铁炉渣施加对稻田甲烷产生、氧化与排放的影响

为了阐明铁炉渣施加对稻田甲烷产生、氧化与排放的影响,采用静态箱-气相色谱法对对照(CK)、2 Mg/hm2(FeⅠ)、4Mg/hm2(FeⅡ)和8 Mg/hm2(FeⅢ)铁炉渣施加后稻田甲烷产生、氧化与排放进行了测定与分析.研究结果表明:观测期内,CK、Fe Ⅰ、FeⅡ和FeⅢ样地甲烷产生量分别为0.06-8.87、0.12-8.28、0.15-7.84、0.17-7.82 mg·m-2·h-1,平均产生量分别为4.68、3.92、3.14、2.76 mg·m-2·h-1;甲烷氧化量分别是0.02-1.27、0.09-0.95、0.09-1.54、0.09-2.79 mg·m-2·h-1,平均氧化量为0.46、0.47、0.59、0.55 mg·m-2·h-1;甲烷排放分别是0.04-7.99、0.03-7.33、0.06-6.30、0.08-5.12 mg· m-2·h-1,平均值分别为3.11、2.29、1.76、1.59 mg·m-2·h-1.铁炉渣的施加降低了甲烷产生量和排放通量,提高了甲烷氧化量.     

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     

Methane emission from a wetland plant: the role of CH4 oxidation in Eriophorum

The importance of plant-mediated CH₄ transport was studied in a northern wetland. CH₄ transport through Eriophorum, a dominant sedge, was found to be the major pathway for CH₄ fluxes. Mean emission from Sphagnum lawns was low (34 g CH₄ m^-2 h^-1) and significantly higher from tussocks of Eriophorum vaginatum (974 g CH₄ m^-2 h^-1; U-test, p < 0.05). Mean flux from single tillers of Eriophorum angustifolium was 92 g CH₄ h^-1. In contrast to other ecosystems, no CH₄ oxidation was associated with Eriophorum. Hence, the lack of oxidation is one reason for the high emission rates from these ecosystems. This finding is a caveat for models of CH₄ emission and may also have consequences for carbon flow models of northern wetlands.     

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.     

Isotopic insights into methane production,oxidation, and emissions in Arctic polygon tundra

Arctic wetlands are currently net sources of atmospheric CH₄. Due to their complex biogeochemical controls and high spatial and temporal variability, current net CH₄ emissions and gross CH₄ processes have been difficult to quantify, and their predicted responses to climate change remain uncertain. We investigated CH₄ production, oxidation, and surface emissions in Arctic polygon tundra, across a wet‐to‐dry permafrost degradation gradient from low‐centered (intact) to flat‐ and high‐centered (degraded) polygons. From 3 microtopographic positions (polygon centers, rims, and troughs) along the permafrost degradation gradient, we measured surface CH₄ and CO₂ fluxes, concentrations and stable isotope compositions of CH₄ and DIC at three depths in the soil, and soil moisture and temperature. More degraded sites had lower CH₄ emissions, a different primary methanogenic pathway, and greater CH₄ oxidation than did intact permafrost sites, to a greater degree than soil moisture or temperature could explain. Surface CH₄ flux decreased from 64 nmol m^?2 s^?1 in intact polygons to 7 nmol m^?2 s^?1 in degraded polygons, and stable isotope signatures of CH₄ and DIC showed that acetate cleavage dominated CH₄ production in low‐centered polygons, while CO₂ reduction was the primary pathway in degraded polygons. We see evidence that differences in water flow and vegetation between intact and degraded polygons contributed to these observations. In contrast to many previous studies, these findings document a mechanism whereby permafrost degradation can lead to local decreases in tundra CH₄ emissions.     

不饱和土壤CH4的吸收与氧化

不饱和土壤是已知唯一的 CH4 生物壑。综述了不饱和土壤 CH4 的吸收、氧化过程及其影响因素。不饱和土壤中 CH4 氧化的临界浓度低 ,因而甲烷氧化菌可氧化大气 CH4 并将其当作唯一的碳源和能源。土壤 CH4 吸收率与土壤湿度通常呈负相关关系。土壤湿度过高 ,大气 CH4 和 O2 向土壤中扩散受阻 ;或土壤湿度过低引起水分胁迫均导致甲烷氧化菌活性下降。NH 4对土壤中 CH4 氧化的抑制作用可归结为 NH3和 CH4 在甲烷单氧酶水平上的竞争、由氧化作用向硝化作用的转移以及 NH 4氧化生成的 NO- 2 的毒性。NH 4对 CH4 氧化的抑制作用与土壤有效氮含量成正比。各类氮肥对 CH4 氧化抑制作用 :化肥 >有机肥 ;铵态氮肥 >尿素。 NO- 3对 CH4 氧化没有抑制效应。阳离子代换量 (CEC)高的土壤 NH 4对 CH4 氧化的抑制作用轻。 CH4 氧化菌对大气 CH4 的高亲和力及 CH4 氧化所需较低的活化能导致其温度系数 Q1 0 较小。地温较低时 ,土壤氧化 CH4 的能力随温度升高而升高。当地温高于 CH4 氧化的最佳温度时 ,CH4 氧化菌难以与硝化细菌及其它微生物竞争利用土壤空气中的 O2 ,导致其活性降低。甲烷氧化菌对 p H值变化不敏感。团粒结构较好的壤土可保护 CH4 氧化菌免受干扰。未受干扰的森林土壤 CH4 氧化率的峰值一般出现在亚表     

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.     

Influence of soil temperature on methane emission from rice paddy fields

Methane emission rates from an Italian rice paddy field showed diel and seasonal variations. The seasonal variations were not closely related to soil temperatures. However, the dieL changes of CH₄ fluxes were significantly correlated with the diel changes of the temperature in a particular soil depth. The soil depths with the best correlations between CH4 flux and temperature were shallow (1–5cm) in May and June, deep (10–15cm) in June and July, and again shallow (1–5 cm) in August. Apparent activation energies (E_a) calculated from these correlations using the Arrhenius model were relatively low (50–150 kJ mol^–1) in May and June, but increased to higher values (80–450 kJ mol^–1) in August. In the laboratory, CH₄ emission from two rice cultures incubated at temperatures between 20 and 38°C showed E . values of 41 and 53 kJ mol^–1) Methane production in anoxic paddy soil suspensions incubated between 7 and 43°C showed E values between 53 and 132 kJ mol^–1 with an average value of 85 kJ mol^–1) and in pure cultures of hydrogenotrophic methanogenic bacteria E _a values between 77 and 173 (average 126) kJ mol^–1. It is suggested that diel changes of soil properties other than temperature affect CH₄ emission rates, e.g. diel changes in root exudation or in efficiency of CH⁴ oxidation in the rhizosphere.     

Bacterial Processes of the Methane Cycle in Bottom Sediments of Lake Baikal

The activity of methanogenic and methanotrophic bacteria was evaluated in bottom sediments of Lake Baikal. Methane concentration in Baikal bottom sediments varied from 0.0053 to 81.7 ml/dm³. Bacterial methane was produced at rates of 0.0004–534.7 l CH₄/(dm³ day) and oxidized at rates of 0.005–1180 l CH₄/(dm³ day). Peak methane production and oxidation were observed in Frolikha Bay near a methane vent. Methane was emitted into water at rates of 49.2–4340 l CH₄/(m² day). Rates of bacterial methane oxidation in near-bottom water layers ranged from 0.002 to 1.78 l/(l day). Methanogens and methanotrophs were found to play an important role in the carbon cycle through all layers of sediments, particularly in the areas of methane vent and gas-hydrate occurrence.     

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.     

Processes involved in formation and emission of methane in rice paddies

The seasonal change of the rates of production and emission of methane were determined under in-situ conditions in an Italian rice paddy in 1985 and 1986. The contribution to total emission of CH₄ of plant-mediated transport, ebullition, and diffusion through the flooding water was quantified by cutting the plants and by trapping emerging gas bubbles with funnels. Both production and emission of CH₄ increased during the season and reached a maximum in August. However, the numbers of methanogenic bacteria did not change. As the rice plants grew and the contribution of plant-mediated CH₄ emission increased, the percentage of the produced CH₄ which was reoxidized and thus, was not emitted, also increased. At its maximum, about 300 ml CH₄ were produced per m² per hour. However, only about 6% were emitted and this was by about 96% via plant-mediated transport. Radiotracer experiments showed that CH, was produced from H₂/CO₂. (30–50%) and from acetate. The pool concentration of acetate was in the range of 6–10 mM. The turnover time of acetate was 12–16 h. Part of the acetate pool appeared to be not available for production of CH₄ or CO₂     

Methane production in rice soil is inhibited by cyanobacteria

The present study was aimed at understanding the role of cyanobacteria and Azolla in methane production and oxidation in laboratory simulation experiments using soil samples from rice fields. All the seven cyanobacterial strains tested effected a significant decrease in the headspace concentration of methane in flooded soil, incubated under light. Synechocystis sp. was the most effective in retarding methane concentration by 10-20 fold over that in controls without cyanobacteria. The decrease in the headspace concentration of methane was negligible in nonsterile soil samples, inoculated with Synechocystis sp. and then incubated under dark. Moist soil cores (0-5 cm depth), collected from rice fields that had been treated with urea in combination with a cyanobacterial mixture, Azolla microphylla, or cyanobacterial mixture plus A. microphylla, effected distinctly more rapid decrease in the headspace concentration of methane added at 200 microl(-1) than did the soil cores from plots treated with urea alone (30, 60, 90 and 120 kg N ha(-1)), irrespective of the rate of chemical nitrogen applied to rice fields. Besides, soil cores from plots treated with urea alone at 60, 90 and 120 kg N ha(-1) oxidised methane more rapidly than did the core samples from plots treated with urea alone at 30kg N ha(-1). Cyanobacteria and A. microphylla, applied to flood water, appear to play a major role in mitigation of methane emission from rice fields-through enhanced methane oxidation.     

Methane consumption in two temperate forest soils

Forest soils are thought to be an important sink for atmospheric methane. To evaluate methane consumption,¹⁴C-labeled methane was added to the headspace of intact soil cores collected from a mixed mesophytic forest and from a red spruce forest located in the central Appalachian Mountains. Both soils consumed the added methane at initially high rates that decreased as the methane mixing ratio of the air decreased. The mixed mesophytic forest soil consumed an average of 2 mg CH₄ m^–2 d^–1 versus 1 mg CH, m^–2 d^–1 for the spruce forest soil. The addition of acetylene to the headspace completely suppressed methane consumption by the soils, suggesting that an aerobic methane-consuming microorganism mediated the process. At both forest sites, methane mixing ratios in soil air spaces were greater than that in the air overlying the soil surface, indicating that these soils had the ability to produce methane. Models of methane emission from forest soils to the atmosphere must represent methane flux as the balance between production and consumption of methane, which are controlled by very different factors     

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.     

Estimation of the increase in CH4 emission from paddy soils by rice straw application

The relationship between the amount of CH₄ emission to the atmosphere from submerged paddy soils with rice plants and the application level (0–8 g kg^-1) of rice straw (RS) in soil was investigated in a pot experiment. Amounts of CH₄ emitted from pots with respective RS levels differed between a clayey yellow soil and a silty gray lowland soil. However, the increase in cumulative amounts of CH₄ emission with the increase in the application level of RS was similar in pattern between the two soils, and the increase (Y) was formulated with a logistic curve: x, application level of RS; k, a coefficient for relative CH₄ emission.Since the seasonal variations in coefficients a, b, and c in the logistic equation were also formulated as the function of the sum of effective temperature (E, (T–15); T, daily average temperature), the increase in cumulative amounts of CH₄ emission from any paddy soil by any level of RS application was known to be estimated by the following equation:     

Methane production capacities of different rice soils derived from inherent and exogenous substrates

Methane production rates were determined at weekly intervals during anaerobic incubation of eleven Philippine rice soils. The average production rates at 25 °C varied in a large range from 0.03 to 13.6 g CH_{4 g(d.w. soil)} ^-1d^-1. The development of methane production rates derived from inherent substrate allowed a grouping of soils in three classes: those with instantaneous development, those with a delay of approximately two weeks, and those with a suppression of methane production of more than eight weeks. Incubation at 30 and 35 °C increased production capacities of all soils, but the grouping of soils was still maintained. The Arrhenius equation provided a good fit for temperature effects on methane production capacities except for those soils with suppressed production. Acetate amendment strongly enhanced methane production rates and disintegrated the grouping. However, the efficiencies in converting acetate to methane differed among soils. Depending on the soil, 16.5–66.7% of the added acetate was utilized within five weeks incubation at 25 °C.Correlation analyses of methane production (over eight weeks) and physico-chemical soil parameters yielded significant correlations for the concentrations of organic carbon (R² = 0.42) and organic nitrogen (R² = 0.52). Correlation indices could substantially be enhanced by using the enriched fraction of organic carbon (R² = 0.94) and organic nitrogen (R² = 0.77), i.e. the differential between topsoil and subsoil concentrations of the respective compounds. The enriched organic material in the topsoil corresponds to the biologically active fraction and thus represents a good indicator of methane production derived from inherent substrate. The best indicators of the conversion rate of acetate in different soils were pH-value (R² = 0.56) and organic carbon content (R² = 0.52).Apparently, soil properties affect methane production through various pathways. Inherent organic substrate represents a considerable source of methane in some soils and is negligible in others. Likewise, soils also differ regarding the response to exogenous substrate. Both mechanisms yield in a distinct spatial variability of methane production in rice soils.     

The processes of methane production and oxidation in the soils of the Russian Arctic tundra

Methane emission from the following types of tundra soils was studied: coarse humic gleyey loamy cryo soil, peaty gleyey soil, and peaty gleyey midloamy cryo soil of the arctic tundra. All the soils studied were found to be potential sources of atmospheric methane. The highest values of methane emission were recorded in August at a soil temperature of 8–10°C. Flooded parcels were the sources of atmospheric methane throughout the observation period. The rates of methane production and oxidation in tundra soils of various types were studied by the radioisotope method at 5 and 15°C. Methane oxidation was found to occur in bog water, in the green part of peat moss, and in all the soil horizons studied. Methane production was recorded in the horizons of peat, in clay with plant roots, and in peaty moss dust of the bogey parcels. At both temperatures, the methane oxidation rate exceeded the rate of methane production in all the horizons of the mossy-lichen tundra and of the hillock tundra with flat-bottom depressions. Methanogenesis prevailed only in a sedge-peat moss bog at 15°C. Bacterial enrichment cultures oxidizing methane at 5 and 15°C were obtained. Different types of methanotrophic bacteria were shown to be responsible for methane oxidation under these conditions. A representative of type I methylotrophs oxidized methane at 5°C, and Methylocella tundrae, a psychroactive representative of an acidophilic methanotrophic genus Methylocella, at 15°C.__________Translated from Mikrobiologiya, Vol. 74, No. 2, 2005, pp. 261–270.Original Russian Text Copyright © 2005 by Berestovskaya, Rusanov, Vasileva, Pimenov.     

植物释放甲烷研究进展

植物是否在有氧条件下自身产生甲烷、其产生机制和释放速率等问题目前还存在很大争议,如果确证植物在有氧条件下产生较大量的甲烷,就必须重新认识和计算全球甲烷的源汇及其收支平衡。已有研究表明,植物排放的甲烷有一部分是由土壤或木本植物的根和树干内部产甲烷微生物产生,再通过植物传输进入大气中的;植物本身产生甲烷的机制可能主要是在活性氧自由基的作用下,将植物细胞壁成分果胶、木质素等中的甲氧基转化为甲烷,这一过程受到高温、强光和UV辐射等环境胁迫的刺激。根据植物排放速率或大气甲烷浓度与碳同位素组成的实测值,对区域和全球植物源甲烷排放率做出的估算还存在相当大的不确定性,需要对更多植物和更多地点开展实测研究,深入了解植物产甲烷的机制和过程,并结合大气传输模型才能进一步提高估算准确性。     

Methanotrophic Activity,Abundance, and Diversity in Forested Swamp Pools: Spatiotemporal Dynamics and Influences on Methane Fluxes

Methane oxidation (methanotrophy) in the water column and sediments of forested swamp pools likely control seasonal and annual emission of CH₄ from these systems, but the methanotrophic microbial communities, their activities, locations, and overall impact, is poorly understood. Several techniques including ¹⁴CH₄ oxidation assays, culture-based most probable number (MPN) estimates of methane-oxidizing bacteria (MOB) and protozoan abundance, MOB strain isolation and characterization, and PCR techniques were used to investigate methanotrophy at a forested swamp near Ithaca, New York. The greatest methanotrophic activity and largest numbers of MOB occurred predominantly at the low oxygen sediment/water interface in the water column. Seasonally, methanotrophic activity was very dynamic, ranging from 0.1 to 61.9 μ moles CH₄ d^?1 g^?1 dry sediment, and correlated most strongly with dissolved inorganic carbon (r = 0.896). Incorporation of methanotrophic variables (abundance and activity) into existing CH₄ flux regression models improved model fit, particularly during mid summer when CH₄ fluxes were most dynamic. Annually integrated methane flux and methanotrophic activity measurements indicate that differences in methanotrophic activity at the sediment/water interface likely accounted for differences in the annual CH₄ emission from the field site. Direct isolations of MOB resulted in the repeated isolation of organisms most closely related to Methylomonas methanica S1. A single acidophilic, type II MOB related to Methylocella palustris K was also isolated. Using a PCR-based MPN method and 16S rRNA genome copy number from isolates and control strains, type I and type II MOB were enumerated and revealed type I dominance of the sediment-associated MOB community.