CH4 Oxidation Rates Research Articles

Exploration of Fe speciation preference for aerobic methane oxidation by using isotopic Fe-modified zeolites

Methane-oxidizing bacteria (MOB) dominate the natural consumption of fugitive methane (CH4), which is largely produced by methanogens in aerobic-anaerobic habitats. Although the influence of iron (Fe) on MOB activities has been examined, the underlying mechanism in terms of the preference remains to be clarified, the key to which may be the extracellular uptake of Fe during CH4 oxidation. In this study, isotopic ferric chloride was used to treat stable raw zeolites (Fe-Z) through a pyrolytic procedure (Fe-Z600). All samples were allowed to develop for 52 d, and the CH4 oxidation rate and contents of Fe and extracellular polymers (EPS) in different MOB periods were determined. The cumulative oxidation mass of CH4 in Fe-Z600 was two times that in Fe-Z, and the abundance of Methylocystis in Fe-Z600 was 21% higher. More uniformly dispersed α-Fe2O3 nanoparticles were formed during the pyrolysis of Fe-Z600, which accelerated the extracellular migration and stimulated MOB growth. In addition, Fe-Z600 presented the strongest extracellular polysaccharides (ECPs) inhibition; the total amount of ECPs was about 53.9% lower than that of the Raw group. Simultaneously, Fe was more likely to be stored in loosely bound EPS (LB-EPS), with a maximum cumulative amount of 38% during the stable period of CH4 oxidation in Fe-Z600, decreasing LB-EPS and tightly bound EPS by 47.4% and 75.4%, respectively. The presented findings can provide guidance for alleviating EPS blockage in long-term activities of MOB to ensure their efficient and durable CH4 oxidation.

Variability and controls of stable carbon isotopic fractionation during aerobic methane oxidation in temperate lakes

The aerobic oxidation of methane (CH4) by methanotrophic bacteria (MOB) is the major sink of this highly potent greenhouse gas in freshwater environments. Yet, CH4 oxidation is one of the largest uncertain components in predicting the current and future CH4 emissions from these systems. While stable carbon isotopic mass balance is a powerful approach to estimate the extent of CH4 oxidation in situ, its applicability is constrained by the need of a reliable isotopic fractionation factor (αox), which depicts the slower reaction of the heavier stable isotope (13C) during CH4 oxidation. Here we explored the natural variability and the controls of αox across the water column of six temperate lakes using experimental incubation of unamended water samples at different temperatures. We found a large variability of αox (1.004–1.038) with a systematic increase from the surface to the deep layers of lake water columns. Moreover, αox was strongly positively coupled to the abundance of MOB in the γ-proteobacteria class (γ-MOB), which in turn correlated to the concentrations of oxygen and CH4, and to the rates of CH4 oxidation. To enable the applicability in future isotopic mass balance studies, we further developed a general model to predict αox using routinely measured limnological variables. By applying this model to δ13C-CH4 profiles obtained from the study lakes, we show that using a constant αox value in isotopic mass balances can largely misrepresent and undermine patterns of the extent of CH4 oxidation in lakes. Our αox model thus contributes towards more reliable estimations of stable carbon isotope-based quantification of CH4 oxidation and may help to elucidate large scale patterns and drivers of the oxidation-driven mitigation of CH4 emission from lakes.

Biocide treatment for mosquito control increases CH4 emissions in floodplain pond mesocosms

Shallow lentic freshwater aquatic systems are globally important emitters of methane (CH4), a highly potent greenhouse gas. Previous laboratory studies indicated that bioturbation by chironomids can reduce CH4 production and increase CH4 oxidation by enhancing oxygen transport into sediment. Thus, reduction in chironomid density by application of biocides for mosquito control, such as Bacillus thuringinesis var. israelensis (Bti), have the potential to affect CH4 emissions. We evaluated the effect of a 41% reduction in chironomid larvae abundance due to Bti applications on CH4 dynamics in the aquatic and aquatic-terrestrial transition zones of 12 floodplain pond mesocosms (FPMs) (half treated, half control). We evaluated short-term (2 months) and seasonal effects by measuring CH4 emissions, dissolved concentrations, and oxidation rates in spring, summer, autumn, and winter. On average, CH4 emissions from the aquatic-terrestrial transition zone of the treated FPMs were 137 % higher than those of the control FPMs. The lack of differences in mean oxidation rates between the treated and control mesocosms suggests that a reduction in bioturbation and the associated decreased oxygen transport into the sediment promoted CH4 production in the treated FPMs. Our findings point to potential effects of Bti on CH4 biogeochemistry through alterations of the chironomid abundance, and highlight the underestimated role of invertebrates in biogeochemical cycling in these ecosystems.

Atmospheric methane oxidation is affected by grassland type and grazing and negatively correlated to total soil respiration in arid and semiarid grasslands in Inner Mongolia

Methane (CH4) is an important trace greenhouse gas and atmospheric CH4 uptake by high-affinity methanotrophs in grassland soil accounts for an important proportion of the terrestrial CH4 sink. However, our understanding of the comprehensive effects of grassland type and grazing treatment on active soil methanotrophs and atmospheric CH4 uptake is still under debate. This study investigates the impact of grazing on CH4 oxidation rate and active atmospheric CH4 oxidizing methanotroph communities in two arid and semiarid grassland ecosystems (meadow and desert) by detecting transcripts of methane monooxygenase (pmoA) genes. Atmospheric CH4 oxidation rates differed according to grassland type and grazing treatment. The highest activity was found in desert grasslands with moderate grazing and the lowest activity in meadow grasslands with exclosures. The differences in activities were linked with changes in abundance, composition and co-occurrence network patterns of active methanotrophs and CO2 production rate. Redundancy, correlation and random forest analyses indicated that pmoA transcripts, available phosphorus (AP), NO3−-N, and CO2 production rate were the most important factors predicting active methanotroph community composition and atmospheric CH4 oxidation activity in these grassland ecosystems. A glucose amendment incubation experiment showed that addition of glucose increased heterotrophic microbial respiration and inhibited atmospheric CH4 oxidation. This study provides evidence that CO2 production rate is an important factor associated with atmospheric CH4 oxidation activity in arid and semiarid grassland ecosystems and suggests that interactions between methanotrophs and other heterotrophs influence methanotroph activity in grassland ecosystems.

Methane oxidation dynamics in a stratified lake: Insights revealed from a mass balance and carbon stable isotopes

AbstractMethane oxidation in lakes removes a large portion of methane (CH4). To date, methane oxidation estimates in lakes have often been derived at low spatial resolution in the water column, preventing understanding of the links to the physicochemical gradients in the stratified regions. We applied a mass balance approach with measured dissolved CH4 and sediment CH4 fluxes to derive high‐resolution depth profiles of specific CH4 oxidation rates () in the water column during the stratified period of a small eutrophic lake (Soppensee, Switzerland). Estimated ranged from 0 to 1 d−1 and the profiles agreed well with previous studies, and were also in agreement with rates from concurrent in situ oxidation experiments. A sensitivity analysis revealed that sediment CH4 flux is the largest source of uncertainty when deriving kox. Although previous studies have estimated methane oxidation based on δ13CCH4, we showed with numerical modeling that δ13CCH4 measurements could not be used to resolve the relative contributions of methane oxidation and sediment fluxes to the water column CH4 balance. Exploration of alternative approaches to derive methane oxidation is needed to reveal potentially unknown or misunderstood drivers of methane oxidation in lakes. The presented mass balance approach has the potential to calculate methane oxidation at high vertical resolution and enhance the spatial limitations of established incubation methods. As methane oxidation is responsible for removing most of the produced methane in lakes, it is as important to accurately resolve the key drivers to predict responses to future climate scenarios.

Biogeochemical versus Conventional Landfill Soil Covers: Analysis of Gas Flow Profiles, Microbial Communities, and Mineralogy

In this study, a novel biogeochemical cover system comprising biochar-amended soil and basic oxygen furnace (BOF) steel slag was explored as a sustainable alternative cover system to mitigate methane (CH4), carbon dioxide (CO2), and hydrogen sulfide (H2S) simultaneously from landfill gas (LFG). Long-term column studies of a simulated biogeochemical cover (BGCC) profile investigated CH4, CO2, and H2S removal potential. The performance of the BGCC system was compared with a conventional soil cover (SC) profile. The CH4 oxidation rates of biochar-amended soil were significantly higher, ranging from 185 to 407 µg CH4/g-day in comparison with the barrier soil in the SC system (6–7.5 µg CH4/g-day), based on the batch incubation of column-exhumed samples. In addition, the biochar-amended soil showed higher relative abundance of methanotrophic bacterial communities (20%–51%) in comparison with soil cover (10%–27%). In both columns, complete attenuation of H2S occurred near the inlet (75 cm bgs) and sulfur oxidizing bacteria (e.g., Thiobacillus) and methanotrophs were both detected. The sulfur content was elevated (0.68%) at the base of both columns and H2S may have imparted an inhibitory effect on CH4 oxidation rate in the SC system. The BOF slag showed a CO2 removal potential of 67 g CO2/kg BOF slag. Overall, the BGCC system outperformed the SC system, effectively mitigating CH4, CO2, and H2S simultaneously.

Potential soil methane oxidation in naturally regenerated oak-dominated temperate deciduous forest stands responds to soil water status regardless of their age—an intact core incubation study

Key messagePotential CH4 oxidation in the top soil layer increased with decreasing soil water content in spring but was inhibited during severe summer drought in naturally-regenerated oak-dominated temperate deciduous forest stands regardless of their age. No direct effect of mineral nitrogen on soil CH4 oxidation was found. Soil CH4 oxidation in temperate forests could be reduced by extreme climatic events.ContextThe oxidation of atmospheric methane (CH4) by methanotrophic bacteria in forest soils is an important but overlooked ecosystem service.AimOur objective was to determine which factors drive variations in soil CH4 oxidation in oak-dominated temperate deciduous forest stands of different ages.MethodsSoil samples were collected in 16 stands aged 20 to 143 years in periods of high and low soil water content (SWC). The potential rate of soil CH4 oxidation was measured by incubating the first five centimetres of intact soil cores at 20 °C.ResultsSWC was the main driver accounting for variations in CH4 oxidation. In spring, a two-fold reduction in SWC greatly increased CH4 oxidation. But when the soil was dry in late summer, a further reduction in SWC led to a decrease in CH4 oxidation in the top soil layer. No direct effect of mineral nitrogen on soil CH4 oxidation was found.ConclusionsWith regard to soil CH4 oxidation, naturally regenerated forest stands contribute equally to climate change mitigation regardless of their age. Considering future climate scenarios for Europe, soil CH4 sink in temperate forests could be reduced, due to both an increase in the number of flooding episodes in spring and drier summers.

Mechanistic understanding of methane combustion over H-SSZ-13 zeolite encapsulated palladium nanocluster catalysts

Catalytic methane (CH4) combustion to CO2 and H2O is of great practical significance and an important prototype catalytic reaction. Extensive experimental studies have suggested that zeolite supported palladium (Pd) catalysts are very active for catalytic CH4 oxidation, while the structure-performance relationship is still not clear. Herein, using H-SSZ-13 zeolite encapsulated Pd nanoclusters as a demonstration case, reaction mechanisms and kinetics of complete CH4 combustion were systematically investigated using first-principles density functional theory (DFT) calculations combined with atomistic thermodynamic analysis and the energetic span model (ESM). Four H-SSZ-13 zeolite encapsulated PdⅡxOmHn model catalysts, i.e., PdⅡ/H-SSZ-13, PdⅡO/H-SSZ-13, PdⅡ2O/H-SSZ-13, and PdⅡ3O3H/H-SSZ-13 were studied. The encapsulated [PdⅡ2O]2+ and [PdⅡ3O3H]+ nanoclusters were identified as the most stable binuclear and trinuclear Pd structures under experimental oxidative conditions. DFT calculations indicated that the most kinetically relevant step in the complete CH4 oxidation reaction over four Pd model catalysts is different. The first, third, third, and fourth C–H bond cleavage were identified as the most kinetically relevant steps over PdⅡ/H-SSZ-13, PdⅡO/H-SSZ-13, PdⅡ2O/H-SSZ-13, and PdⅡ3O3H/H-SSZ-13, respectively. Using the energetic span model, the relative turnover frequencies of CH4 oxidation over four PdⅡxOmHn/H-SSZ-13 model catalysts were calculated. Consistent with recent experimental observation, the encapsulated PdⅡ3O3H/H-SSZ-13 nanocluster was found to be the most active catalyst for complete CH4 oxidation. On the basis of DFT results and the ESM model analysis, it is noted that the reaction rate of complete CH4 oxidation over the PdⅡxOmHn/H-SSZ-13 catalysts is not completely dependent on one “highest” activation barrier, i.e., the barrier for the most kinetically relevant step. Neither one specific transition state nor one reaction step carried all the kinetics information that determines the catalytic performance of the PdⅡxOmHn/H-SSZ-13 catalysts. The present work sheds light on the structure-performance relationship for CH4 combustion over the zeolite supported noble metal catalysts.

Excess soil moisture and fresh carbon input are prerequisites for methane production in podzolic soil

Abstract. Boreal upland forests are generally considered methane (CH4) sinks due to the predominance of CH4 oxidizing bacteria over the methanogenic archaea. However, boreal upland forests can temporarily act as CH4 sources during wet seasons or years. From a landscape perspective and in annual terms, this source can be significant as weather conditions may cause flooding, which can last a considerable proportion of the active season and because often, the forest coverage within a typical boreal catchment is much higher than that of wetlands. Processes and conditions which change mineral soils from acting as a weak sink to a strong source are not well understood. We measured soil CH4 fluxes from 20 different points from regularly irrigated and control plots during two growing seasons. We also estimated potential CH4 production and oxidation rates in different soil layers and performed a laboratory experiment, where soil microcosms were subjected to different moisture levels and glucose addition simulating the fresh labile carbon (C) source from root exudates. The aim was to find the key controlling factors and conditions for boreal upland soil CH4 production. Probably due to long dry periods in both summers, we did not find occasions of CH4 production following the excess irrigation, with one exception in July 2019 with emission of 18 200 µg CH4 m−2 h−1. Otherwise, the soil was always a CH4 sink (median CH4 uptake rate of 260–290 and 150–170 µg CH4 m−2 h−1, in control and irrigated plots, respectively). The median soil CH4 uptake rates at the irrigated plot were 88 % and 50 % lower than at the control plot in 2018 and 2019, respectively. Potential CH4 production rates were highest in the organic layer (0.2–0.6 nmol CH4 g−1 d−1), but some production was also observed in the leaching layer, whereas in other soil layers, the rates were negligible. Potential CH4 oxidation rates varied mainly within 10–40 nmol CH4 g−1 d−1, except in deep soil and the organic layer in 2019, where potential oxidation rates were almost zero. The laboratory experiment revealed that high soil moisture alone does not turn upland forest soil into a CH4 source. However, a simple C source, e.g., substrates coming from root exudates with high moisture, switched the soil into a CH4 source. Our unique study provides new insights into the processes and controlling factors on CH4 production and oxidation, and the resulting net efflux that should be incorporated in process models describing global CH4 cycling.

Sheep grazing impacts on soil methanotrophs and their activity in typical steppe in the Loess Plateau China

Soil methanotrophs are the only biological sink for atmospheric methane (CH4). The activity and the diversity of methanotrophic communities in grassland soils can be impacted by animal grazing. In this study, soils were collected from a 17-year running grazing intensity trial in a typical steppe in the Loess Plateau, China to examine the responses of CH4 oxidation rate and active methanotroph composition and abundance to different levels of sheep grazing. Soils were collected from summer grazed pastures at no (0 sheep ha−1), low (2.67 sheep ha−1), moderate (5.33 sheep ha−1) and high (8.67 sheep ha−1) levels of grazing. We found that all soils were able to oxidize atmospheric and high concentration (10%) CH4 through soil methanotrophs. Compared to ungrazed soils, the atmospheric CH4 oxidation rate and methanotroph abundance were higher in low and high grazed soils but lower in moderately grazed soils. Across all soils, the in-situ methanotroph community was dominated by upland soil cluster gamma methanotrophs and potentially contributed to the atmospheric concentration CH4 oxidation. Soil methanotroph community responses to the high 13CH4 concentration revealed an increase in methanotroph abundance and a transition of the active methanotroph community to traditional Type I (Methylocaldum and Methylobacter) and Type II (Methylocystis) methanotrophs. Methylocaldum responses to the high CH4 concentration were greater in highly grazed soil than in ungrazed soil. The activity of Methylobacter and Methylocystis under high CH4 concentrations was reduced by high grazing intensity. This study provides evidence that recent intensifications of animal grazing can result in particular community assembly of microbial methane oxidizers responsible for CH4 sink capabilities of grassland soils.

Coupled CH4 production and oxidation support CO2 supersaturation in a tropical flood pulse lake (Tonle Sap Lake, Cambodia)

Carbon dioxide (CO2) supersaturation in lakes and rivers worldwide is commonly attributed to terrestrial-aquatic transfers of organic and inorganic carbon (C) and subsequent, in situ aerobic respiration. Methane (CH4) production and oxidation also contribute CO2 to freshwaters, yet this remains largely unquantified. Flood pulse lakes and rivers in the tropics are hypothesized to receive large inputs of dissolved CO2 and CH4 from floodplains characterized by hypoxia and reducing conditions. We measured stable C isotopes of CO2 and CH4, aerobic respiration, and CH4 production and oxidation during two flood stages in Tonle Sap Lake (Cambodia) to determine whether dissolved CO2 in this tropical flood pulse ecosystem has a methanogenic origin. Mean CO2 supersaturation of 11,000 ± 9,000 μatm could not be explained by aerobic respiration alone. 13C depletion of dissolved CO2 relative to other sources of organic and inorganic C, together with corresponding 13C enrichment of CH4, suggested extensive CH4 oxidation. A stable isotope-mixing model shows that the oxidation of 13C depleted CH4 to CO2 contributes between 47 and 67% of dissolved CO2 in Tonle Sap Lake. 13C depletion of dissolved CO2 was correlated to independently measured rates of CH4 production and oxidation within the water column and underlying lake sediments. However, mass balance indicates that most of this CH4 production and oxidation occurs elsewhere, within inundated soils and other floodplain habitats. Seasonal inundation of floodplains is a common feature of tropical freshwaters, where high reported CO2 supersaturation and atmospheric emissions may be explained in part by coupled CH4 production and oxidation.

Field study on the effect of vegetation on the performance of soil methanotrophy-based engineered systems – Column experiments

Current literature provides conflicting information on the role vegetation plays when considering methane (CH4) oxidation potential of engineered Biosystems, such as landfill biocovers (LBCs), bio-windows and methane biofilters. The primary objective of this study was to determine whether the impact of vegetation on biological CH4 oxidation was positive or negative and to explain the reasons for the observations using a variety of experiments. A total of eight flow-through columns (two alfalfa, two native grass, two canola and two bare-soil replicates) were set up outdoors to simulate field operation in cold climatic conditions. Each column was layered with 18 cm of topsoil and 32 cm of compost mixture (compost 30%: topsoil 70% v/v) as packing material and treated with CH4 fluxes ranging from 180 to 815 g CH4 m−2 d−1. The bare-soil columns exhibited the highest CH4 oxidation rate of 455 g CH4 m−2 d−1, while the maximum CH4 oxidation rates for the vegetated columns ranged between 147 and 171 g CH4 m−2 d−1 with the alfalfa column showing the lowest. Gas profiles of vegetated columns showed high concentrations of nitrogen (N2) and oxygen (O2) at all depths, possibly due to increased permeability created by the plant root systems. Quantitative Polymerase Chain Reaction (q-PCR) assessment showed that pmoA gene copy numbers, indicative of methanotrophic population levels, were higher in bare-soil columns than in vegetated columns. The Illumina based sequencing of 16S rRNA gene showed that Type I methanotrophs dominated both vegetated and bare-soil columns. Soil incubation experiments conducted to determine oxidation kinetic parameters also indicated greater methanotrophic activity in the bare-soil columns than in vegetated columns. The plant data collected at the end of the column experiments provided clear evidence of CH4 escape due to preferential pathways created by plant roots (reaching 38 cm, 31.5 cm and 26.5 cm by alfalfa, canola and native grass, respectively) that resulted in decreased CH4 oxidation in vegetated columns. The study results clearly demonstrate that vegetation decreases CH4 oxidation at high loading rates, notwithstanding the type of vegetation.

Insight into SO4(-II)- dependent anaerobic methane oxidation in landfill: Dual-substrates dynamics model, microbial community, function and metabolic pathway

In anaerobic landfill, SO42- could serve as electron receptor for methane oxidation. In theory, concentrations of both methane and SO42- should be related to methane oxidation rate. However, the dynamics process has yet to be discovered, and the understanding of metabolic pathways of the sulfate-dependent anaerobic methane oxidation (S-DAMO) process in landfill remains limited. In this study, S-DAMO dynamics was investigated by observing the CH4 oxidation rates under different CH4/ SO42-counter-gradients. The CH4-SO42- dual-substrate model based on MichaeliseMenten equation was got (maximum substrate degradation rate Vmax [22.9 ± 1.31] µmol/[kg·d], half-saturation constantsKm,CH446.095±0.41ppmv, andKm,so42-(7.26±0.605)g/g). High-throughput sequencing analysis indicated Methanobacterials, Methanosarcinales, and Soil Crenarchaeotic were the main functional microorganisms for S-DAMO in landfill. The metabolic pathway of S-DAMO was speculated as the reverse methanogenesis pathway through Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUST) analysis, while methanogenesis was the methyl nutrition way based on methanol. The enzymes related to the carbon and sulfur cycles and their relative abundances in the microcosms were analyzed to graph the methane metabolic pathway and the sulfur metabolic pathway. The findings provide important parameters for CH4 mitigation in landfills, and give a new insight for understanding S-DAMO metabolic pathway in landfill.

Use of methanotrophically activated biochar in novel biogeochemical cover system for carbon sequestration: Microbial characterization

Biochar-amended soils have been explored to enhance microbial methane (CH4) oxidation in landfill cover systems. Recently, research priorities have expanded to include the mitigation of other components of landfill gas such as carbon dioxide (CO2) and hydrogen sulfide (H2S) along with CH4. In this study, column tests were performed to simulate the newly proposed biogeochemical cover systems, which incorporate biochar-amended soil for CH4 oxidation and basic oxygen furnace (BOF) slag for CO2 and H2S mitigation, to evaluate the effect of cover configuration on microbial CH4 oxidation and community composition. Biogeochemical covers included a biochar-amended soil (10% w/w), and methanotroph-enriched activated biochar amended soil (5% or 10% w/w) as a biocover layer or CH4 oxidation layer. The primary outcome measures of interest were CH4 oxidation rates and the structure and abundance of methane-oxidation bacteria in the covers. All column reactors were active in CH4 oxidation, but columns containing activated biochar-amended soils had higher CH4 oxidation rates (133 to 143 μg CH4 g−1 day−1) than those containing non-activated biochar-amended soil (50 μg CH4 g−1 day−1) and no-biochar soil or control soil (43 μg CH4 g−1 day−1). All treatments showed significant increases in the relative abundance of methanotrophs from an average relative abundance of 5.6% before incubation to a maximum of 45% following incubation. In activated biochar, the abundance of Type II methanotrophs, primarily Methylocystis and Methylosinus, was greater than that of Type I methanotrophs (Methylobacter) due to which activated biochar-amended soils also showed higher abundance of Type II methanotrophs. Overall, biogeochemical cover profiles showed promising potential for CH4 oxidation without any adverse effect on microbial community composition and methane oxidation. Biochar activation led to an alteration of the dominant methanotrophic communities and increased CH4 oxidation.

Methane oxidation in a landfill biowindow under wide seasonally fluctuating climatic conditions.

In the current study, a pilot biowindow was constructed in a closed cell of a Canadian Landfill, undergoing high seasonal fluctuations in the temperature from -30 in winter to 35 in summer. The biowindow was filled with biosolids compost amended with yard waste and leaf compost with the ratio of 4:1 as the substrate layer. Two years of monitoring of methane (CH4) oxidation in the biowindow led to remarkable expected observations including a thick, solid winter frost cover affecting gas exchange in winter and temperatures above 45 ℃ in the biowindow in late summer. A high influx compared to the reported values was observed into the biowindow with an average value of 1137 g.m-2.d-1, consisting of 64% of CH4 and 36% of carbon dioxide (CO2) in the landfill gas. The variations in the temperature and moisture content (MC) of the compost layer in addition to the influx fluctuations affected CH4 oxidation efficiency; however, a high average CH4 oxidation rate of 237 g.m-2.d-1 was obtained, with CH4 being mostly oxidized at top layers. The laboratory batch experiments verified that thermophilic methane-oxidizing bacteria (MOB) were active throughout the study period and oxidized CH4 with a higher rate than mesophilic MOB. The methanotrophic potential of the compost mixture showed an average value of 282 µmol.g-1.d-1 in the entire period of the study which is in the range of the highest reported maximum CH4 oxidation rates. The adopted compost mixture was suitable for CH4 oxidation if the MC was above 30%. The significance of MC variations on CH4 oxidation rate depended on the temperature range within the biowindow. At temperatures below 2 ℃, between 29 and 31℃, and above 45 ℃, MC was not a controlling factor for mesophilic CH4 oxidation.

Active Methanotrophs and Their Response to Temperature in Marine Environments: An Experimental Study

Aerobic methane (CH4) oxidation plays a significant role in marine CH4 consumption. Temperature changes resulting from, for example, global warming, have been suggested to be able to influence methanotrophic communities and their CH4 oxidation capacity. However, exact knowledge regarding temperature controls on marine aerobic methane oxidation is still missing. In this study, CH4 consumption and the methanotrophic community structure were investigated by incubating sediments from shallow (Bohai Bay) and deep marine environments (East China Sea) at 4, 15, and 28 °C for up to 250 days. The results show that the abundance of the methanotrophic population, dominated by the family Methylococcaceae (type I methanotrophs), was significantly elevated after all incubations and that aerobic methane oxidation for both areas had a strong temperature sensitivity. A positive correlation between the CH4 oxidation rate and temperature was witnessed in the Bohai Bay incubations, whereas for the East China Sea incubations, the optimum temperature was 15 °C. The systematic variations of pmoA OTUs between the Bohai Bay and East China Sea incubations indicated that the exact behaviors of CH4 oxidation rates with temperature are related to the different methanotrophic community structures in shallow and deep seas. These results are of great significance for quantitatively evaluating the biodegradability of CH4 in different marine environments.

The Influence of Above-Ground Herbivory on the Response of Arctic Soil Methanotrophs to Increasing CH4 Concentrations and Temperatures.

Rising temperatures in the Arctic affect soil microorganisms, herbivores, and peatland vegetation, thus directly and indirectly influencing microbial CH4 production. It is not currently known how methanotrophs in Arctic peat respond to combined changes in temperature, CH4 concentration, and vegetation. We studied methanotroph responses to temperature and CH4 concentration in peat exposed to herbivory and protected by exclosures. The methanotroph activity was assessed by CH4 oxidation rate measurements using peat soil microcosms and a pure culture of Methylobacter tundripaludum SV96, qPCR, and sequencing of pmoA transcripts. Elevated CH4 concentrations led to higher CH4 oxidation rates both in grazed and exclosed peat soils, but the strongest response was observed in grazed peat soils. Furthermore, the relative transcriptional activities of different methanotroph community members were affected by the CH4 concentrations. While transcriptional responses to low CH4 concentrations were more prevalent in grazed peat soils, responses to high CH4 concentrations were more prevalent in exclosed peat soils. We observed no significant methanotroph responses to increasing temperatures. We conclude that methanotroph communities in these peat soils respond to changes in the CH4 concentration depending on their previous exposure to grazing. This “conditioning” influences which strains will thrive and, therefore, determines the function of the methanotroph community.

Constraining models for methane oxidation based on long-term continuous chamber measurements in a temperate forest soil

Upland soils are thought to be a sink of CH4, the second most important anthropogenic greenhouse gas, owing to oxidation by methanotrophs. To better understand CH4 fluxes in upland forests, we quasi-continuously measured CH4 fluxes using an automated closed chamber system over seven years on a larch plantation in a volcanic soil in Japan. We hypothesized that the long-term data sufficiently can calibrate modules for CH4 fluxes, and aimed to predict future pathways of CH4 uptake and their uncertainties in the forest. Based on the observations, a thinning of the overstory only marginally influenced the CH4 fluxes measured by the chambers. Using the data with a Bayesian method, we calibrated four modules for CH4 fluxes in forest soils, which were embedded in the process-based ecosystem model VISIT. The modules well reproduced the observed seasonality, annual budgets, and interannual variability in the CH4 fluxes after calibrating the following parameters: the diffusion coefficient or base CH4 oxidation rate constant and temperature sensitivity. The CH4 fluxes were predicted to increase in the future under the RCP8.5 scenario but to decrease under the RCP 2.6 scenario. The contrasting trajectory was caused by rising and decreasing CH4 concentrations under the RCP 8.5 and 2.6 scenarios, respectively. Furthermore, the magnitudes of the future changes in the fluxes differed in each module because the responses to the changes in the CH4 concentrations were inconsistent among the modules. The observed CH4 fluxes increased with increasing atmospheric CH4 concentration (4.95 mg CH4 m−2 d−1 ppm−1), which was greater in magnitude than those in the modules. Considering the uncertainties in the modules and potential confounding effects in the observations, we conclude that further understanding the responses of CH4 uptake to rising CH4 concentrations is required.

Divergent responses of wetland methane emissions to elevated atmospheric CO2 dependent on water table

Elevated atmospheric CO2 may have consequences for methane (CH4) emissions from wetlands, yet the magnitude and direction remain unpredictable, because the associated mechanisms have not been fully investigated. Here, we established an in situ macrocosm experiment to compare the effects of elevated CO2 (700 ppm) on the CH4 emissions from two wetlands: an intermittently inundated Calamagrostis angustifolia marsh and a permanently inundated Carex lasiocarpa marsh. The elevated CO2 increased CH4 emissions by 27.6–57.6% in the C. angustifolia marsh, compared to a reduction of 18.7–23.5% in the C. lasiocarpa marsh. The CO2-induced increase in CH4 emissions from the C. angustifolia marsh was paralleled with (1) increased dissolved organic carbon (DOC) released from plant photosynthesis and (2) reduced (rate of) CH4 oxidation due to a putative shift in methanotrophic community composition. In contrast, the CO2-induced decrease in CH4 emissions from the C. lasiocarpa marsh was associated with the increases in soil redox potential and pmoA gene abundance. We synthesized data from worldwide wetland ecosystems, and found that the responses of CH4 emissions to elevated CO2 was determined by the wetland water table levels and associated plant oxygen secretion capacity. In conditions with elevated CO2, plants with a high oxygen secretion capacity suppress CH4 emissions while plants with low oxygen secretion capacity stimulate CH4 emissions; both effects are mediated via a feedback loop involving shifts in activities of methanogens and methanotrophs.

Methane production and oxidation potentials along a fen-bog gradient from southern boreal to subarctic peatlands in Finland.

Methane (CH4 ) emissions from northern peatlands are projected to increase due to climate change, primarily because of projected increases in soil temperature. Yet, the rates and temperature responses of the two CH4 emission-related microbial processes (CH4 production by methanogens and oxidation by methanotrophs) are poorly known. Further, peatland sites within a fen-bog gradient are known to differ in the variables that regulate these two mechanisms, yet the interaction between peatland type and temperature lacks quantitative understanding. Here, we investigated potential CH4 production and oxidation rates for 14 peatlands in Finland located between c. 60 and 70°N latitude, representing bogs, poor fens, and rich fens. Potentials were measured at three different temperatures (5, 17.5, and 30℃) using the laboratory incubation method. We linked CH4 production and oxidation patterns to their methanogen and methanotroph abundance, peat properties, and plant functional types. We found that the rich fen-bog gradient-related nutrient availability and methanogen abundance increased the temperature response of CH4 production, with rich fens exhibiting the greatest production potentials. Oxidation potential showed a steeper temperature response than production, which was explained by aerenchymous plant cover, peat water holding capacity, peat nitrogen, and sulfate content. The steeper temperature response of oxidation suggests that, at higher temperatures, CH4 oxidation might balance increased CH4 production. Predicting net CH4 fluxes as an outcome of the two mechanisms is complicated due to their different controls and temperature responses. The lack of correlation between field CH4 fluxes and production/oxidation potentials, and the positive correlation with aerenchymous plants points toward the essential role of CH4 transport for emissions. The scenario of drying peatlands under climate change, which is likely to promote Sphagnum establishment over brown mosses in many places, will potentially reduce the predicted warming-related increase in CH4 emissions by shifting rich fens to Sphagnum-dominated systems.