CH4 Oxidation Capacity Research Articles

Spatial variation of methane oxidation and carbon dioxide sequestration in landfill biogeochemical cover

ABSTRACT The study investigated the spatial variation of potential methane (CH4) oxidation and residual carbon dioxide (CO2) sequestration in biogeochemical cover (BGCC) system designed to remove CH4, CO2, and hydrogen sulfide (H2S) from landfill gas (LFG) emissions. A 50 cm x 50 cm x 100 cm tank simulated BGCC system, comprising a biochar-amended soil (BAS) layer for CH4 oxidation, a basic oxygen furnace (BOF) slag layer for CO2 and H2S sequestration, and an upper topsoil layer. Synthetic LFG was flushed through the system in five phases, with each corresponding to different compositions and flow rates. Following monitoring, the system was dismantled, and samples were extracted from different depths and locations to analyze spatial variations, focusing on moisture content (MC), organic content (OC), pH, and electrical conductivity (EC). Additionally, batch tests on selected samples from BAS and BOF slag layers were performed to assess potential CH4 oxidation and residual carbonation capacity. The aim of study was to evaluate the BGCC's effectiveness in LFG mitigation, however this study focused on assessing spatial variations in physico-chemical properties, CH4 oxidation in the BAS layer, and residual carbonation in the BOF slag layer. Findings revealed CH4 oxidation in the BAS layer varied between 22.4 and 277.9 µg CH4/g-day, with higher rates in the upper part, and significant spatial variations at 50 cm below ground surface (bgs) compared to 85 cm bgs. The BOF slag layer showed a residual carbonation capacity of 40-49.3 g CO2/kg slag, indicating non-uniform carbonation. Overall, CH4 oxidation and CO2 sequestration capacities varied spatially and with depth in the BGCC system.

Metabolic diversity shapes vegetation-enhanced methane oxidation in landfill covers: Multi-omics study of rhizosphere microorganisms

Vegetation root exudates have the ability to shape soil microbial community structures, thereby enhancing CH4 bio-oxidation capacity in landfill cover systems. In this study, the CH4 oxidation capacity of indigenous vegetation rhizosphere microorganisms within operational landfill covers in Chongqing, China, was investigated for the first time, with the objective of identifying suitable plant candidates for CH4 mitigation initiatives within landfill cover systems. Furthermore, a multi-omics methodology was employed to explore microbial community structures and metabolic variances within the rhizospheric environment of diverse vegetation types. The primary aim was to elucidate the fundamental factors contributing to divergent CH4 oxidation capacities observed in rhizosphere soils. The findings demonstrated that herbaceous vegetation predominated in landfill covers. Notably, Rumex acetosa exhibited the highest CH4 oxidation capacity in the rhizosphere soil, approximately 20 times greater than that in non-rhizosphere soil. Root exudates played a crucial role in inducing the colonization of CH4-oxidizing functional microorganisms in the rhizosphere, subsequently prompting the development of specific metabolic pathways. This process, in turn, enhanced the functional activity of the microorganisms while concurrently bolstering their tolerance to microbial pollutants. Consequently, the addition of substances like Limonexic acid strengthened the CH4 bio-oxidation process, thereby underscoring the suitability of Rumex acetosa and similar vegetation species as preferred choices for landfill cover vegetation restoration.

Enhanced methane oxidation efficiency by digestate biochar in landfill cover soil: Microbial shifts and carbon metabolites insights

The ability of biochar to enhance the oxidation of methane (CH4) in landfill cover soil by promoting the growth and activity of methane-oxidizing bacteria (MOB) has attracted significant attention. However, the optimal characteristics of digestate-derived biochar (DBC) for promoting the MOB community and CH4 removal performance remain unclear. This study examined how the CH4 oxidation capacity and respiratory metabolism of MOB life process are affected by the application of DBC compared with the most commonly used woody-derived biochar (WBC). The addition of both WBC and DBC enhanced CH4 oxidation, with DBC exhibiting a nearly twofold increase in cumulative CH4 oxidation mass (7.14 mg CH4 g−1) compared to WBC. The high ion-exchange capacity of DBC was found to be more favorable for the growth of Type I MOB, which have more efficient metabolic pathways for CH4 oxidation. Type I MOB which are abundant in DBC may prefer monovalent positive ions, while the charge-rich nature of DBC may also have hindered extracellular protein aggregation. The superiority of DBC in terms of CH4 oxidation thus highlights the underlying mechanisms of biochar-MOB interactions, offering potential biochar options for landfill cover soil.

Experimental Characterization of the Engineering Properties of Landfill Compost-Biocover

A landfill biocover system optimizes environmental conditions for biotic methane (CH4) consumption that controls the fugitive and residual emissions from landfills. Research shows that wasted compost material has more (CH4) oxidation potential than other materials. Thus, in this study, the authors investigate the engineering properties of compacted compost to test its suitability for CH4 oxidation capacity. Different laboratory and analytical approaches are employed to attain the set objectives. The biochemical tests show that the studied material indicates the presence of methanotrophs with sufficient organic contents. The compacted compost also shows adequate diffusivity potential to free air space for a wide range of water content. The data also imply that compacting compost to low hydraulic conductivity can be accomplished for a wide range of water content, according to the suggested values for a landfill hydraulic barrier. Furthermore, the low thermal properties of compost as compared to other mineral materials seem more beneficial, as specifically, during the winter season, when the atmospheric temperature is low, low thermal conductivity enables it to sustain a stable temperature for the activities of the microbial organisms, which therefore extends the CH4 oxidation process right through a long period in the winter.

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.

Impact of hydrogen sulfide on biochar in stimulating the methane oxidation capacity and microbial communities of landfill cover soil

Hydrogen sulfide (H2S) can influence methanotrophic activities and be adsorbed by biochar (BC); however, the impact of H2S on BC in stimulating the methane (CH4) oxidation capacity of landfill cover soil (LCS) has not been clarified. Thus, batch incubation experiments were conducted to observe the effect of H2S on the CH4 oxidation capacity of and microbial communities in BC-amended LCS. Three landfill gas conditions were considered: 5 % CH4 and 15 % oxygen (O2) (5 M), 10 % CH4 and 10 % O2, and 20 % CH4 and 5 % O2 (20 M) by volume, with H2S concentrations of 0, 100, 250, and 1000 ppm, respectively. Another series was conducted using LCS subjected to pre-H2S saturation under the 20 M gas condition. In the 5 M gas condition suitable for the dominant methanotroph Methylocaldum (type I), the BC retained its ability to stimulate the CH4 oxidation capacity of LCS (enhancement of 41–108 %) in the presence of H2S. Additionally, when H2S ≤ 250 ppm, the BC exhibited a relatively consistent impact of H2S on both CH4 oxidation capacity and microbial communities in LCS, independent of the CH4 or O2 concentrations. This result could be attributed to the different pathways of H2S metabolism for the LCS and BC-amended LCS. Furthermore, when saturated adsorption of H2S occurred for the LCS, the CH4 oxidation capacity for BC-amended LCS was higher than that for non-amended LCS, which demonstrated the ability of BC in alleviating the inhibition of H2S on CH4 oxidation due to its excellent H2S adsorption under even anoxic environments.

Validation and optimization of key biochar properties through iron modification for improving the methane oxidation capacity of landfill cover soil

Knowledge of various BC properties in stimulating the methane (CH4) oxidation capacity of landfill cover soil (LCS) is still limited, restricting the optimization of BC performance. To validate key BC properties and seek a feasible way for enhancing BC performance, this study prepared BCs with distinctly varying characteristics through iron (Fe) modification. Then, batch incubation experiments under different CH4 and oxygen concentrations were conducted. Pore volume, cation exchange capacity (CEC), and surface area of BC collectively accounted for 78.5% of the variances in the microbial community structures, with pore volume being the most important factor. These correlated well with the differences in the CH4 oxidation capacities among the different BC-amended LCS. At a low ratio of 15% (v/v) in LCS, BCs' pH not affected their performance but homogeneity could be a limiting factor. Fe modification proved a promising approach to more efficiently improve the three key BC properties (especially pore volume) and thus optimize BC performance than increasing pyrolysis temperature did. Fe-modified BCs encouraged a bacterial consortium (methanotroph, methylotrophs, and nitrogen-fixing bacteria) in the soil with significantly improved CH4 oxidation capacities by up to 26%–74% compared to that of pristine BC.

Carbon-dependent growth, community structure and methane oxidation performance of a soil-derived methanotrophic mixed culture.

Soil-borne methane-oxidizing microorganisms act as a terrestrial methane (CH4) sink and are potentially useful in decreasing global CH4 emissions. Understanding the ecophysiology of methanotrophs is crucial for a thorough description of global carbon cycling. Here, we report the in situ balance of soils from abandoned landfills, meadows and wetlands, their capacities to produce and oxidize CH4 at laboratory-scale and the isolation of a soil-borne methanotrophic-heterotrophic mixed culture that was used for carbon (C1 and C2) feeding experiments. We showed that even with similar soil properties, the in situ CH4 balance depends on land-use. Different soils had different potentials to adapt to increased CH4 availability, leading to the highest CH4 oxidation capacities for landfill and wetland soils. The most efficient mixed culture isolated from the landfill was dominated by the methanotrophs Methylobacter sp. and Methylosinus sp., which were accompanied by Variovorax sp. and Pseudomonas sp. and remained active in oxidizing CH4 when supplied with additional C-sources. The ratios between type I and type II methanotrophs and between methanotrophic and heterotrophic bacteria changed when C-sources were altered. A significant effect of the application of the mixed culture on the CH4 oxidation of soils was established but the extent varied depending on soil type.

Numerical modeling of methane oxidation and emission from landfill cover soil coupling water-heat-gas transfer: Effects of meteorological factors

CH4 emissions from landfills present temporal and spatial variation due to the changing meteorological conditions. The combined effect of meteorological parameter on CH4 transport, oxidation and emission are not fully understood. In this study a water-heat-gas transfer model embedding a meteorological module and CH4 oxidation was built to elucidate the CH4 transport, oxidation and emissions in landfill cover soils. A series of parametric studies are carried out to investigate the influence of barometric pressure, air temperature and rainfall on CH4 transport, oxidation and emissions in landfill cover soil. Finally, a real-time climatic condition was conducted to predict the daily CH4 oxidation and emissions from different climatic zones. The simulated results indicated that a low atmospheric pressure would convert the dominant gas transport mechanism from diffusion to advection and therefore result in a lower CH4 oxidation efficiency. In a landfill, the cover soil would also have a higher CH4 oxidation capacity because of the heat conduction from the waste layer under cold conditions. A significant decrease of CH4 emission flux was observed during the rainfall. After rainfall event, the CH4 emission flux was significantly positively correlated with rain intensity. This study proved that building a real-time landfill CH4 emission model considering varying climatic conditions is necessary to improve the accuracy of model predicting.

Comparison of the methane-oxidizing capacity of landfill cover soil amended with biochar produced using different pyrolysis temperatures

The in-situ mitigation of methane (CH4) in landfill gas using landfill cover soil (LCS) is a cost-effective approach, but its efficiency needs to be enhanced. In this study, we incorporated an enriched methane-oxidizing bacteria (MOB) consortium into LCS and established four biochar-amended LCS groups with biochar produced at 300 °C (BC300), 400 °C (BC400), 500 °C (BC500), and 600 °C (BC600). The purpose was to evaluate the CH4 oxidation capacity of biochar-amended LCS after inoculation with MOB and to investigate how the physicochemical properties of biochar that are influenced by the pyrolysis temperature affect the performance and microbial activity of biochar-amended LCS. It was found that a 15% volume ratio (representing a mass ratio of 2.49%–2.78%) for biochar amendment in LCS enhanced CH4 removal efficiency, with the highest removal observed to be 46% for BC400-amended LCS compared to 30% for the original LCS. In addition, CH4 adsorption by the biochar was not observed, and a 15% mass ratio for biochar in the LCS had no or a negative impact. Besides improving the water-holding capacity and gas permeability of LCS, other possible advantages of biochar amendment in terms of CH4 oxidization include greater retention of nutrients, electron acceptors, and exchangeable cations, as well as introducing iron ions. It was also found that CH4 oxidation capacity and the methanotroph activity of biochar-amended LCS did not continue to increase with higher pyrolysis temperatures, even though higher micropore volumes and surface areas were obtained at higher pyrolysis temperatures. From this study, BC400 was identified as the optimal choice for the best performance in terms of enhancing both the CH4 oxidation capacity of the amended LCS and the growth of type II methanotroph Methylocystaceae, which can possibly be attributed to having the highest cation exchange capacity of the four biochars.

Treatment of landfill gas with low methane content by biocover systems

Landfills are significant sources of anthropogenic atmospheric methane (CH4), which contributes to climate change. Large amounts of CH4 are emitted from landfills in dilute form due to mixing with air in leachate collection systems, or during lateral migration away from landfills. The objective of this study was to investigate the CH4 oxidation efficiency of a compost material subject to LFG diluted with atmospheric air resulting in CH4 concentrations of 5–10% v/v. CH4 oxidation rates and carbon dioxide (CO2) production were measured through batch and dynamic column experiments where two laboratory scale biofilters were constructed. The columns were run at increasing flow rates. Column gas concentration profiles for each of five flow campaigns were compared to each other. This showed that oxygen (O2) was present through the entire column and elevated CO2 concentrations throughout the biofilters were found. Moreover, the oxidation process tended to be centred in the lower parts of both columns. It was observed that the biofilters performed better once they had adapted to the increasing loads of CH4. In both columns, the maximum removal rate of CH4 was found to be 98–100%. Using CH4 mass balances the maximum oxidation rate was 238 g CH4 m−2 d−1 in Column 1 and 483 g CH4 m−2 d−1 in Column 2 (equal to the load). None of the biofilters reached their maximum CH4 oxidation capacity, hence they could have been exposed to a larger CH4 load. It was found that the retention time in the columns was not a factor limiting the oxidation process. High O2 consumption and carbon mass balances underlined the strong microbial activity in the biofilters and it was not suspected that the methane oxidising bacteria were O2 limited. The results of this study suggest that biofilters have great potential for reducing CH4 in diluted LFG.

Dose-response relationships between environmentally-relevant H2 concentrations and the biological sinks of H2, CH4 and CO in soil

Local H2 accumulations can be found in soil, especially within legume crop agroecosystems, where H2 is an obligate by-product of nitrogen fixation. Recent investigations show that diffusive fluxes of H2 act as additional energy inputs shaping microbial community structure and function in soil. The goal of this study is thus to define dose-response relationships between H2 exposure and soil microbial community dynamics. Community structure and trace gases (i.e. H2, CH4 and CO) oxidation activities were investigated following soil incubation to environmentally-relevant H2 mixing ratios. Despite no evidence of an alteration of microbial diversity, coordinated dose-response relationships between trace gases oxidation rates and H2 exposure were recorded. Measured H2 oxidation rates were implemented into a theoretical framework modeling H2 decay as a function of distance from H2-emitting point sources. Theoretical H2 concentration profiles and dose-response relationships between H2 concentration and trace gases oxidation rates were integrated to predict the impact of H2 on microbial community functioning. While most H2 is oxidized within 1 cm of H2 point sources, trace gases oxidation is predicted to be altered within a 10 cm radius. High-affinity CH4 and CO oxidation capacities dropped by up to 78% and 84% along H2 concentration gradients, respectively. Theoretical distances from H2-emitting point sources required to reactivate 50% of maximal CH4 oxidation rate were 0.3 cm in farmland soil and 0.2 cm in poplar soil. A longer distance was required to reactivate 50% of CO oxidation rate, i.e. 1.5 and 2.7 cm in farmland and poplar soils, respectively. Loss of CH4 oxidation potential observed under elevated H2 exposure was correlated with a gain of low-affinity H2 oxidation activity, while a substrate inhibition of high affinity H2 oxidation rate was paralleled with a decreasing trend of CO oxidation activity. In addition to shedding light on potential interactions between H2, CH4 and CO biogeochemical processes in soil, these novel findings provide evidence that H2 supports metabolic and energetic flexibility in microorganisms supplying a variety of ecosystem services.

Responses of mixed methanotrophic consortia to variable Cu2+/Fe2+ ratios

Methane mitigation in landfill top cover soils is mediated by methanotrophs whose optimal methane (CH4) oxidation capacity is governed by environmental and complex microbial community interactions. Optimization of CH4 remediating bio-filters need to take microbial responses into account. Divalent copper (Cu2+) and iron (Fe2+) are present in landfills at variable ratios and play a vital role in methane oxidation capacity and growth of methanotrophs. This study, as a first of its kind, therefore quantified effects of variable Cu2+ and Fe2+ (5:5, 5:25 and 5:50 μM) ratios on mixed methanotrophic communities enriched from landfill top cover (LB) and compost soils (CB). CH4 oxidation capacity, CH4 removal efficiencies, fatty acids content/profiles and polyhydroxybutyrate (PHB; a biopolymer) contents were also analysed to quantify performance and potential co-product development. Mixed methanotroph cultures were raised in 10 L continuous stirred tank reactors (CSTRs, Bioflo® & Celligen® 310 Fermentor/Bioreactor; John Morris Scientific, Chatswood, NSW, Australia). Community structure was determined by amplifying the V3-V4 region of 16s rRNA gene. Community structure and, consequently, fatty acid-profiles changed significantly with increasing Cu2+/Fe2+ ratios, and responses were different for LB and CB. Effects on methane oxidation capacities and PHB content were similar in the LB- and CB-CSTR, decreasing with increasing Cu2+/Fe2+ ratios, while biomass growth was unaffected. In general, high Fe2+ concentration favored growth of the type -II methanotroph Methylosinus in the CB-CSTR, but methanotroph abundances decreased in the LB-CSTR. Increase in Cu2+/Fe2+ ratio increased the growth of Sphingopyxis in both systems, while Azospirllum was co-dominant in the LB- but absent in the CB-CSTR. After 13 days, methane oxidation capacities and PHB content decreased by ∼50% and more in response to increasing Fe2+ concentrations. Although methanotroph abundance was ∼2% in the LB- (compared to >50% in CB-CSTR), methane oxidation capacities were comparable in the two systems, suggesting that methane oxidation capacity was maintained by the dominant Azospirllum and Sphingopyxis in the LB-CSTR. Despite similar methanotroph inoculum community composition and controlled environmental variables, increasing Cu2+/Fe2+ ratios resulted in significantly different microbial community structures in the LB- and CB-CSTR, indicative of complex microbial interactions. In summary, our results suggest that a detailed understanding of allelopathic interactions in mixed methanotrophic consortia is vital for constructing robust bio-filters for CH4 emission abatement.

Response of mixed methanotrophic consortia to different methane to oxygen ratios

Methane (CH4) and oxygen (air) concentrations affect the CH4 oxidation capacity (MOC) and mixed methanotrophic community structures in compost (fresh) and landfill (age old) top cover soils. A change in the mixed methanotrophic community structure in response has implications for landfill CH4 bio-filter remediation and possible bio-product outcomes (i.e., fatty acid methyl esters (FAME) content and profiles and polyhydroxybutyrate (PHB) contents). Therefore the study aimed to evaluate the effect of variable CH4 to oxygen ratios (10–50% CH4 in air) on mixed methanotrophic community structures enriched from landfill top cover (LB) and compost soils (CB) and to quantify flow on impacts on MOC, total FAME contents and profiles, and PHB accumulation. A stable consortium developed achieving average MOCs of 3.0±0.12, 4.1±0.26, 6.9±0.7, 7.6±1.3 and 9.2±1.2mgCH4g−1DWbiomassh−1 in LB and 2.9±0.04, 5.05±0.32, 6.7±0.31, 7.9±0.61 and 8.6±0.48mgCH4g−1DWbiomassh−1 in CB for a 20day cultivation period at 10, 20, 30, 40 and 50% CH4, respectively. CB at 10% CH4 had a maximal FAME content of 40.5±0.8mgFAMEg−1DWbiomass, while maximal PHB contents (25mgg−1DWbiomass) was observed at 40% CH4 in LB. Despite variable CH4/O2 ratios, the mixed methanotrophic community structures in both LB and CB were relatively stable, dominated by Methylosarcina sp., and Chryseobacterium, suggesting that a resilient consortium had formed which can now be tested in bio-filter operations for CH4 mitigations in landfills.

Importance of vegetation classes in modeling CH4 emissions from boreal and subarctic wetlands in Finland

Boreal/arctic wetlands are dominated by diverse plant species, which vary in their contribution to CH4 production, oxidation and transport processes. Earlier studies have often lumped the processes all together, which may induce large uncertainties into the results. We present a novel model, which includes three vegetation classes and can be used to simulate CH4 emissions from boreal and arctic treeless wetlands. The model is based on an earlier biogeophysical model, CH4MODwetland. We grouped the vegetation as graminoids, shrubs and Sphagnum and recalibrated the vegetation parameters according to their different CH4 production, oxidation and transport capacities. Then, we used eddy-covariance-based CH4 flux observations from a boreal (Siikaneva) and a subarctic fen (Lompolojänkkä) in Finland to validate the model. The results showed that the recalibrated model could generally simulate the seasonal patterns of the Finnish wetlands with different plant communities. The comparison between the simulated and measured daily CH4 fluxes resulted in a correlation coefficient (R2) of 0.82 with a slope of 1.0 and an intercept of −0.1mgm−2h−1 for the Siikaneva site (n=2249, p<0.001) and an R2 of 0.82 with a slope of 1.0 and an intercept of 0.0mgm−2h−1 for the Lompolojänkkä site (n=1826, p<0.001). Compared with the original model, the recalibrated model in this study significantly improved the model efficiency (EF), from −5.5 to 0.8 at the Siikaneva site and from −0.4 to 0.8 at the Lompolojänkkä site. The simulated annual CH4 emissions ranged from 7 to 24gm−2yr−1, which was consistent with the observations (7–22gm−2yr−1). However, there are some discrepancies between the simulated and observed daily CH4 fluxes for the Siikaneva site (RMSE=50.0%) and the Lompolojänkkä site (RMSE=47.9%). Model sensitivity analysis showed that increasing the proportion of the graminoids would significantly increase the CH4 emission levels. Our study demonstrated that the parameterization of the different vegetation processes was important in estimating long-term wetland CH4 emissions.

Geotechnical characterization of peat-based landfill cover materials

Natural methane (CH4) oxidation that is carried out through the use of landfill covers (biocovers) is a promising method for reducing CH4 emissions from landfills. Previous studies on peat-based landfill covers have mainly focused on their biochemical properties (e.g. CH4 oxidation capacity). However, the utilization of peat as a cover material also requires a solid understanding of its geotechnical properties (thermal, hydraulic, and mechanical), which are critical to the performance of any biocover. Therefore, the objective of this context is to investigate and assess the geotechnical properties of peat-based cover materials (peat, peat–sand mixture), including compaction, consolidation, and hydraulic and thermal conductivities. The studied materials show high compressibility to the increase of vertical stress, with compression index (Cc) values ranging from 0.16 to 0.358. The compressibility is a function of sand content such that the peat–sand mixture (1:3) has the lowest Cc value. Both the thermal and hydraulic conductivities are functions of moisture content, dry density, and sand content. The hydraulic conductivity varies from 1.74 × 10−9 m/s to 7.35 × 10−9 m/s, and increases with the increase in sand content. The thermal conductivity of the studied samples varies between 0.54 W/(m K) and 1.41 W/(m K) and it increases with the increases in moisture and sand contents. Increases in sand content generally increase the mechanical behavior of peat-based covers; however, they also cause relatively high hydraulic and thermal conductivities which are not favored properties for biocovers.

Influence of nutrients on oxidation of low level methane by mixed methanotrophic consortia.

Low-level methane emissions from coal mine ventilation air (CMV-CH4; i.e., 1 % CH4) can significantly contribute to global climate change, and therefore, treatment is important to reduce impacts. To investigate CMV-CH4 abatement potential, five different mixed methanotrohic consortia (MMCs) were established from soil/sediment sources, i.e., landfill top cover soil, bio-solid compost, vegetated humus soil, estuarine and marine sediments. Enrichment conditions for MMCs were as follows: nitrate mineral salt (NMS) medium, pH ~ 6.8; 25 °C; 20-25 % CH4; agitation 200 rpm; and culture period 20 days, in mini-bench-top bioreactors. The enriched cultures were supplemented with extra carbon (methanol 0.5-1.5 %, formate 5-15 mM, and acetate 5-15 mM), nitrogen (nitrate 0.5-1.5 g L(-1), ammonium 0.1-0.5 g L(-1), or urea: 0.1-0.5 g L(-1)), and trace elements (copper 1-5 μM, iron 1-5 μM, and zinc 1-5 μM) in different batch experiments to improve low-level CH4 abatement. Average CH4 oxidation capacities (MOCs) of MMCs varied between 1.712 ± 0.032 and 1.963 ± 0.057 mg g(-1)DWbiomass h(-1). Addition of formate improved the MOCs of MMCs, but the dose-response varied for different MMCs. Acetate, nitrate and copper had no significant effect on MOCs, while addition of methanol, ammonium, urea, iron and zinc impacted negatively. Overall, MMCs enriched from marine sediments and landfill top cover soil showed high MOCs which were largely resilient to nutrient supplementation, suggesting a strong potential for biofilter development for industrial low-level CH4 abatement, such as those present in CMV.

Microbial methane cycling in the bed of a chalk river: oxidation has the potential to match methanogenesis enhanced by warming

Summary Many rivers are oversaturated in methane (CH4) and carbon dioxide (CO2) relative to the atmosphere, but we know little about the biological controls on the balance between these two important greenhouse gases and how they might respond to warming. We characterise the potential response to temperature in the biological production of CO2 and CH4 and the subsequent microbial oxidation of that CH4, that is the sink and source components of the CH4 cycle, in contrasting river bed sediments: fine sediments, which are largely anoxic, and oxic, coarse gravels. In the fine sediments, anaerobic production of both CH4 and CO2 increased with temperature, with apparent activation energies for each being 0.51 eV and 0.24 eV, respectively. The difference between the two resulted in a 4% increase in the ratio of CH4:CO2 production for a 1 °C increase in temperature. In the coarse gravels, microbial CH4 oxidation showed no response to temperature at CH4 concentrations characteristic of these gravel beds (30–200 nmol CH4 L−1), due to strong substrate limitation. In contrast, at higher (although still rate limiting) CH4 concentrations, more characteristic of the fine sediment patches (2–4 μmol CH4 L−1), CH4 oxidation exhibited an increasingly strong response to temperature, eventually exceeding that for CH4 production. In the fine sediment, the surface layers had a CH4 oxidation capacity over 100 times greater than the gravels and the kinetic response to differing pore water CH4 concentrations meant CH4 was oxidised some 2000 times faster in the fine sediment patches compared with the coarse gravels. The calculated kinetic and temperature responses showed that with warming, methanogenesis is unlikely to outstrip methanotrophy and the ratio of CO2 to CH4 emitted could be conserved. Consequently, any changes in the efflux ratio of CH4 to CO2 are unlikely to be due to the incapacity of methanotrophy to respond to CH4 production, but rather to a physical bypassing of the methanotrophic community (e.g. through ebullition or transport via plant stems) or contraction of the oxic layer.

Methane oxidation kinetics and diffusivity in soils under conventional tillage and long-term no-till

Long-term no-till (NT) farming can improve the methane (CH4) oxidation capacity of agricultural soils, but the relative contribution of soil biological and physical properties on this outcome remains unclear. This understanding is especially important in fine-textured and poorly-drained soils where gas transport can limit expression of the potential for biological CH4 oxidation. A study was conducted to assess gaseous diffusivity (Ds), CH4 oxidation capacity (initial CH4: ~3μLL−1) and other oxidation kinetic parameters (Km, Vmax, Th) in a well-drained (WD) and a poorly-drained (PD) soil under forest (WL), plow till (PT) and long-term NT (50y). Regardless of study site, CH4 oxidation capacity (μgCH4-Ckg−1h−1) was significantly higher in NT (0.26) than in PT soils (0.06), and approached (74%) the level in forest soils. Half saturation constants (Km) and thresholds for CH4 oxidation (Th) were lower in NT (Km: 100.5; Th: 0.6μLCH4L−1) than PT soils (Km: 134; Th: 2.7μLCH4L−1) suggesting a shift in the affinity of methanotrophs for CH4 in soils under long-term NT. Irrespective of soil drainage, Ds was significantly higher in NT (2.97×10−3cm2s−1) than PT soils (1.32×10−3cm2s−1). CH4 oxidation rates in intact soil cores were positively related to Ds (r2=0.64, P<0.002), and were also lower than in sieved soils, suggesting diffusion restriction of CH4 oxidation. This limitation was most pronounced in the PD soils under PT (CH4 consumption in cores was only 6% of that in sieved soils); thus, this combination of soil type and tillage would be least favorable to CH4 uptake at the field-scale. Overall these results confirm the positive impact of NT on both the biological and physical soil attributes essential for CH4 uptake in croplands. Long-term implementation of NT farming could therefore contribute to the mitigation of CH4 emission from agriculture.

Conversion of cropland to forest increases soil CH4 oxidation and abundance of CH4 oxidizing bacteria with stand age

We investigated CH4 oxidation in afforested soils over a 200-year chronosequence in Denmark including different tree species (Norway spruce, oak and larch) and ages. Samples of the top mineral soil (0–5cm and 5–15cm depth) were incubated and analyzed for the abundance of the soil methane-oxidizing bacteria (MOB) and ammonia-oxidizing bacteria (AOB) and archaea (AOA) based on quantitative PCR (qPCR) on pmoA and amoA genes. Our study showed that CH4 oxidation rates and the abundance of MOB increased simultaneously with time since afforestation, suggesting that the methanotrophic activity is reflected in the abundance of this functional group.The development of forest soils resulted in increased soil organic carbon and reduced bulk density, and these were the two variables that most strongly related to CH4 oxidation rates in the forest soils. For the top mineral soil layer (0–5cm) CH4 oxidation rates did not differ between even aged stands from oak and larch, and were significantly smaller under Norway spruce. Compared to the other tree species Norway spruce caused a decrease in the abundance of MOB over time that could explain the decreased oxidation rates. However, the cause for the lower abundance remains unclear. The abundance of ammonia-oxidizers along the chronosequence decreased over time, oppositely to the MOB. However, our study did not indicate a direct link between CH4 oxidation rates and ammonia-oxidizers. Here, we provide evidence for a positive impact of afforestation of former cropland on CH4 oxidation capacity in soils most likely caused by an increased population size and activity of MOB.