A numerical study on the effect of CO2 addition for methane explosion reaction kinetics in confined space

To explore the influence of the CO2 volume fraction on methane explosion in confined space over wide equivalent ratios, the explosion temperature, the explosion pressure, the concentration of the important free radicals, and the concentration of the catastrophic gas generated after the explosion in confined space were studied. Meanwhile, the elementary reaction steps dominating the gas explosion were identified through the sensitivity analysis. With the increase of the CO2 volume fraction, the explosion time prolongs, and the explosion pressure and temperature decrease monotonously. Moreover, the concentrations of the investigated free radicals also decrease as the increase of the CO2 volume fraction. For the catastrophic gas, the concentration of the gas product CO increases and the concentrations of CO2, NO, and NO2 decrease as the volume fraction of CO2 increases. When 7% methane is added with 10% CO2, the increase rate of CO is 76%, and the decrease rates of CO2, NO, and NO2 are 27%, 37%, and 39%, respectively. If the volume fraction of CO2 is constant, the larger the volume fraction of methane in the blend gas, the greater the mole fraction of radical H and the lower the mole fraction of radical O. For radical OH, its mole fraction first increases, and then decreases with the location of peak value of 9.5%, while the CO concentration increases with the increase of the methane concentration. For all the investigated volume fraction of methane, the addition of CO2 reduces the sensitivity coefficients of each key elementary reaction step, and the sensitivity coefficient of reaction promoting methane consumption decreases faster than that of the reaction inhibit methane consumption, which indicates that the addition of CO2 effectively suppresses the methane explosion.

Does stand age affect methane consumption of forest soil? A study in a chronosequence of sessile oak.

<p>Soils play an important role of atmospheric methane sink, consuming about 30 Tg year<sup>-1</sup>. Methane consumption is carried out by methanotrophic bacteria whose activity can be affected by different environmental factors. One of the most important factors that impact on methane consumption is the air-filled porosity of soil (AFP), which depends on its total porosity (P) and its water content (SWC). A high AFP enhances gas diffusion in soil, and therefore methane consumption. In forests, P is thought to increase with stand age because of soil decompaction by tree roots and SWC is thought to decrease because of a high evapotranspiration. Another factor that can affect methane consumption and thought to decrease with the aging of forest stands is mineral nitrogen (Nmin) and particularly ammonium that competes with methane for the active site of methane monooxygenase, thus reducing methane oxidation. However only few studies have addressed the effects of stand aging on soil methane consumption.</p><p>Our objective was to confirm the hypothesis that methane consumption by forest soil increases with stand age, in relation with an increase AFP and a decrease Nmin. We carried out this study in a chronosequence of 16 stands of sessile oak divided into six age classes (20-30, 40-60, 60-70, 85-90, 125-130 and 140-145). Three sampling campaigns were conducted in late summer 2018 and 2019 (periods of maximum AFP) and in early spring 2018 (period of minimum AFP). Soil methane consumption was measured by incubating the five first centimetres of soil cores at 20°C and by measuring the decrease of CH<sub>4</sub> concentration in the incubation chamber with a laser-based CH<sub>4</sub> analyser.</p><p>In contrast to our hypothesis, we did not find any significant effect of stand age on Nmin, P, SWC and AFP, nor on methane consumption. However, methane consumption was higher in stands with high values of AFP and low value of SWC, whatever their age. AFP, through differences in SWC, appeared to be the main driver of soil methane consumption in our study site, explaining both seasonal variations and variations among stands, that could not however be related to their age.</p>

Conversion of marginal land into switchgrass conditionally accrues soil carbon and reduces methane consumption

Switchgrass (Panicum virgatum L.) is a perennial C4 grass native to tallgrass prairies of the Central US, and a promising bioenergy feedstock. Switchgrass can be cultivated on soils with low nutrient contents and its rooting depth, of up to 2 m, has brought attention to the crop as a potential mechanism to sequester and build soil carbon (C). Switchgrass, therefore, offers multifaceted benefits on degraded soils by enhancing soil organic matter content. However, to evaluate the sustainability of switchgrass-based biofuel production, it is crucial to understand the impacts of land conversion and switchgrass establishment on biotic/abiotic characteristics of various soils. In this study, we characterized the ecosystem-scale consequences of switchgrass growing at two highly-eroded, ‘Dust Bowl’ remnant field sites from Oklahoma US, with silt-loam (SL) or clay-loam (CL) soil textures having low nitrogen (N), phosphorus (P), and C contents. Paired plots at each site, including fallow control and switchgrass-cultivated, were assessed. Our results indicated that switchgrass significantly increased soil C at the SL site and reduced microbial diversity at the CL site. The CL site exhibited significantly higher CO2 flux and higher respiration from switchgrass plots. Strikingly, switchgrass significantly reduced the CH4 consumption by an estimated 39% for the SL site and 47% for the CL site. Structural equation modeling identified soil temperature, P content, and soil moisture levels as the most influential factors regulating both CO2 and CH4 fluxes. CO2 flux was also influenced by microbial biomass while CH4 flux was influenced by microbial diversity. Together, our results suggest that site selection by soil type is a crucial factor in improving soil C stocks and mitigating greenhouse gas (GHG) fluxes, especially considering our finding that switchgrass reduced methane consumption, implying that carbon balance considerations should be accounted for to fully evaluate the sustainability of switchgrass cultivation.

Methane consumption potential of soybean-wheat, maize-wheat and maize-gram cropping systems under conventional and no-tillage agriculture in a tropical vertisol

Methane (CH4) consumption in agricultural soil is imperative for the mitigation of climate change. However, the effect of tillage and cropping systems on CH4 consumption is less studied. Experiments were carried out in Madhya Pradesh, India with soybean-wheat (SW), maize-wheat (MW) and maize-gram (MG) cropping systems under conventional tillage (CT) and no-tillage (NT). Soybean/maize was cultivated during the kharif season (July–October) and wheat/chickpea in the rabi season (October–March) for 9 years consecutively. Soil samples were collected during vegetative growth stages of soybean and maize from different cropping systems. Methane consumption, the abundance of methanotrophs as particulate methane monooxygenase (pmoA) gene copies, soil and crop parameters were estimated. Methane consumption rate was higher in NT and upper soil layer (0–5 cm) than CT and 5–15 cm depth. Methane consumption rate k ranged from 0.35 to 0.56 μg CH4 consumed/g soil/d in the order of MW>SW>MG in 0–5 cm. The abundance of pmoA gene copies ranged from 43 × 104/g soil to 13 × 104/g soil and was highest in MW-NT and lowest in MG-CT. Available nitrogen, phosphorus and potassium were higher in 0–5 cm than in 5–15 cm depth. Soil and plant parameters and abundance of pmoA genes correlated significantly and positively with CH4 consumption rate. No-tillage stimulated CH4 consumption compared to CT irrespective of cropping system and CH4 consumption potential was highest in MW and lowest in MG. However, the magnitude of the positive effect of NT towards CH4 consumption was higher in SW and MG than MW.

Hemerythrins enhance aerobic respiration in Methylomicrobium alcaliphilum 20ZR, a methane-consuming bacterium

ABSTRACT Numerous hemerythrins, di-iron proteins, have been identified in prokaryote genomes, but in most cases their function remains elusive. Bacterial hemerythrin homologs (bacteriohemerythrins, Bhrs) may contribute to various cellular functions, including oxygen sensing, metal binding and antibiotic resistance. It has been proposed that methanotrophic Bhrs support methane oxidation by supplying oxygen to a core enzyme, particulate methane monooxygenase. In this study, the consequences of the overexpression or deletion of the Bhr gene (bhr) in Methylomicrobiam alcaliphillum 20ZR were investigated. We found that the bhrknockout (20ZRΔbhr) displays growth kinetics and methane consumption rates similar to wild type. However, the 20ZRΔbhr accumulates elevated concentrations of acetate at aerobic conditions, indicating slowed respiration. The methanotrophic strain overproducing Bhr shows increased oxygen consumption and reduced carbon-conversion efficiency, while its methane consumption rates remain unchanged. These results suggest that the methanotrophic Bhr proteins specifically contribute to oxygen-dependent respiration, while they have minimal, if any, input of oxygen for the methane oxidation machinery.

Members of the GenusMethylobacterAre Inferred To Account for the Majority of Aerobic Methane Oxidation in Oxic Soils from a Freshwater Wetland

ABSTRACTMicrobial carbon degradation and methanogenesis in wetland soils generate a large proportion of atmospheric methane, a highly potent greenhouse gas. Despite their potential to mitigate greenhouse gas emissions, knowledge about methane-consuming methanotrophs is often limited to lower-resolution single-gene surveys that fail to capture the taxonomic and metabolic diversity of these microorganisms in soils. Here our objective was to use genome-enabled approaches to investigate methanotroph membership, distribution, andin situactivity across spatial and seasonal gradients in a freshwater wetland near Lake Erie. 16S rRNA gene analyses demonstrated that members of the methanotrophicMethylococcaleswere dominant, with the dominance largely driven by the relative abundance of four taxa, and enriched in oxic surface soils. Three methanotroph genomes from assembled soil metagenomes were assigned to the genusMethylobacterand represented the most abundant methanotrophs across the wetland. Paired metatranscriptomes confirmed that these Old Woman Creek (OWC)Methylobactermembers accounted for nearly all the aerobic methanotrophic activity across two seasons. In addition to having the capacity to couple methane oxidation to aerobic respiration, these new genomes encoded denitrification potential that may sustain energy generation in soils with lower dissolved oxygen concentrations. We further show thatMethylobactermembers that were closely related to the OWC members were present in many other high-methane-emitting freshwater and soil sites, suggesting that this lineage could participate in methane consumption in analogous ecosystems. This work contributes to the growing body of research suggesting thatMethylobactermay represent critical mediators of methane fluxes in freshwater saturated sediments and soils worldwide.IMPORTANCEHere we used soil metagenomics and metatranscriptomics to uncover novel members within the genusMethylobacter. We denote these closely related genomes as members of the lineage OWCMethylobacter. Despite the incredibly high microbial diversity in soils, here we present findings that unexpectedly showed that methane cycling was primarily mediated by a single genus for both methane production (“CandidatusMethanothrix paradoxum”) and methane consumption (OWCMethylobacter). Metatranscriptomic analyses revealed that decreased methanotrophic activity rather than increased methanogenic activity possibly contributed to the greater methane emissions that we had previously observed in summer months, findings important for biogeochemical methane models. Although members of thisMethylococcalesorder have been cultivated for decades, multi-omic approaches continue to illuminate the methanotroph phylogenetic and metabolic diversity harbored in terrestrial and marine ecosystems.