Quantification of methane oxidation in the rhizosphere of emergent aquatic macrophytes: defining upper limits

Rates of rhizospheric methane oxidation were evaluated by aerobic incubations of subcores collected in flooded anoxic soils populated by emergent macrophytes, by greenhouse whole plant incubations, and by CH₄ stable isotopic analysis. Subcore incubations defined upper limits for rhizospheric methane oxidation on an areal basis which were equal to or greater than emission rates. These rates are considered upper limits because O₂ did not limit CH₄ uptake as is likely to occur in situ. The ratio of maximum potential methane oxidation (MO) to methane emission (ME) ranged from 0.7 to 1.9 in Louisiana rice (Oryza sativa), from 1.0 to 4.0 in a N. Florida Sagittaria lancifolia marsh, and from 5.6 to 51 in Everglades Typha domingensis and Cladium jamaicense areas. Methane oxidation/methane emission ratios determined in whole plant incubations of Sagittaria lancifolia under oxic and anoxic conditions ranged from 0.5 to 1.6. Methane oxidation activity associated with emergent aquatic macrophytes was found primarily in fine root material. A weak correlation was observed between live root biomass and CH₄ uptake in Typha. Rhizomes showed small or zero rates of methane uptake and no uptake was associated with plant stems. Methane stable isotope data from a S. lancifolia marsh were as follows: CH₄ emitted from plants: −61.6 ± 0.3%; CH₄ within stems: −42.0 ± 0.2%; CH₄ within sedimentary bubbles: −51.7 ± 0.3%). The ¹³C enrichment observed relative to emitted CH₄ could be due to preferential mobilization of CH₄ containing the lighter isotope and/or the action of methanotrophic bacteria.     

Production and oxidation of methane in a boreal mire after a decade of increased temperature and nitrogen and sulfur deposition

Natural wetlands are the single largest source of atmospheric methane (CH₄). Both a changed climate and deposition of anthropogenic nitrogen and sulfur can alter the production and oxidation of CH₄ respectively and thereby also CH₄ exchange. We used a long‐term (12 years) factorial field experiment in a boreal oligotrophic mire to evaluate the effects of greenhouse cover and addition of ammonium nitrate and sodium sulfate on the production and oxidation of CH₄ by applying laboratory incubations of samples from five depths in the mire. The rates of CH₄ production were measured without amendments and after the addition of either glucose or sulfate. Twelve years of increased nitrogen deposition has changed the mire from a Sphagnum‐dominated plant community to one dominated by sedges and dwarf shrubs. The deposition of nitrogen to the field plots caused increased production of CH₄ in incubations without amendments (34%), and also after amendments with glucose (40%) or sulfate (42%). This indicates increased substrate availability (without amendments) but also a greater abundance of methanogens (glucose amendment). The greenhouse cover caused a decrease in CH₄ production in incubations without amendments (34%), after glucose amendment (20%) and after sulfate amendment (31%). These responses indicate decreased substrate availability (without amendment) accompanied by the reduced abundance of methanogens (glucose amendment). The field application of sulfur had no effect on CH₄ production at the depth where maximal CH₄ production occurred. Closer to the mire surface, however, the rate of CH₄ production was significantly reduced by 32–45%. These results suggest that the deposition of sulfate has altered the vertical distribution of methanogens and sulfate‐reducing bacteria. The oxidation of CH₄ was not significantly affected by any of the long‐term field treatments.     

Distribution and Rate of Methane Oxidation in Sediments of the Florida Everglades

Rates of methane emission from intact cores were measured during anoxic dark and oxic light and dark incubations. Rates of methane oxidation were calculated on the basis of oxic incubations by using the anoxic emissions as an estimate of the maximum potential flux. This technique indicated that methane oxidation consumed up to 91% of the maximum potential flux in peat sediments but that oxidation was negligible in marl sediments. Oxygen microprofiles determined for intact cores were comparable to profiles measured in situ. Thus, the laboratory incubations appeared to provide a reasonable approximation of in situ activities. This was further supported by the agreement between measured methane fluxes and fluxes predicted on the basis of methane profiles determined by in situ sampling of pore water. Methane emissions from peat sediments, oxygen concentrations and penetration depths, and methane concentration profiles were all sensitive to light-dark shifts as determined by a combination of field and laboratory analyses. Methane emissions were lower and oxygen concentrations and penetration depths were higher under illuminated than under dark conditions; the profiles of methane concentration changed in correspondence to the changes in oxygen profiles, but the estimated flux of methane into the oxic zone changed negligibly. Sediment-free, root-associated methane oxidation showed a pattern similar to that for methane oxidation in the core analyses: no oxidation was detected for roots growing in marl sediment, even for roots of Cladium jamaicense, which had the highest activity for samples from peat sediments. The magnitude of the root-associated oxidation rates indicated that belowground plant surfaces may not markedly increase the total capacity for methane consumption. However, the data collectively support the notion that the distribution and activity of methane oxidation have a major impact on the magnitude of atmospheric fluxes from the Everglades.     

Does the accelerated soil N cycling sustain N demand of <Emphasis Type="Italic">Quercus mongolica</Emphasis> after decade-long elevated CO<Subscript>2</Subscript> treatment?

Net methane (CH₄) flux across the ecosystem-atmosphere boundary is governed by two counteracting processes, CH₄ oxidation and production. Recent research on CH₄ cycling has focused on net CH₄ fluxes, however, the separate processes of CH₄ oxidation and production may vary at local scales and respond differently to environmental change. Here, we separate CH₄ oxidation and production, measured as emission, in situ using CH₄ oxidation inhibition combined with a novel in situ ¹³CH₄ labeling experiment to determine the rate of soil oxidation of atmospheric CH₄. The study was conducted in a subarctic heath ecosystem with three characteristic plant community types: moist mixed species heath, dry Carex-dominated heath, and wet Eriophorum-dominated fen. We further explored the projected climate change effects of increased temperature and enhanced leaf litter input. The CH₄ oxidation inhibition experiment revealed significant potential CH₄ emission despite net CH₄ uptake. Total CH₄ oxidation and potential CH₄ emission rates differed significantly between plant communities, demonstrating high local-scale variation in CH₄ fluxes. Climate treatments did not affect CH₄ oxidation rates, however, warming tended to increase potential CH₄ emission, indicating that climate change may affect oxidation and production rates asymmetrically. Near-surface soil oxidation of atmospheric CH₄ was successfully traced using ¹³C stable isotope labeling in situ. CH₄ oxidation rates ranged widely, yet preliminarily suggested some degree of substrate limitation. Accounting for the local-scale variation in CH₄ fluxes and the relative importance of the separate processes of CH₄ oxidation and production will contribute importantly to predicting changes in landscape-scale CH₄ budgets and climate feedbacks.     

Effects of N-fertilisation on CH4 oxidation and production, and consequences for CH4 emissions from microcosms and rice fields

The world's growing human population causes an increasing demand for food, of which rice is one of the most important sources. In rice production nitrogen is often a limiting factor. As a consequence increasing amounts of fertiliser will have to be applied to maximise yields. There is an ongoing discussion on the possible effects of fertilisation on CH₄ emissions. We therefore investigated the effects of N‐fertiliser (urea) on CH₄ emission, production and oxidation in rice microcosms and field experiments. In the microcosms, a substantial but short‐lived reduction of CH₄ emission was observed after N‐addition to 43‐d‐old rice plants. Methane oxidation increased by 45%, demonstrated with inhibitor measurements and model calculations based on stable carbon isotope data (δ¹³CH₄). A second fertilisation applied to 92‐d‐old plants had no effect on CH₄ emission rates. The positive effect of additional N on methanotrophic bacteria was also found in vitro for potential CH₄ oxidation rates in soil and root samples from the microcosm and field experiments, indicated by elevated initial oxidation rates and reduced lag‐phases. Fertilisation did not affect methane production in the microcosms. In the field, the effects were diverse: methane production was inhibited in the topsoil, but stimulated instead in the bulk soil. Stimulation occurred probably in the anaerobic food chain at the level of hydrolytic or fermenting bacteria, because acetate, a methanogenic precursor, increased simultaneously. Combining field, microcosm and laboratory experiments we conclude that any agricultural treatment improving the N‐supply to the rice plants will also be favourable for the CH₄ oxidising bacteria. However, N‐fertilisation had only a transient influence and was counter‐balanced in the field by an elevated CH₄ production. A negative effect of the fertilisation was a transient increase of N₂O emissions from the microcosms. However, integrating over the season the global warming potential (GWP) of N₂O emitted after fertilisation was still negligible compared to the GWP of emitted CH₄.     

Effect of rice plants on CH4 production,transport, oxidation and emission in rice paddy soil

To understand the integrated effects of rice plants (variety Wuyugeng 2) on CH₄ emission during the typical rice growth stage, the production, oxidation and emission of methane related to rice plants were investigated simultaneously through laboratory and greenhouse experiments. CH₄ emission was significantly higher from the rice planted treatment than from the unplanted treatment. In the rice planted treatment, CH₄ emission was higher at tillering stage than at panicle initiation stage. An average of 36.3% and 54.7% of CH₄ produced was oxidized in the rhizosphere at rice tillering stage and panicle initiation stage, respectively, measured by using methyl fluoride (MF) technique. In the meantime, CH₄ production in the planted treatments incubated under O₂-free N₂ condition was reduced by 44.9 and 22.3%, respectively, compared to unplanted treatment. On the contrary, the presence of rice plants strongly stimulated CH₄ production by approximately 72.3% at rice ripening stage. CH₄ emission through rice plants averaged 95% at the tillering stage and 89% at the panicle initiation stage. Based on these results, conclusions are drawn that higher CH₄ emission from the planted treatment than from unplanted treatment could be attributed to the function of rice plants for transporting CH₄ from belowground to the atmosphere at tillering and panicle initiation stage, and that a higher CH₄ emission at tillering stage than at panicle initiation stage is due to the lower rhizospheric CH₄ oxidation and more effective transport mediated by rice plants.     

Emission and oxidation of methane in Equisetum fluviatile stands growing on organic sediment and sand bottoms

Methane emission and rhizospheric CH₄ oxidation were studied in stands of Equisetum fluviatile, a common cryptogam in boreal lakes. The experiment was performed in mesocosms with organic sediment or sand bottoms under natural variation of temperature and light using the light-oxic – dark-anoxic chamber (LO/DA) technique. Net CH₄ emission from the organic sediment during the growing season varied between 3.4 and 19.0 mg m^–2 h^–1, but from sand the net CH₄ emission was only 3–10% of that measured from the organic sediment. In the organic sediment net CH₄ emission was very significantly correlated with sediment temperature (r² = 0.92). In the sand mesocosms the variation of net CH₄ emission was better correlated with the shoot biomass than with sediment temperature variation during the growing season, indicating that methanogens were severely limited by substrate availability and were probably dependent on substrates produced by E. fluviatile. The proportion of the methane oxidized of the potential CH₄ emission in summer did not differ significantly between the bottom types. The net CH₄ emission during the growing season as a proportion of the seasonal maximum of the shoot biomass was significantly higher in the organic sediment mesocosms (6.5%) than in sand (1.7%). The high CH₄ emissions observed from dense well-established E. fluviatile stands in the field appear to be more related to temperature-regulated turnover of detritus in the anaerobic sediment and less to CH₄ oxidation and seasonal variation in plant growth dynamics     

Depth distribution of microbial production and oxidation of methane in northern boreal peatlands

The depth distributions of anaerobic microbial methane production and potential aerobic microbial methane oxidation were assessed at several sites in both Sphagnum- and sedge-dominated boreal peatlands in Sweden, and compared with net methane emissions from the same sites. Production and oxidation of methane were measured in peat slurries, and emissions were measured with the closed-chamber technique. Over all eleven sites sampled, production was, on average, highest 12 cm below the depth of the average water table. On the other hand, highest potential oxidation of methane coincided with the depth of the average water table. The integrated production rate in the 0–60 cm interval ranged between 0.05 and 1.7 g CH4 m ^–2 day^– and was negatively correlated with the depth of the average water table (linear regression: r ² = 0.50, P = 0.015). The depth-integrated potential CH₄-oxidation rate ranged between 3.0 and 22.1 g CH₄ m^–2 day^–1 and was unrelated to the depth of the average water table. A larger fraction of the methane was oxidized at sites with low average water tables; hence, our results show that low net emission rates in these environments are caused not only by lower methane production rates, but also by conditions more favorable for the development of CH₄-oxidizing bacteria in these environments. Correspondence to: I. Sundh.     

Effects of free-air CO2 enrichment (FACE) on CH4 emission from a rice paddy field

Methane (CH₄) is a particularly potent greenhouse gas with a radiative forcing 23 times that of CO₂ on a per mass basis. Flooded rice paddies are a major source of CH₄ emissions to the Earth's atmosphere. A free‐air CO₂ enrichment (FACE) experiment was conducted to evaluate changes in crop productivity and the crop ecosystem under enriched CO₂ conditions during three rice growth seasons from 1998 to 2000 in a rice paddy at Shizukuishi, Iwate, Japan. To understand the influence of elevated atmospheric CO₂ concentrations on CH₄ emission, we measured methane flux from FACE rice fields and rice fields with ambient levels of CO₂ during the 1999 and 2000 growing seasons. Methane production and oxidation potentials of soil samples collected when the rice was at the tillering and flowering stages in 2000 were measured in the laboratory by the anaerobic incubation and alternative propylene substrates methods, respectively. The average tiller number and root dry biomass were clearly larger in the plots with elevated CO₂ during all rice growth stages. No difference in methane oxidation potential between FACE and ambient treatments was found, but the methane production potential of soils during the flowering stage was significantly greater under FACE than under ambient conditions. When free‐air CO₂ was enriched to 550 ppmv, the CH₄ emissions from the rice paddy field increased significantly, by 38% in 1999 and 51% in 2000. The increased CH₄ emissions were attributed to accelerated CH₄ production potential as a result of more root exudates and root autolysis products and to increased plant‐mediated CH₄ emissions because of the larger rice tiller numbers under FACE conditions.     

Influence of Ni, Co, Fe, and Na additions on methane production in Sphagnum-dominated Northern American peatlands

Although Sphagnum (moss)-dominated, northern peatlandecosystems harbor methane (CH₄)-producing microorganisms(methanogens) and are a significant source of atmosphericCH₄, rates of CH₄ production vary widely amongdifferent systems. Very little work has been done to examine whetherconcentrations of cations and metal elements may account for thevariability. We examined rates of CH₄ production in peat fromfive geographically and functionally disparateSphagnum-dominated peatlands by incubating peat samples invitro with and without additions of trace metals (Fe, Ni, Co) andbase cations (Ca, Li, Na). In peat from the most mineral poor sites, theaddition of metals and Na enhanced CH₄ production beyond thatobserved in controls. The same treatments in mineral rich sites yieldedno effect or an inhibition of CH₄ production. None of thetreatments affected anaerobic respiration, measured as CO₂production, in the in vitro incubations of peat, except addedcitrate, suggesting that methanogens, and not the entire anaerobiccommunity, can be limited by the availability of metal elements andcations.     

CH4 emission from a hollow-ridge complex in a raised bog: The role of CH4 production and oxidation

The aim of this study was to correlate magnitude andcontrols of CH₄ fluxes with the microtopographyand the vegetation in a hollow-ridge complex of araised bog. High CH₄ emission rates were measuredfrom hollows and mud-bottom hollows, while hummocksconsumed atmospheric CH₄ at a low rate. Thehighest emissions were measured from plots with Eriophorum vaginatum and Scheuchzeriapalustris. CH₄ emission ceased after Scheuchzeria had been clipped below the water table,indicating the importance of this aerenchymatic plantas a conduit for CH₄.Peat in the upper catotelm of hollows was younger andless decomposed than in hummocks. Potential CH₄production in vitro was higher and themethanogenic association was better adapted to highertemperatures in hollow than in hummock peat. Highertemperatures in hollows resulted in a strongerCH₄ source in hollows than in hummocks. Negativefluxes from hummocks indicated that even in wetlandsmethanotrophic bacteria exist that are able to oxidizeCH₄ at atmospheric mixing ratios, and thatoxidation controls CH₄ emission completely. TheCH₄ mixing ratio was low in the acrotelm, but itincreased within the catotelm. Comparing fluxesmeasured in static chambers with fluxes calculatedfrom the porewater CH₄ profiles it was deducedthat the zone of methane oxidation was located closeto the water table.In hollows, CH₄ production at in situtemperature was far higher than emission into theatmosphere, corresponding to an oxidation rate ofnearly 99%. The CH₄ flux between the catotelmand the acrotelm of hollows was also higher than theemission, indicating the importance of CH₄oxidation in the aerobic acrotelm, too. CH₄microprofiles showed that CH₄ oxidation inmud-bottom hollows was confined to the topmost 2 mm,and that in Sphagnum-covered hollows CH₄oxidation occurred at the lower edge of green Sphagnum-parts.     

Seasonal variation in pathways of CH4 production and in CH4 oxidation in rice fields determined by stable carbon isotopes and specific inhibitors

Flooded rice fields, which are an important source of the atmospheric methane, have become a model system for the study of interactions between various microbial processes. We used a combination of stable carbon isotope measurements and application of specific inhibitors in order to investigate the importance of various methanogenic pathways and of CH₄ oxidation for controlling CH₄ emission. The fraction of CH₄ produced from acetate and H₂/CO₂ was calculated from the isotopic signatures of acetate, carbon dioxide (CO₂) and methane (CH₄) measured in porewater, gas bubbles, in the aerenchyma of the plants and/or in incubation experiments. The calculated ratio between both pathways reflected well the ratio determined by application of methyl fluoride (CH₃F) as specific inhibitor of acetate‐dependent methanogenesis. Only at the end of the season, the theoretical ratio of acetate: H₂ = 2 : 1 was reached, whereas at the beginning H₂/CO₂‐dependent methanogenesis dominated. The isotope discrimination was different between rooted surface soil and unrooted deep soil. Root‐associated CH₄ production was mainly driven by H₂/CO₂. Porewater CH₄ was found to be a poor proxy for produced CH₄. The fraction of CH₄ oxidised was calculated from the isotopic signature of CH₄ produced in vitro compared to CH₄ emitted in situ, corrected for the fractionation during the passage from the aerenchyma to the atmosphere. Isotope mass balances and in situ inhibition experiments with difluoromethane (CH₂F₂) as specific inhibitor of methanotrophic bacteria agreed that CH₄ oxidation was quantitatively important at the beginning of the season, but decreased later. The seasonal pattern was consistent with the change of potential CH₄ oxidation rates measured in vitro. At the end of the season, isotope techniques detected an increase of oxidation activity that was too small to be measured with the flux‐based inhibitor technique. If porewater CH₄ was used as a proxy of produced CH₄, neither magnitude nor seasonal pattern of in situ CH₄ oxidation could be reproduced. An oxidation signal was also found in the isotopic signature of CH₄ from gas bubbles that were released by natural ebullition. In contrast, bubbles stirred up from the bulk soil had preserved the isotopic signature of the originally produced CH₄.     

Methane‐derived carbon flow through microbial communities in arctic lake sediments

Aerobic methane (CH₄) oxidation mitigates CH₄ release and is a significant pathway for carbon and energy flow into aquatic food webs. Arctic lakes are responsible for an increasing proportion of global CH₄ emissions, but CH₄ assimilation into the aquatic food web in arctic lakes is poorly understood. Using stable isotope probing (SIP) based on phospholipid fatty acids (PLFA‐SIP) and DNA (DNA‐SIP), we tracked carbon flow quantitatively from CH₄ into sediment microorganisms from an arctic lake with an active CH₄ seepage. When 0.025 mmol CH₄ g^?1 wet sediment was oxidized, approximately 15.8–32.8% of the CH₄‐derived carbon had been incorporated into microorganisms. This CH₄‐derived carbon equated to up to 5.7% of total primary production estimates for Alaskan arctic lakes. Type I methanotrophs, including Methylomonas, Methylobacter and unclassified Methylococcaceae, were most active at CH₄ oxidation in this arctic lake. With increasing distance from the active CH₄ seepage, a greater diversity of bacteria incorporated CH₄‐derived carbon. Actinomycetes were the most quantitatively important microorganisms involved in secondary feeding on CH₄‐derived carbon. These results showed that CH₄ flows through methanotrophs into the broader microbial community and that type I methanotrophs, methylotrophs and actinomycetes are important organisms involved in using CH₄‐derived carbon in arctic freshwater ecosystems.     

Methane emission from a wetland rice field as affected by salinity

The impact of salinity on CH₄ emission was studied by adding salt to a Philippine rice paddy, increasing pore water EC to approx. 4 dS.m^-1 Methane emission from the salt-amended plot and adjacent control plots was monitored with a closed chamber technique. The addition of salt to the rice field caused a reduction by 25% in CH₄ emission. Rates of methane emissions from intact soil cores were measured during aerobic and anaerobic incubations. The anaerobic CH₄ fluxes from the salt-amended soil cores were three to four times lower than from cores of the control plot, whereas the aerobic CH₄ fluxes were about equal. Measurements of the potential CH₄ production with depth showed that the CH₄ production in the salt-amended field was strongly reduced compared to the control field. Calculation of the percentage CH₄ oxidized of the anaerobic flux indicated that CH₄ oxidation in the salt-amended plot was even more inhibited than CH₄ production. The net result was about equal aerobic CH₄ fluxes from both salt-amended plots and non-amended plots. The data illustrate the importance of both CH₄ production and CH₄ oxidation when estimating CH₄ emission and show that the ratio between CH₄ production and CH₄ oxidation may depend on environmental conditions. The reduction in CH₄ emission from rice paddies upon amendment with salt low in sulfate is considerably smaller than the reduction in CH₄ emission observed in a similar study where fields were amended with high-sulfate containing salt (gypsum). The results indicate that CH₄ emissions from wetland rice fields on saline, low-sulfate soils are lower than CH₄ emissions from otherwise comparable non-saline rice tields. However, the reduction in CH₄ emission is not proportional to the reduction in CH₄ production     

Active Methanotrophs in Two Contrasting North American Peatland Ecosystems Revealed Using DNA-SIP

The active methanotroph community was investigated in two contrasting North American peatlands, a nutrient-rich sedge fen and nutrient-poor Sphagnum bog using in vitro incubations and ¹³C-DNA stable-isotope probing (SIP) to measure methane (CH₄) oxidation rates and label active microbes followed by fingerprinting and sequencing of bacterial and archaeal 16S rDNA and methane monooxygenase (pmoA and mmoX) genes. Rates of CH₄ oxidation were slightly, but significantly, faster in the bog and methanotrophs belonged to the class Alphaproteobacteria and were similar to other methanotrophs of the genera Methylocystis, Methylosinus, and Methylocapsa or Methylocella detected in, or isolated from, European bogs. The fen had a greater phylogenetic diversity of organisms that had assimilated ¹³C, including methanotrophs from both the Alpha- and Gammaproteobacteria classes and other potentially non-methanotrophic organisms that were similar to bacteria detected in a UK and Finnish fen. Based on similarities between bacteria in our sites and those in Europe, including Russia, we conclude that site physicochemical characteristics rather than biogeography controlled the phylogenetic diversity of active methanotrophs and that differences in phylogenetic diversity between the bog and fen did not relate to measured CH₄ oxidation rates. A single crenarchaeon in the bog site appeared to be assimilating ¹³C in 16S rDNA; however, its phylogenetic similarity to other CO₂-utilizing archaea probably indicates that this organism is not directly involved in CH₄ oxidation in peat.     

Methane Biogeochemistry and Methanogen Communities in Two Northern Peatland Ecosystems,New York State

Rates of methane (CH₄) production vary considerably among northern peat-forming wetlands, and it is not clear whether variability is caused by environmental factors affecting CH₄ production or differences in methanogen communities. We investigated CH₄ production and emission dynamics concomitantly with 16S rRNA gene sequence-based community analysis of Archaea in two contrasting peat-forming northern wetlands, an ombrotrophic bog and a minerotrophic conifer swamp. Individual measurements of CH₄ emissions to the atmosphere followed a lognormal distribution pattern in both sites, and mean rates were 30× greater in the bog site. Rates of CH₄ production measured in vitro were initially 3× greater in the bog than in the conifer swamp; although, after 30 days of incubation, production rates were similar suggesting that in situ environmental conditions limited production in the conifer swamp. Amplified ribosomal DNA restriction analysis (ARDRA) and rarefaction techniques indicated that both sites had similar levels of archaeal richness, with 27 unique taxa in the bog and 23 taxa in the conifer swamp. However, the bog had more pronounced dominance of a few taxa, whereas the conifer swamp had more even distribution among taxa. A 16S rRNA gene sequence-based phylogenetic analysis indicated high levels of diversity with similarity to known methanogenic families Methanosarcinaceae, Methanosaetaceae, Methanobacteriaceae, and likely Methanomicrobiaceae as well as two additional lineages previously characterized as groups of yet uncultivated Euryarchaeota commonly occurring in flooded rice soils. Therefore, sites with low and high rates of CH₄ production supported very diverse methanogenic communities.     

Soil environmental variables affecting the flux of methane from a range of forest, moorland and agricultural soils

Measurements of the net methane exchange over a range of forest, moorland, and agricultural soils in Scotland were made during the period April to June 1994 and 1995. Fluxes of CH₄ ranged from oxidation –12.3 to an emission of 6.8 ng m^–2 s^–1. The balance between CH₄ oxidation and emission depended on the physical conditions of the soil, primarily soil moisture. The largest oxidation rates were found in the mineral forest soils, and CH₄ emission was observed in several peat soils. The smallest oxidation rate was observed in an agricultural soil. The relationship between CH₄ flux and soil moisture observed in peats (Flux_{CH 4} = 0.023 × %H₂O (dry weight) – 7.44, p > 0.05) was such that CH₄ oxidation was observed at soil moistures less than 325%( ± 80%). CH₄ emission was found at soil moistures exceeding this value. A large range of CH₄ oxidation rates were observed over a small soil moisture range in the mineral soils. CH₄ oxidation in mineral soils was negatively correlated with soil bulk density (Flux_{CH 4} = –37.35 × bulk density (g cm^–3) + 48.83, p > 0.05). Increased nitrogen loading of the soil due to N fixation, atmospheric deposition of N, and fertilisation, were consistently associated with decreases in the soil sink for CH₄, typically in the range 50 to 80%, on a range of soil types and land uses.     

Methane emissions from rice microcosms: The balance of production, accumulation and oxidation

In rice microcosms (Oryza sativa, var. Roma, type japonica),CH₄ emission, CH₄ production, CH₄oxidation and CH₄ accumulation were measured over an entirevegetation period. Diffusive CH₄ emission was measured inclosed chambers, CH₄ production was measured in soil samples,CH₄ oxidation was determined from the difference between oxicand anoxic emissions, and CH₄ accumulation was measured byanalysis of porewater and gas bubbles. The sum of diffusiveCH₄ emission, CH₄ oxidation, andCH₄ accumulation was only 60% of the cumulativeCH₄ production. The two values diverged during the first 50days (vegetative phase) and then again during the last 50 days (latereproductive phase and senescence) of the 150 day vegetation period. Duringthe period of day 50–100 (early reproductive phase/flowering), theprocesses were balanced. Most likely, gas bubbles and diffusion limitationare responsible for the divergence in the early and late phases. The effectof rice on CH₄ production rates and CH₄concentrations was studied by measuring these processes also in unplantedmicrocosms. Presence of rice plants lowered the CH₄concentrations, but had no net effect on the CH₄ productionrates.     

Niche separation within aerobic methanotrophic bacteria across lakes and its link to methane oxidation rates

Lake methane (CH₄) emissions are largely controlled by aerobic methane-oxidizing bacteria (MOB) which mostly belong to the classes Alpha- and Gammaproteobacteria (Alpha- and Gamma-MOB). Despite the known metabolic and ecological differences between the two MOB groups, their main environmental drivers and their relative contribution to CH₄ oxidation rates across lakes remain unknown. Here, we quantified the two MOB groups through CARD-FISH along the water column of six temperate lakes and during incubations in which we measured ambient CH₄ oxidation rates. We found a clear niche separation of Alpha- and Gamma-MOB across lake water columns, which is mostly driven by oxygen concentration. Gamma-MOB appears to dominate methanotrophy throughout the water column, but Alpha-MOB may also be an important player particularly in well-oxygenated bottom waters. The inclusion of Gamma-MOB cell abundance improved environmental models of CH₄ oxidation rate, explaining part of the variation that could not be explained by environmental factors alone. Altogether, our results show that MOB composition is linked to CH₄ oxidation rates in lakes and that information on the MOB community can help predict CH₄ oxidation rates and thus emissions from lakes.