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.     

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     

盐分对湿地甲烷排放影响的研究进展

当前在全球气候变化和人类活动双重作用下,湿地正在或者将要面临着显著的盐分变化形势,尤其是内陆和滨海咸化湿地。湿地是大气甲烷的重要排放源。甲烷排放是甲烷产生、氧化和传输过程综合作用的结果。盐分变化将影响湿地水-土环境,降低植物群落初级生产力和有机物积累速率,改变微生物主导的有机物矿化速率和途径等,进而改变湿地生态系统的结构和功能,影响湿地甲烷产生、氧化、传输和排放系列过程。本文综述了盐分(浓度与组成)对湿地甲烷产生与排放的影响结果,从底物供给、微生物(产甲烷菌和甲烷氧化菌等)数量、活性与群落组成、酶活性、植物、电子受体、p H和氧化还原电位等几个关键方面分析了盐分影响湿地甲烷排放过程的内在机制。在此基础上提出了今后需重点关注的5个方面:1)加强盐分浓度与组成对湿地甲烷产生、氧化、传输与排放影响的系统性、框架性研究;2)深入探讨盐分背景、变化幅度与速率的耦合如何影响湿地甲烷系列过程;3)不同离子组成及其交互效应如何影响湿地甲烷动态过程;4)结合生物学、基因组学及同位素技术等,加强湿地产甲烷菌与甲烷氧化菌与盐分的关系及其响应研究;5)湿地甲烷对盐分变化响应的时空分异规律。     

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.     

The geochemistry of methane in Lake Fryxell,an amictic,permanently ice-covered,antarctic lake

The abundance and distribution of dissolved CH₄ were determined from 1987–1990 in Lake Fryxell, Antarctica, an amictic, permanently ice-covered lake in which solute movement is controlled by diffusion. CH₄ concentrations were < 1 υM in the upper oxic waters, but increased below the oxycline to 936 μM at 18 m. Sediment CH₄ was 1100 μmol (1 sed)⁻¹ in the 0–5 cm zone. Upward flux from the sediment was the source of the CH₄, NH₄ ⁺, and DOC in the water column; CH₄ was 27% of the DOC+CH₄ carbon at 18 m. Incubations with surficial sediments indicated that H¹⁴CO₃ ⁻ reduction was 0.4 μmol (1 sed)⁻¹ day⁻¹ or 4× the rate of acetate fermentation to CH₄. There was no measurable CH₄ production in the water column. However, depth profiles of CH₄, NH₄, and DIC normalized to bottom water concentrations demonstrated that a significant CH₄ sink was evident in the anoxic, sulfate-containing zone of the water column (10–18 m). The δ¹³CH₄ in this zone decreased from −72 % at 18 m to −76% at 12 m, indicating that the consumption mechanism did not result in an isotopic enrichment of ¹³CH₄. In contrast, δ¹³CH₄ increased to −55 % at 9 m due to aerobic oxidation, though this was a minor aspect of the CH₄ cycle. The water column CH₄ profile was modeled by coupling diffusive flux with a first order consumption term; the best-fit rate constant for anaerobic CH₄ consumption was 0.012 yr⁻¹. On a total carbon basis, CH₄ consumption in the anoxic water column exerted a major effect on the flux of carbonaceous material from the underlying sediments and serves to exemplify the importance of CH₄ to carbon cycling in Lake Fryxell.     

甲烷单加氧酶的研究进展

甲烷氧化细菌在转化甲烷制造新型燃料、单细胞蛋白和新功能酶生产、污水处理等方面有着潜在的应用前景,因此,甲烷单加氧酶作为其代谢过程中重要的酶系也受到人们的广泛关注。我们简要综述了近年来对甲烷单加氧酶的性质、结构、催化机理等方面的研究,特别是对颗粒性甲皖单加氧酶的相关性质进行了详细的阐述。     

Oxidation of methane in boreal forest soils: a comparison of seven measures

Methane oxidation rates were measured in boreal forest soils using seven techniques that provide a range of information on soil CH₄ oxidation. These include: (a) short-term static chamber experiments with a free-air (1.7 ppm CH₄) headspace, (b) estimating CH₄ oxidation rates from soil CH₄ distributions and (c)²²²Rn-calibrated flux measurements, (d) day-long static chamber experiments with free-air and amended (+20 to 2000 PPM CH₄) headspaces, (e) jar experiments on soil core sections using free-air and (f) amended (+500 ppm CH₄) headspaces, and (g) jar experiments on core sections involving tracer additions of¹⁴CH₄. Short-term unamended chamber measurements,²²²Rn-calibrated flux measurements, and soil CH₄ distributions show independently that the soils are capable of oxidizing atmospheric CH₄ at rates ranging to < 2 mg m^–2 d^–1. Jar experiments with free-air headspaces and soil CH₄ profiles show that CH₄ oxidation occurs to a soil depth of 60 cm and is maximum in the 10 to 20 cm zone. Jar experiments and chamber measurements with free-air headspaces show that CH₄ oxidation occurs at low (< 0.9 ppm) thresholds. The¹⁴CH₄-amended jar experiments show the distribution of end products of CH₄ oxidation; 60% is transformed to CO₂ and the remainder is incorporated in biomass. Chamber and jar experiments under amended atmospheres show that these soils have a high capacity for CH₄ oxidation and indicate potential CH₄ oxidation rates as high as 867 mg m^–2 d^–1. Methane oxidation in moist soils modulates CH₄ emission and can serve as a negative feedback on atmospheric CH₄ increases.     

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.     

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.     

Dimethylsulfide and methane thiol in sediment porewater of a Danish estuary

Seasonal variation of dimethylsulfide (DMS) and methane thiol (MSH) concentrations in sediment porewater was determined in a Danish estuary. Dimethylsulfide (DMDS) was never found. Detectable DMS levels of up to 0.1 M were found only in the summer and only within the upper 5 cm of the sediment. The DMS accumulation was probably associated with decomposing fragments of macro-algae in the surface layer. Significant MSH accumulation of up to 1 M was found only in the deep, CH₄-rich sediment below the SO₄ ^2- zone. With depth, a detectable MSH level could thus be observed below the 1 mM SO₄ ^2--isopleth which also marked the SO₄ ^2--CH₄ transition. The transition zone was located deeper in the sediment in winter (20–25 cm depth) than in summer (5–10 cm depth). The absence of MSH in the SO₄ ^2- zone could be due to rapid utilization of the compound by SO₄ ^2--reducing bacteria. A possible involvement of MSH in anaerobic CH₄ oxidation at the transition zone is discussed; CH₄ and sulfide (HS^- form, pH 7) are proposed to form MSH and H₂ which in turn may be metabolized by, e.g. SO₄ ^2--reducing bacteria.     

Methane production and methane consumption: a review of processes underlying wetland methane fluxes

Potential rates of both methane production and methane consumptionvary over three orders of magnitude and their distribution is skew.These rates are weakly correlated with ecosystem type, incubationtemperature, in situ aeration, latitude, depth and distanceto oxic/anoxic interface. Anaerobic carbon mineralisation is amajor control of methane production. The large range in anaerobicCH₄:CO₂ production rates indicate that a largepart of the anaerobically mineralised carbon is used for reduction ofelectron acceptors, and, hence, is not available for methanogenesis.Consequently, cycling of electron acceptors needs to be studied tounderstand methane production. Methane and oxygen half saturationconstants for methane oxidation vary about one order of magnitude.Potential methane oxidation seems to be correlated withmethanotrophic biomass. Therefore, variation in potential methaneoxidation could be related to site characteristics with a model ofmethanotrophic biomass.     

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.     

Intensity of the Microbiological Processes of the Methane Cycle in Different Types of Baltic Lakes

The intensity of the microbiological processes of methane formation (MF) and methane oxidation (MO) was determined in the sediments and water of different types of Baltic lakes. The emission of methane from the lake sediments and methane distribution in the water column of the lakes were studied as functions of the lake productivity and hydrologic conditions. During summers, the intensity of MF in the lake sediments and waters varied from 0.001 to 106 ml CH₄/(dm³ day) and from 0 to 3.2 ml CH₄/(l day), respectively, and the intensity of MO in the sediments and water varied from 0 to 11.2 ml CH₄/(dm³ day) and from 0 to 1.1 ml CH₄/(l day), respectively. The total methane production (MP) in the lakes varied from 15 to 5000 ml CH₄/(m² day). In anoxic waters, the MP comprised 9–18% of the total PM in the lakes. The consumption of organic carbon for methanogenesis varied from 0.03 to 9.7 g/(m² day). The role of the methane cycle in the degradation of organic matter in the lakes increased with their productivity.     

好氧甲烷氧化菌生理生态特征及其在自然湿地中的群落多样性研究进展

甲烷氧化菌是一类可以利用甲烷作为唯一碳源和能源的细菌,在全球变化和整个生态系统碳循环过程中起着重要的作用。近年来,对甲烷氧化菌的生理生态特征及其在自然湿地中的群落多样性研究取得了较大进展。在分类方面,疣微菌门、NC10门及两个丝状菌属甲烷氧化菌的发现使其分类体系得到了进一步的完善;在单加氧酶方面,发现甲烷氧化菌可以利用pM MO和sM MO两种酶进行氧化甲烷的第一步反应,Ⅱ型甲烷氧化菌中pM MO2的发现证实甲烷氧化菌可以利用这种酶氧化低浓度的甲烷;在底物利用方面,已经发现了越来越多的兼性营养型甲烷氧化菌,证实它们可以利用的底物比之前认为的更广泛,其中包括乙酸等含有碳碳键的化合物;在生存环境方面,能在不同温度、酸度和盐度的环境中生存的甲烷氧化菌不断被分离出来。全球自然湿地甲烷氧化菌群落多样性的研究目前主要集中在北半球高纬度的酸性泥炭湿地,Ⅱ型甲烷氧化菌Methylocystis、Methylocella和Methylocapsa是这类湿地主要的甲烷氧化菌类群,尤其以Methylocystis类群最为广泛,而Ⅰ型甲烷氧化菌尤其是Methylobacter在北极寒冷湿地中占优势。随着高通量测序时代的到来和新的分离技术的发展,对甲烷氧化菌的现有认识将面临更多的挑战和发展。     

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     

Comparison of Methanotrophic Community and Methane Oxidation between Rhizospheric and Non-Rhizospheric Soils

Using particulate methane monooxygenase (pMMO) encoding gene, pmoA-based terminal-restrict fragment length polymorphism (T-RFLP), the methanotrophic communities between rhizospheric soils (RSs) and non-rhizospheric soil (NRSs) of landfill cover (LC), riparian wetland (RW) and rice paddy (RP) were compared before and after pre-incubation of 90 days. The ultimate potential of methane oxidation rate (UPMOR) and gene copy number of pmoA were evaluated in the soil samples after pre-incubation. Compared to the methanotrophic community in the soil samples before pre-incubation, type II methanotrophs, the Methylocystis-Methylosinus group, was mostly increased after pre-incubation, regardless of the soil type. The UPMOR (11.82 ± 0.27 μmol-CH₄· g^?1 _soil-DW·h^?1) in the LC-RS was significantly higher than that (9.57 ± 0.14 μmol-CH₄· g^?1 _soil-DW·h^?1) in the LC-NRS. However, no significant difference was found between RSs and NRSs in the RW (15.28 ± 0.91 and 13.23 ± 0.69 μmol-CH₄· g^?1 _soil-DW·h^?1, respectively) and RP (13.81 ± 1.04 and 12.81 ± 2.40 μmol-CH₄· g^?1 _soil-DW·h^?1, respectively) soils. There was no significantly difference in the gene copy numbers of pmoA in the RSs compared with those in the NRSs at all of the sampling sites. This study provides basic metagenomic information about both rhizospheric and non-rhizospheric methanotrophs, which will be helpful in developing a better strategy of biological methane removal from both natural and anthropogenic major methane sources.     

生物分子机器——甲烷单加氧酶的研究进展

甲烷氧化菌中的甲烷单加氧酶能够在生理条件下选择性地以甲烷和氧气为底物生成甲醇,麻省理工学院的Lippard教授称它为"神奇的生物分子机器"。本文重点对生物分子机器甲烷单加氧酶的结构、编码基因及调控机制、催化反应机理等进行了综述,此外也简要介绍了甲烷单加氧酶的产生菌甲烷氧化菌的研究历史及分类。生物分子机器甲烷单加氧酶可催化甲烷氧化成甲醇,不仅为甲醇的生产提供了一种新颖的生产方法,而且对生物分子机器的设计也有借鉴意义。     

Methane emission from Arctic tundra

Concerns about a possible feedback effect on global warming following possible increased emissions of methane from tundra environments have lead to series of methane flux studies of northern wetland/tundra environments. Most of these studies have been carried out in boreal sub-Arctic regions using different techniques and means of assessing representativeness of the tundra. Here are reported a time series of CH₄ flux measurements from a true Arctic tundra site. A total of 528 independent observations were made at 22 fixed sites during the summers of 1991 and 1992. The data are fully comparable to the most extensive dataset yet produced on methane emissions from sub-Arctic tundra-like environments. Based on the data presented, from a thaw-season with approximately 55% of normal precipitation, a global tundra CH₄ source of 18–30 Tg CH₄ yr⁻¹ is estimated. This is within the range of 42±26 Tg CH₄ yr⁻¹ found in a similar sub-Arctic tundra environment. No single-parameter relationship between one environmental factor and CH₄ flux covering all sites was found. This is also in line with conclusions drawn in the sub-Arctic. However, inter-season variations in CH₄ flux at dry sites were largely controlled by the position of the water table, while flux from wetter sites seemed mainly to be controlled by soil temperature.     

Release of fossil methane from mineral soil particles, and its implication for estimation of methane oxidation in a mineral subsoil

In a preliminary experiment we found that methane evolved from a sandy subsoil during aerobic incubation of shaken soil slurries. In the study presented here the methane was found to be released from the sand particles by mechanical weathering, caused by the grinding effect of the shaking. Large amounts of gas (about 0.5 ml gas g^–1 soil) were extracted by intense grinding of the soil in gas tight serum vials. Methane was the main hydrocarbon in the emitted gas, but also a considerable amount of ethane was present, as well as minor amounts of heavier hydrocarbons (up to C₆). The ¹³C-values of the emitted methane and ethane were –33 and –29 , respectively. Together these results demonstrate a thermogenic origin of the gas. This paper also reports the results of an incubation experiment where possible methane oxidation was looked for. If a possible release of methane is not accounted for, methane oxidation may be overlooked, as illustrated in this paper. Methane consumption was detected only in soil from 40 cm, in contrast to soil sampled at 100 cm and deeper where a slight production was measured. When methane oxidation was inhibited by dimethyl-ether, a significant release of methane was seen. The release was probably caused by chemical weathering. When this methane release was taken into account, methane oxidation was found to be present at all measured depths (40 to 200 cm). Fertilization with urea inhibited the methane oxidation at 40 cm but not at deeper layers. It is hypothesized that ammonia oxidizing bacteria were the main methane oxidizers in this mineral subsoil (deeper than 1 m), and that oxidation of methane might be a survival mechanism for ammonia oxidizers in ammonia limited environments.