in landfills with aeration.

Landfills are the third largest source of anthropogenic CH4 emissions. Anaerobic oxidation of methane (AOM) activity and communities of methane-oxidizing bacteria were investigated in three informal landfills in this study, namely, BJ, CH and SZ landfills, among which BJ and CH represent traditional anaerobic landfills, while the SZ landfill was subjected to aeration to accelerate waste stabilization. The AOM rates of the investigated landfilled wastes ranged from 3.66 to 23.91 nmol g-1 h-1. Among the three landfills, the AOM rate was highest in the SZ-1-Top sample, which was closest to the aeration pipe. Among the possible electron acceptors for AOM, including NO3-, NO2-, SO42- and Fe3+, the NO2--N content was the only variable that was positively correlated with the AOM rate. Compared with α-Proteobacteria methanotrophs, γ-Proteobacteria methanotrophs were more abundant in the landfilled waste, especially Methylobacter, which was detected in nearly all samples. Members of the family Methylomirabilaceae, including Candidatus Methylomirabilis, were also detected in the SZ-1 and SZ-2-Bot samples. The relative abundance of the main methanotrophs in the families Methylomonadaceae, Methylococcaceae, Rokubacteriales and Methylomirabilaceae, the genus Methylocystis and the phylum NC10 were all positive correlations with the contents of NO2--N in the landfilled waste samples. Additionally, significantly positive correlations were observed between the AOM rates and the relative abundance of the main methanotrophs except for the family Methylococcaceae. This indicated that aeration could enhance the conversion of nitrogen compounds in the landfilled waste, in which the high contents of NO2--N could stimulate the growth of methanotrophs and increase AOM rate. These findings are helpful for understanding the mechanisms of CH4 oxidation in landfills and for taking effective measures to mitigate CH4 emissions from landfills.

[Inhibition effect of biocontrol bacteria NJ13 and its mixture with chemical fungicides against ginseng root rot caused by Fusarium solani].

This study was aimed to clarify the toxicity indoor and inhibition effect of biocontrol strain NJ13 and its mixture with chemical fungicides against Fusarium solani causing ginseng root rot. The method of mycelial growth rate and Sun Yunpei method were used to determine the indoor toxicity and co-toxicity coefficient of strain NJ13 and their mixture with chemical pesticides against F. solani. The dual culture assay method,mixed culture method and microscopic observation were used to determine the sporulation and germination of spores and mycelial growth and morphological change of hyphae of F. solani treated by strain NJ13. The results of toxicity indoor showed that strain NJ13 had the best inhibitory effect on pathogen,and its EC_(50) value was 0. 071 mg·L~(-1). It was all synergistic for antifungal effect that strain NJ13 was mixed with propiconazole and difenoconazole respectively with a range from 1 ∶4 to 4 ∶1( volume ratio). Both of optimal ratios were 1 ∶1,and the co-toxicity coefficients were 848. 70 and 859. 73,respectively. The strain NJ13 could inhibit the sporulation,germination and mycelial growth of F. solani. The biocontrol strain NJ13 had an inhibition effect on F. solani,and the optimal antifungal ratio of strain NJ13 mixed with propiconazole and difenoconazole was obtained.

[Evaluation of isolation and field effect of ginseng disease resistance and growth-promoting bacteria].

This study was aimed to isolate the strains with both disease resistance and growth-promoting, and clarify the field application effects of the strain for laying the further application foundation. The strains with good antagonistic effect were isolated from the 298 strains in Panax ginseng and the soil by plate confrontation method. The nitrogen fixation potential was verified by Ashby medium. The Salkowski method was used to determine the ability of producing IAA. Silicate medium screening and flame spectrophotometry was used to determine the ability of dissolving potassium. CAS method was applied to detect the ability of producing siderophores to determine its growth characteristics. The morphological, physiological and biochemical and 16S rRNA sequences were used to identify the species. The method of root irrigation was used to determine the effects of its disease control and growth-promoting on ginseng. A strain TY15 with broad spectrum of antimicrobial effect, nitrogen fixation, potassium-dissolving and the capacity of producing IAA and siderophores was obtained by screening. And the strain TY15 was identified as Pantoea agglomerans. The control effect of TY15 on the disease of ginseng in the field was 68.02%, which was equivalent to 68.94% of 30 billion per gram of beneficial microecological bacterium agent. The fresh weight of P. ginseng treated with TY15 strain was increased by 22.73% compared with the control group treated with water. And finally a strain TY15 with good application prospects was obtained.

Estimation of sulfur fate and contribution to VSC emissions from lakes during algae decay.

Algae decay is an important process influencing environmental variables and emissions of volatile sulfur compounds (VSCs) in eutrophic lakes. However, effects of algae decay on VSC emissions from eutrophic lakes as well as fate of algae-derived sulfur remain poorly understood. In this study, simulated algae-sediment systems were used to explore the flow and distribution of sulfur during algae decay. VSCs including hydrogen sulfide (H2S), methanethiol (CH3SH), carbon disulfide (CS2) and dimethyl sulfide ((CH3)2S) were detected during algae decay, which increased with algae biomass and eutrophic levels in lakes. During algae decay, the highest H2S, CH3SH and (CH3)2S emission rates of 10.45, 21.82 and 43.26 µg d-1 occurred in the first 1-2 days, respectively, while the highest CS2 emission rates were observed between days 8 and 11. The maximum emissions of H2S and CS2 from algae decay were estimated at 0.51 and 0.35 mg m-2 d-1 in Lake Taihu, accounting for 1.57% and 0.69% of the total H2S and CS2 emissions of in situ, respectively. Algae decay could significantly increase the contents of total sulfur and total carbon in sediments by 2.90%-21.11% and 4.23%-45.05%, respectively. The VSC emissions during algae decay could be predicted using the multiple regression models with the contents of total carbon, total nitrogen and sulfur-containing compounds in sediments. Partial least squares path modelling demonstrated that algae decay had a low direct effect on VSC emissions with a strength of 0.06, while it had a significant influence on environmental variables with a strength of 0.63, which could affect VSC emissions with a strength of 0.85, indicating VSC emissions from eutrophic lakes were affected by the environmental variables rather than the direct influence of algae decay. These findings illustrated the mechanisms of VSC emissions during algae decay and provided insights into VSC control and mitigation for eutrophic lakes.

Eutrophication levels increase sulfur biotransformation and emissions from sediments of Lake Taihu.

Eutrophication can stimulate the emissions of volatile sulfur compounds (VSCs) accompanied by variations in environmental variables in lakes. However, the effects of eutrophication on VSC emissions from lake sediments as well as the underlying mechanisms remain unclear. In this study, depth gradient sediments at different eutrophication levels and seasons were collected from Lake Taihu to investigate the response of sulfur biotransformation in the sediments to eutrophication based on the analysis of environmental variables, microbial activity, abundance and community structure. H2S and CS2 were the main VSCs produced from the lake sediments, with the production rates of 2.3-7.9 and 1.2-3.9 ng g-1 h-1 in August, respectively, which were higher than those in March, mainly due to the increasing activity and abundance of sulfate-reducing bacteria (SRB) at high temperatures. The VSC production rates from the sediments increased with lake eutrophication level. Higher VSC production rates were detected in surface sediments in eutrophic regions but in deep sediments in oligotrophic regions. Sulfuricurvum, Thiobacillus and Sulfuricella were the main sulfur-oxidizing bacteria (SOB) in the sediments, while Desulfatiglans and Desulfobacca were the predominant SRB. Organic matter, Fe3+, NO3--N and total sulfur had significant influences on the microbial communities in the sediments. Partial least squares path modelling showed that the trophic level index could stimulate VSC emissions from lake sediments by influencing the activities and abundances of SOB and SRB. These findings indicated that sediments contributed substantially to VSC emissions from eutrophic lakes, especially surface sediments, and sediment dredging might be an effective way to mitigate VSC emissions from eutrophic lakes.

Characterization of methanotrophic community and activity in landfill cover soils under dimethyl sulfide stress.

Landfill cover soil is the environmental interface between landfills and the atmosphere and plays an important role in mitigating CH4 emission from landfills. Here, stable isotope probing microcosms with CH4 or CH4 and dimethyl sulfide (DMS) were carried out to characterize activity and community structure of methanotrophs in landfill cover soils under DMS stress. The CH4 oxidation activity in the landfill cover soils was not obviously influenced at the DMS concentration of 0.05%, while it was inhibited at the DMS concentrations of 0.1% and 0.2%. DMS-S was mainly oxidized to sulfate (SO42-) in the landfill cover soils. In the landfill cover soils, DMS could inhibit the expression of bacteria and decrease the abundances of pmoA and mmoX genes, while it could prompt the expression of pmoA and mmoX genes. γ-Proteobacteria methanotrophs including Methylocaldum, Methylobacter, Crenothrix and unclassified Methylococcaceae and α-Proteobacteria methanotrophs Methylocystis dominated in assimilating CH4 in the landfill cover soils. Of them, Methylobacter and Crenothrix had strong tolerance to DMS or DMS could promote the growth and activity of Methylobacter and Crenothrix, while Methylocaldum had weak tolerance to DMS and showed an inhibitory effect. Metagenomic analyses showed that methanotrophs had the genes of methanethiol oxidation and could metabolize CH4 and methanethiol simultaneously in the landfill cover soils. These findings suggested that methanotrophs might metabolize sulfur compounds in the landfill cover soils, which may provide the potential application in engineering for co-removal of CH4 and sulfur compounds.

Additional ratios of hydrolysates from lignocellulosic digestate at different hydrothermal temperatures influencing anaerobic digestion performance.

Hydrothermal treatment (HT) is envisaged as a promising technology to treat the lignocellulosic biomass. HT temperature is an important parameter influencing the hydrolysate compositions such as organic compounds and potential inhibitors, and therefore affect the subsequential anaerobic digestion (AD) process. Herein, HT-AD was employed to treat the wheat straw-derived digestate. The HT temperature of 190 °C was proved to be the best performance with a higehst reducing sugar yield (45.05 mg g-1) in the hydrolysate and a highest methane yield (120.8 mL gTS-1) from the AD of the hydrolysate, which was 42.5% higher than the methane yield in the control without the hydrolysate addition (84.8 mL gTS-1). 3-Furaldehyde was the dominant organic in the hydrolysates. The HT temperature of 210 °C led to the presence of AD inhibitory moieties (e.g., phenols and furans) in the hydrolysate, resulting in a low methane yield. Although the treatments with the addition of 100% hydrolysate outperformed those of 50% hydrolysate in the methane yields in the late stage, the latter had higher methane yields in the first stage, suggesting that the additional ratios of hydrolysates should be carefully considered in AD, especially the detrimental effects of inhibitors and adaptability issues of AD consortia. The MiSeq sequencing showed that the hydrolysis/acidogenesis was dominant in the first stage, while methanogenesis became dominant in the late stage with the acetoclastic/hydrogenotrophic methanogens (Methanosarcina and Methanobacterium) enriched in the hydrolysate-feeding reactors. These findings demonstrated that a integration of HT-AD was a promising approach for the digestate valorization and to reduce the potential carbon emission from waste treatments.

Characteristic pollutants and microbial community in underlying soils for evaluating landfill leakage.

Leachate leakage poses a serious environmental risk to the safety of surrounding soils and groundwater. A much faster approach to reflect landfill leakage is the premise to mitigate the ecological risk of landfills. In this study, two landfills (BJ and WZ) were selected to investigate the leaching characteristics of various pollutants along the vadose soil depths. The physiochemical properties of underlying soils including NO3--N, NO2--N, NH4+-N, OM, TN, EC and Cl- exhibited a typical leaching dynamic along the depths. Among them, TN, NH4+-N, OM, NO3--N, and EC might be used as characteristic pollutants to evaluate the leachate leakage issues in landfilled sites. The genera Thiopseudomonas, Acinetobacter, Pseudomonas, and Hydrogenispora dominated in underlying soils. Compared to BJ samples, a more diverse and active microbiome capable of carbon and nitrogen cycles was observed in WZ samples, which was mainly ascribed to nutrients and elements contained in different types of soils. Among the environmental factors, nitrogenous compounds, SO42-, pH and EC had significant effects on the microbial community structures in the underlying soils. The relative abundances of Hydrogenispora and Caldicoprobacter might be used as characteristic microorganisms to evaluate the leachate leakage issues in landfilled sites. These results provided a deep insight into effects of leachate leakage in underlying soils, especially the pollutants vertical distribution and the corresponding microbial community structures.

Intermittent aeration reducing N2O emissions from bioreactor landfills with gas-water joint regulation.

Landfills are important emission sources of atmospheric N2O, especially bioreactor landfills with leachate recirculation. In this study, N2O emissions were characterized in four bioreactor landfills with different ventilation methods, including intermittent (2-h aeration per 12 h or 4 h/d in continuous) and continuous aeration (20 h/d), in comparison to a traditional landfill without aeration. During the experiment, the N2O emissions from the landfill reactors with intermittent aeration were 7.48 and 7.15 mg, accounting for only 20.8% and 19.9% of those with continuous aeration, respectively. Continuous aeration was more favorable for the biodegradation of organic matter than intermittent aeration in the landfilled waste and leachate. Intermittent and continuous aeration could both effectively remove total nitrogen (TN) and NH4+-N with removal efficiencies above 64% in the leachate. In the experimental landfill reactors with gas-water joint regulation, the proportion of N2O-N to TN loss ranged from 0.02% to 0.75%. Luteimonas, Pseudomonas, Thauera, Pusillimonas and Comamonas were the dominant denitrifying bacteria in the landfill reactors. The denitrifying bacterial community in the landfilled waste was closely related to its degree of stabilization and nitrogenous compound concentrations in the landfilled waste and leachate. The NO3--N and NO2--N concentrations of leachate were the most important environmental factors affecting the succession of nirS-type and nirK-type denitrifying microbial communities in the landfilled waste. These findings indicated that intermittent aeration was an economical and effective way to accelerate the stabilization of landfilled waste and reduce the pollutants in leachate and N2O emissions during landfill mining and reclamation.

Eutrophic levels and algae growth increase emissions of methane and volatile sulfur compounds from lakes.

Eutrophic lakes are hot spots of CH4 and volatile sulfur compound (VSC) emissions, especially during algal blooms and decay. However, the response of CH4 and VSC emissions to lake eutrophication and algae growth as well as the underlying mechanisms remain unclear. In this study, the emissions of CH4 and VSCs from four regions of Lake Taihu with different eutrophic levels were investigated in four months (i.e., March, May, August and December). The CH4 emissions ranged from 20.4 to 126.9 mg m-2 d-1 in the investigated sites and increased with eutrophic levels and temperature. H2S and CS2 were the dominant volatile sulfur compounds (VSCs) emitted from the lake. The CH4 oxidation potential of water ranged from 2.1 to 14.9 µg h-1 L-1, which had positive correlations with trophic level index and the environmental variables except for the NH4+-N concentration. Eutrophic levels could increase the abundances of bacteria and methanotrophs in lake water. α-Proteobacteria methanotroph Methylocystis was more abundant than γ-Proteobacteria methanotrophs in March and May, while the latter was more abundant in August and November. The relative abundance of Cyanobacteria, including Microcystis, A. granulata var. angustissima and Cyanobium had significantly positive correlations with temperature, turbidity, SO42--S, and total sulfur. Partial least squares path modelling revealed that the algal growth could promote VSC emissions, which had a positive correlation with CH4 oxidation potential, likely due to the positive correlation between the CH4 and VSC emissions from lakes. These findings indicate that water eutrophication and algae growth could increase the emissions of CH4 and VSCs from lakes. Controlling algae growth might be an effective way to mitigate the emissions of CH4 and VSCs from freshwater lakes.

Metabolic flexibility of aerobic methanotrophs under anoxic conditions in Arctic lake sediments.

Methane (CH4) emissions from Arctic lakes are a large and growing source of greenhouse gas to the atmosphere with critical implications for global climate. Because Arctic lakes are ice covered for much of the year, understanding the metabolic flexibility of methanotrophs under anoxic conditions would aid in characterizing the mechanisms responsible for limiting CH4 emissions from high-latitude regions. Using sediments from an active CH4 seep in Lake Qalluuraq, Alaska, we conducted DNA-based stable isotope probing (SIP) in anoxic mesocosms and found that aerobic Gammaproteobacterial methanotrophs dominated in assimilating CH4. Aerobic methanotrophs were also detected down to 70 cm deep in sediments at the seep site, where anoxic conditions persist. Metagenomic analyses of the heavy DNA from 13CH4-SIP incubations showed that these aerobic methanotrophs had the capacity to generate intermediates such as methanol, formaldehyde, and formate from CH4 oxidation and to oxidize formaldehyde in the tetrahydromethanopterin (H4MPT)-dependent pathway under anoxic conditions. The high levels of Fe present in sediments, combined with Fe and CH4 profiles in the persistent CH4 seep site, suggested that oxidation of CH4, or, more specifically, its intermediates such as methanol and formaldehyde might be coupled to iron reduction. Aerobic methanotrophs also possessed genes associated with nitrogen and hydrogen metabolism, which might provide potentially alternative energy conservation options under anoxic conditions. These results expand the known metabolic spectrum of aerobic methanotrophs under anoxic conditions and necessitate the re-assessment of the mechanisms underlying CH4 oxidation in the Arctic, especially under lakes that experience extended O2 limitations during ice cover.

Reduction in VOC emissions by intermittent aeration in bioreactor landfills with gas-water joint regulation.

Landfill mining and reclamation is a new strategy for addressing the lack of space available for new landfills and realizing the sustainable development of landfills. A gas-water joint bioreactor landfill is regulated by injecting water and/or recirculating leachate, and a blasting aeration system to optimize waste stabilization. In this study, four landfill reactors were constructed to investigate the effects of ventilation methods, including continuous (20 h d-1) and intermittent aeration (4 h d-1 in continuous or 2-h aeration per 12 h, twice a day), on the degradation of organic matter and volatile organic compound (VOC) emissions in comparison with traditional landfills. A total of 62 VOCs were detected in the landfill reactors. Among them, halogenated compounds had the highest abundance (39.8-65.4 %), followed by oxygenated compounds, alkanes and alkenes, and aromatic compounds. Both intermittent and continuous aeration could accelerate the degradation of landfilled waste and increase the volatilization rate of VOCs. Compared with intermittent aeration, the degradation of landfilled waste was more quickly in the landfill reactor with continuous aeration. However, intermittent aeration could create anaerobic-anoxic-aerobic conditions, which were conducive to the growth and metabolism of anaerobic and aerobic microorganisms in landfills and thereby reduced more than 63.4 % of total VOC emissions from the landfill reactor with continuous aeration. Moreover, intermittent aeration could reduce the ventilation rate and decrease the cost of aeration by 80 % relative to continuous aeration. Firmicutes, Bacteroidetes, Proteobacteria and Tenericutes predominated in the landfill reactors. The environmental variables including organic matter and VOCs concentrations had significant influences on microbial community structure in the landfilled waste. These findings indicated that intermittent aeration was an effective way to accelerate the stabilization of landfilled waste and reduce the cost and environmental risks in bioreactor landfills with gas-water joint regulation.

Effects of different pretreatment methods on biogas production and microbial community in anaerobic digestion of wheat straw.

The pretreatment of wheat straw has been recognized to be an essential step prior to anaerobic digestion, owing to the high abundance of lignocellulosic materials. In order to choose economical and effective techniques for the disposal of wheat straw, effects of five pretreatment methods including acid, alkali, co-pretreatment of acid and alkali, CaO2, and liquid digestate of municipal sewage sludge on anaerobic digestion of wheat straw were investigated by analyzing biogas production and organic matter degradation in the study. The results showed that among these pretreatment methods, the methane yield was highest in the liquid digestate pretreated-wheat straw with 112.6 mL gTS-1, followed by the acid, alkali, and CaO2 pretreatments, and the lowest was observed in the co-pretreatment of acid and alkali. Illumina MiSeq sequencing of the microbial communities in the anaerobic digesters revealed that the genera Ruminiclostridium including Ruminiclostridium and Ruminiclostridium 1, Hydrogenispora, and Capriciproducens were the main hydrolytic bacteria, acidogenic bacteria, and acetogenic bacteria, respectively, in the anaerobic digesters. Capriciproducens and Hydrogenispora dominated in the first and the later stages, respectively, in the anaerobic digesters, which could work as indicators of the anaerobic co-digestion stage of sludge and wheat straw. The total solid and SO42--S contents of the solid digestate and the NH4+-N concentration of the liquid digestate had a significant influence on the microbial community in the digesters. These findings indicated that liquid digestate pretreatment was a potential option to improve the anaerobic digestion of wheat straw, due to the low cost without additional chemical agents.

Stable-isotopic and metagenomic analyses reveal metabolic and microbial link of aerobic methane oxidation coupled to denitrification at different O2 levels.

Aerobic methane (CH4) oxidation coupled to denitrification (AME-D) can not only mitigate CH4 emission into the atmosphere, but also potentially alleviate nitrogen pollution in surface waters and engineered ecosystems, and it has attracted substantial research interest. O2 concentration plays a key role in AME-D, yet little is understood about how it impacts microbial interactions. Here, we applied isotopically labeled K15NO3 and 13CH4 and metagenomic analyses to investigate the metabolic and microbial link of AME-D at different O2 levels. Among the four experimental O2 levels of 21%,10%, 5% and 2.5% and a CH4 concentration of 8% (i.e., the O2/CH4 ratios of 2.62, 1.26, 0.63 and 0.31), the highest NO3--N removal occurred in the AME-D system incubated at the O2 concentration of 10%. Methanol and acetate may serve as the trophic linkage between aerobic methanotrophs and denitrifers in the AME-D systems. Methylotrophs including Methylophilus, Methylovorus, Methyloversatilis and Methylotenera were abundant under the O2-sufficient condition with the O2 concentration of 21%, while denitrifiers such as Azoarcus, Thauera and Thiobacillus dominated in the O2-limited environment with the O2 concentration of 10%. The competition of denitrifiers and methylotrophs in the AME-D system for CH4-derived carbon, such as methanol and acetate, might be influenced by chemotactic responses. More methane-derived carbon flowed into methylotrophs under the O2-sufficient condition, while more methane-derived carbon was used for denitrification in the O2-limited environment. These findings can aid in evaluating the distribution and contribution of AME-D and in developing strategies for mitigating CH4 emission and nitrogen pollution in natural and engineered ecosystems.

Effects of oxygen tension on the microbial community and functional gene expression of aerobic methane oxidation coupled to denitrification systems.

Aerobic CH4 oxidation coupled to denitrification (AME-D) can not only mitigate the emission of greenhouse gas (e.g., CH4) to the atmosphere, but also reduce NO3- and/or NO2- and alleviate nitrogen pollution. The effects of O2 tension on the community and functional gene expression of methanotrophs and denitrifiers were investigated in this study. Although higher CH4 oxidation occurred in the AME-D system with an initial O2 concentration of 21% (i.e., the O2-sufficient condition), more NO3--N was removed at the initial O2 concentration of 10% (i.e., the O2-limited environment). Type I methanotrophs, including Methylocaldum, Methylobacter, Methylococcus, Methylomonas, and Methylomicrobium, and type II methanotrophs, including Methylocystis and Methylosinus, dominated in the AME-D systems. Compared with type II methanotrophs, type I methanotrophs were more abundant in the AME-D systems. Proteobacteria and Actinobacteria were the main denitrifiers in the AME-D systems, and their compositions varied with the O2 tension. Quantitative PCR of the pmoA, nirS, and 16S rRNA genes showed that methanotrophs and denitrifiers were the main microorganisms in the AME-D systems, accounting for 46.4% and 24.1% in the O2-limited environment, respectively. However, the relative transcripts of the functional genes including pmoA, mmoX, nirK, nirS, and norZ were all less than 1%, especially the functional genes involved in denitrification under the O2-sufficient condition, likely due to the majority of the denitrifiers being dormant or even nonviable. These findings indicated that an optimal O2 concentration should be used to optimize the activity and functional gene expression of aerobic methanotrophs and denitrifiers in AME-D systems.

Low O2 level enhances CH4-derived carbon flow into microbial communities in landfill cover soils.

CH4 oxidation in landfill cover soils plays a significant role in mitigating CH4 release to the atmosphere. Oxygen availability and the presence of co-contaminants are potentially important factors affecting CH4 oxidation rate and the fate of CH4-derived carbon. In this study, microbial populations that oxidize CH4 and the subsequent conversion of CH4-derived carbon into CO2, soil organic C and biomass C were investigated in landfill cover soils at two O2 tensions, i.e., O2 concentrations of 21% ("sufficient") and 2.5% ("limited") with and without toluene. CH4-derived carbon was primarily converted into CO2 and soil organic C in the landfill cover soils, accounting for more than 80% of CH4 oxidized. Under the O2-sufficient condition, 52.9%-59.6% of CH4-derived carbon was converted into CO2 (CECO2-C), and 29.1%-39.3% was converted into soil organic C (CEorganic-C). A higher CEorganic-C and lower CECO2-C occurred in the O2-limited environment, relative to the O2-sufficient condition. With the addition of toluene, the carbon conversion efficiency of CH4 into biomass C and organic C increased slightly, especially in the O2-limited environment. A more complex microbial network was involved in CH4 assimilation in the O2-limited environment than under the O2-sufficient condition. DNA-based stable isotope probing of the community with 13CH4 revealed that Methylocaldum and Methylosarcina had a higher relative growth rate than other type I methanotrophs in the landfill cover soils, especially at the low O2 concentration, while Methylosinus was more abundant in the treatment with both the high O2 concentration and toluene. These results indicated that O2-limited environments could prompt more CH4-derived carbon to be deposited into soils in the form of biomass C and organic C, thereby enhancing the contribution of CH4-derived carbon to soil community biomass and functionality of landfill cover soils (i.e. reduction of CO2 emission).