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.     

Enhanced CH4 emission from a wetland soil exposed to Elevated CO2

Methane emissions from wetland soils are generally a positive function ofplant size and primary productivity, and may be expected to increase dueto enhanced rates of plant growth in a future atmosphere of elevatedCO₂. We performed two experiments with Orontium aquaticum, acommon emergent aquatic macrophyte in temperate and sub-tropical wetlands, todetermine if enhanced rates of photosynthesis in elevated CO₂atmospheres would increase CH₄ emissions from wetland soils.O. aquaticum was grown from seed in soil cores under ambient and elevated(ca. 2-times ambient) concentrations of CO₂ in an initialglasshouse study lasting 3 months and then a growth chamber study lasting 6months. Photosynthetic rates were 54 to 71% higher underelevated CO₂ than ambient CO₂, but plantbiomass was not significantly different at the end of the experiment. Ineach case, CH₄ emissions were higher under elevated thanambient CO₂ levels after 2 to 4 months of treatment, suggestinga close coupling between photosynthesis and methanogenesis in our plant-soilsystem. Methane emissions in the growth chamber study increased by 136%. We observed a significant decrease in transpirationrates under elevated CO₂ in the growth chamber study, andspeculate that elevated CO₂ may also stimulate CH₄ emissions by increasing the extent and duration offlooding in some wetland ecosystems. Elevated CO₂ maydramatically increase CH₄ emissions from wetlands, a sourcethat currently accounts for 40% of global emissions.     

CH4 production from H2 and CO2 byMethanobacterium thermoautotrophicum cells fixed on hollow fibers

Summary Methane was produced from H₂ and CO₂ byMethanobacterium thermoautotrophicum cells fixed on the surface of hollow fibers. The mineral solution permeated through the inside of fibers was consumed by the cells, while the gaseous substrate flowing outside the fibers was directly metabolized to methane. Methane production was proportional to hollow fiber length i.e., contact area between cell layer and gas phase. In repeated batch cultures, the production rates of methane and cell mass were 33.1 L/L reactor/day and 1.75 g cells/L reactor/day, respectively with 90% conversion rate.     

Seasonal variation in CH4 emissions and production and oxidation potentials at microsites on an oligotrophic pine fen

 Temporal and spatial variation in CH₄ emissions was studied at hummock, Eriophorum lawn, flark and Carex lawn microsites in an oligotrophic pine fen over the growing season using a static chamber method, and CH₄ production and oxidation potentials in peat profiles from hummock and flark were determined in laboratory incubation experiments. Emissions were lowest in the hummocks, and decreased with increasing hummock height, while in the lawns and flarks they increased with increasing sedge cover. Statistical response functions with water table and peat temperature as independent variables were calculated in order to reconstruct seasonal CH₄ emissions by reference to the time series for peat temperature and water table specific to each microsite type. Mean CH₄ emissions in the whole area in the snow-free period of 1993, weighted in terms of the proportions of the microsites, were 1.7 mol CH₄ m^–2. Potential CH₄ production and oxidation rates were very low in the hummocks rising above the groundwater table, but were relatively similar when expressed per dry weight of peat both in the hummocks and flarks below the water table. The CH₄ production potential increased in autumn at both microsites and CH₄ oxidation potential seemed to decrease. The decrease in temperature in autumn certainly reduced in situ decomposition processes, possibly leaving unused substrates in the peat, which would explain the increase in CH₄ production potential. Received: 16 August 1996 / Accepted: 30 November 1996     

灌水量对温室番茄土壤CO2、N2O和CH4排放的影响

为揭示不同灌水量对温室番茄土壤CO₂、N₂O和CH₄排放及作物产量的影响,提出有效的减排措施,试验设置充分灌溉(1.0W,W_1.0;W为充分供水的灌水量)、亏缺20%灌溉(0.8W,W_0.8)和亏缺40%灌溉(0.6W,W_0.6)3个灌水水平,采用静态暗箱/气相色谱法于2017年4—12月对两茬温室番茄土壤CO₂、N₂O和CH₄进行全生长季监测,分析土壤CO₂、N₂O和CH₄排放对不同灌水量的响应.结果表明: 番茄两个生长季中,土壤CO₂、N₂O和CH₄排放量均随着灌水量增加呈现逐渐增加的趋势(W_1.0＞W_0.8＞W_0.6),除W_0.6和W_1.0处理间土壤N₂O排放具有显著差异外,其他各处理间气体排放差异均不显著.与W_1.0处理相比,W_0.6和W_0.8处理土壤CO₂排放分别减小了12.2%和8.3%,N₂O分别减小了19.1%和8.0%,CH₄分别减小了11.0%和6.2%.番茄产量和由土壤N₂O和CH₄引起的全球增温潜势(GWP)均随灌水量增加而增加;与W_1.0处理相比,W_0.6处理产量和GWP显著减小,降幅分别为17.0%和22.9%,而W_0.8处理对两者未产生显著影响.单位产量GWP随灌水量增加表现为先增加后降低的趋势(W_0.8＞W_1.0＞W_0.6),处理间差异不显著.灌溉水利用效率(IWUE)随灌水量增加而降低,与W_1.0处理相比,W_0.6和W_0.8处理IWUE分别增加了38.3%和9.4%.回归分析表明,土壤CO₂排放通量与土壤水分呈指数负相关关系;土壤CH₄通量与土壤水分呈线性正相关关系;当土壤温度小于18 ℃和大于18 ℃时,土壤N₂O排放通量与土壤温度间均呈指数负相关关系.灌水增加了番茄产量和温室气体排放,但降低了IWUE.综合考虑番茄产量、IWUE和温室效应,推荐W_0.8处理为较佳的灌溉模式.     

开放式空气CO2增高对稻田CH4和N2O排放的影响

在FACE(free aircarbondioxideenrichment)平台上 ,采用静态暗箱 气相色谱法观测研究了大气CO2 浓度增加对稻田CH4和N2 O排放的影响 .结果表明 ,在 15 0和 2 5 0kgN·hm-2 两种氮肥水平下大气CO2 浓度增加 2 0 0 μmol·mol-1均明显促进水稻生长 ,水稻生物量积累 .大气CO2 浓度增加对 15 0和 2 5 0kgN·hm-2 两种氮肥水平下稻田CH4排放均无显著影响 ,并简要分析了与现有文献报道结果不一致的原因 .大气CO2 浓度增加也未导致 15 0和 2 5 0kgN·hm-2 两种氮肥水平下稻田N2 O排放的明显变化 ,与大多数研究结果一致 .     

Essential prerequisites for successful bioprocess development of biological CH4 production from CO2 and H2

The production and storage of energy from renewable resources steadily increases in importance. One opportunity is to utilize carbon dioxide (CO₂)-type hydrogenotrophic methanogens, which are an intriguing group of microorganisms from the domain Archaea, for conversion of hydrogen and CO₂ to methane (CH₄). This review summarizes the current state of the art of bioprocess development for biological CH₄ production (BMP) from pure cultures with pure gasses. The prerequisites for successful quantification of BMP by using closed batch, as well as fed-batch and chemostat culture cultivation, are presented. This review shows that BMP is currently a much underexplored field of bioprocess development, which mainly focuses on the application of continuously stirred tank reactors. However, some promising alternatives, such as membrane reactors have already been adapted for BMP. Moreover, industrial-based scale-up of BMP to pilot scale and larger has not been conducted. Most crucial parameters have been found to be those, which influence gas-limitation fundamentals, or parameters that contribute to the complex effects that arise during medium development for scale-up of BMP bioprocesses, highly stressing the importance of holistic BMP quantification by the application of well-defined physiological parameters. The much underexplored number of different genera, which is mainly limited to Methanothermobacter spp., offers the possibility of additional scientific and bioprocess development endeavors for the investigation of BMP. This indicates the large potential for future bioprocess development considering the possible application of bioprocessing technological aspects for renewable energy storage and power generation.     

Effects of extreme experimental drought and rewetting on CO2 and CH4 exchange in mesocosms of 14 European peatlands with different nitrogen and sulfur deposition

The quantitative impact of intense drought and rewetting on gas exchange in ombrotrophic bogs is still uncertain. In particular, we lack studies investigating multitudes of sites with different soil properties and nitrogen (N) and sulfur (S) deposition under consistent environmental conditions. We explored the timing and magnitude of change in CO₂ (Respiration, Gross Primary Production – GPP, and Net Exchange – NE) and CH₄ fluxes during an initial wet, a prolonged dry (~100 days), and a subsequent wet period (~230 days) at 12 °C in 14 Sphagnum peat mesocosms collected in hollows from bogs in the UK, Ireland, Poland, and Slovakia. The relationship of N and S deposition with GPP, respiration, and CH₄ exchange was investigated. Nitrogen deposition increased CO₂ fluxes and GPP more than respiration, at least up to about 15 kg N ha^?1 yr^?1. All mesocosms became CO₂ sources during drying and most of them when the entire annual period was considered. Response of GPP to drying was faster than that of respiration and contributed more to the change in NE; the effect was persistent and few sites recovered “predry” GPP by the end of the wet phase. Respiration was higher during the dry phase, but did not keep increasing as WT kept falling and peaked within the initial 33 days of drying; the change was larger when differences in humification with depth were small. CH₄ fluxes strongly peaked during early drought and water table decline. After rewetting, methanogenesis recovered faster in dense peats, but CH₄ fluxes remained low for several months, especially in peats with higher inorganic reduced sulfur content, where sulfate was generated and methanogenesis remained suppressed. Based on a range of European sites, the results support the idea that N and S deposition and intense drought can substantially affect greenhouse gas exchange on the annual scale.     

The combined effects of warming and drying suppress CO2 and N2O emission rates in an alpine meadow of the eastern Tibetan Plateau

The eastern Tibetan Plateau has become increasingly warmer and drier since the 1990s. Such warming and drying has a great impact on ecosystem processes on the eastern Tibetan Plateau. To determine their combined effects on CO₂ and N₂O emission rates, we conducted a field manipulative experiment in an alpine meadow of the eastern Tibetan Plateau during the growing season of 2009. The experiment showed that warming manipulation increased soil temperature by 1?°C, and drying manipulation decreased soil water content by 6.8?%. We found that by counteracting the effect of low temperature in the area, experimental warming significantly increased soil microbial biomass, the number of bacteria, fungi, actinomycetes, ammonifying bacteria, nitrobacteria and denitrifying bacteria, and facilitated the emission rates of CO₂ and N₂O by 33.4 and 31.5?%, respectively. However, decreased precipitation further aggravated soil water stress and inhibited the numbers of these organisms, and reduced the emission rates of CO₂ and N₂O by 47.4 and 37.9?%, respectively. So decreased soil water content tended to offset the positive effect of warming. Compared to the positive effects of warming, decreased soil water content was shown in our study to have even greater impact on the eastern Tibetan Plateau during the growing season. Therefore, inhibition of CO₂ and N₂O emission rates (32.3 and 29.3?%, respectively) by warming and drying will intensify if the combined effects of these climatic trends persist in the region.     

Effects of increased CO2 and N on CH4 efflux from a boreal mire: a growth chamber experiment

Increases in the supply of atmospheric CO₂ and N are expected to alter the carbon cycle, including CH₄ emissions, in boreal peatlands. These effects were studied in a glasshouse experiment with peat monoliths cored from an oligotrophic pine fen. The cores with living plants were kept in 720 ppm_v and 360 ppm_v CO₂ atmospheres for about 6 months under imitated natural temperature cycle. Fertilisation with NH₄NO₃ (3 g m⁻² for 25 weeks) was applied to 18 of the 36 monoliths. The rate of CH₄ flux was non-linearly dependent on the number of Eriophorum vaginatum shoots growing in the monoliths, probably due to the gas transport properties of the aerenchyma. The average CH₄ efflux rate, standardised by the number of shoots, was increased by a maximum of 10–20% in response to the raised CO₂ level. In the raised-NH₄NO₃ treatment, the increase in CH₄ release was lower. The effect of combined CO₂+NH₄NO₃ on CH₄ release was negligible and even lower than in the single treatments. Both potential CH₄ production and oxidation rates at 5, 15 and 25°C were higher near the surface than at the bottom of the core. As expected, the rates clearly depended on the incubation temperature, but the different treatments did not cause any consistent differences in either CH₄ production or oxidation. The determination of potential CH₄ production and oxidation in the laboratory is evidently too crude a method of differentiating substrate-induced differences in CH₄ production and oxidation in vivo. These results indicate that an increase in atmospheric CO₂ or N supply alone, at least in the short term, slightly enhances CH₄ effluxes from boreal peatlands; but together their effect may even be restrictive. Received: 18 June 1998 / Accepted: 25 January 1999     

加气灌溉对不同施氮水平的设施甜瓜土壤CO2和N2O排放的影响

为探讨不同加气灌溉施氮模式下设施甜瓜土壤CO₂和N₂O排放的动态变化规律及其与土壤温度、湿度的关系,本研究采用密闭静态箱-气相色谱法对加气灌溉不同施氮水平下土壤CO₂和N₂O排放进行监测,并分析了加气灌溉对不同施氮量下土壤CO₂和N₂O排放的影响.试验采用加气灌溉(AI)和不加气灌溉(CK)两种灌溉方式,施氮量设不施氮(N₁)、传统施氮量的2/3(150 kg·hm^-2,N₂)和传统施氮量(225 kg·hm^-2,N₃)3个施氮水平.结果表明:加气灌溉土壤CO₂和N₂O排放量高于不加气灌溉处理,但是差异不显著;相同灌溉模式下,CO₂和N₂O排放量随施氮量的增加而显著增加,施氮量是土壤CO₂和N₂O排放的主要影响因素.加气灌溉条件下,不同施氮处理N₂O排放通量与土壤温度和湿度呈显著正相关,CO₂排放通量与土壤温度呈显著正相关.加气减氮处理在氮肥减少1/3的情况下,甜瓜产量提高了6.9%,温室气体排放引起的增温潜势值从9544.82 kg·hm^-2下降到9340.72 kg·hm^-2.综上,通过加气灌溉减少氮肥施用量来抑制农业生产系统中温室气体排放是可行的.     

Kinetics of CH4 production from H2 and CO2 in a hollow fiber reactor by plug flow reaction model

For microbial production of CH₄ from H₂ and CO₂, a hollow fiber reactor had been developed to increase an interfacial area between liquid and gas phases. The CH₄ production with the hollow fiber reactor was analyzed by applying a plug flow reaction model of a tubular reactor. It was possible to apply the model to the reaction of CH₄ production. The relationships between influent gas velocity, length of reactor and reaction yield were simulated by the reaction model. The plug flow reaction model was useful to design a hollow fiber bioreactor for the biomethanation of H₂ and CO₂.     

Responses of CO2, CH4 and N2O fluxes to livestock exclosure in an alpine steppe on the Tibetan Plateau, China

Background and aims

The photochemical reflectance index (PRI) is correlated to photosynthetic efficiency and has been successfully applied at multiple scales for remote estimation of physiological functioning. However, interpretation of the PRI signal can be confounded by many different variables including declines in photochemical pigments. Our study was aimed at investigating PRI in response to salinity stress, and evaluating physiological and pigment responses of two co-occurring shrubs, Baccharis halimifolia and Myrica cerifera in laboratory studies.

Methods

Photosynthesis, water relations, chlorophyll fluorescence, hyperspectral reflectance and leaf pigment contents were measured following salinity treatment.

Results

Physiological measurements indicated that both species exhibit adaptations which protect PSII during periods of stress. Chlorophyll fluorescence parameters were affected in both species, but indicated that other photochemical reactions (e.g. photorespiration) were important for energy dissipation in absence of chlorophyll changes. After many days of reduced photosynthesis, photochemical changes were detectable using PRI indicating chronic stress.

Conclusions

Variations in PRI were not related to changes in pigments but strongly related to tissue chlorides indicating salinity effects on the PRI signal. Thus, PRI is an indicator of salinity stress in these coastal species and may be as an early signal for increasing salt exposure associated with rising sea-level and climate change.     

A pan‐Arctic synthesis of CH4 and CO2 production from anoxic soil incubations

Permafrost thaw can alter the soil environment through changes in soil moisture, frequently resulting in soil saturation, a shift to anaerobic decomposition, and changes in the plant community. These changes, along with thawing of previously frozen organic material, can alter the form and magnitude of greenhouse gas production from permafrost ecosystems. We synthesized existing methane (CH₄) and carbon dioxide (CO₂) production measurements from anaerobic incubations of boreal and tundra soils from the geographic permafrost region to evaluate large‐scale controls of anaerobic CO₂ and CH₄ production and compare the relative importance of landscape‐level factors (e.g., vegetation type and landscape position), soil properties (e.g., pH, depth, and soil type), and soil environmental conditions (e.g., temperature and relative water table position). We found fivefold higher maximum CH₄ production per gram soil carbon from organic soils than mineral soils. Maximum CH₄ production from soils in the active layer (ground that thaws and refreezes annually) was nearly four times that of permafrost per gram soil carbon, and CH₄ production per gram soil carbon was two times greater from sites without permafrost than sites with permafrost. Maximum CH₄ and median anaerobic CO₂ production decreased with depth, while CO₂:CH₄ production increased with depth. Maximum CH₄ production was highest in soils with herbaceous vegetation and soils that were either consistently or periodically inundated. This synthesis identifies the need to consider biome, landscape position, and vascular/moss vegetation types when modeling CH₄ production in permafrost ecosystems and suggests the need for longer‐term anaerobic incubations to fully capture CH₄ dynamics. Our results demonstrate that as climate warms in arctic and boreal regions, rates of anaerobic CO₂ and CH₄ production will increase, not only as a result of increased temperature, but also from shifts in vegetation and increased ground saturation that will accompany permafrost thaw.     

温度对亚热带地区常见树种叶片甲烷排放的影响

以亚热带常见树种米槠、木荷、浙江桂、罗浮栲、杉木和柑橘为对象,利用控制试验研究了温度对树木叶片甲烷(CH4)排放的影响.结果表明:当温度在10℃时,供试的6种树木中,仅木荷、柑橘和罗浮栲的叶片排放CH4;温度高于20℃时,所有树木叶片均可排放CH4.温度高于30℃时,叶片排放CH4的平均排放速率(1.010ngCH4·g-1DM·h-1)是10～30℃时平均排放速率(0.255ngCH4·g-1DM·h-1)的3.96倍.增温对柑橘和杉木CH4排放速率的影响显著高于其他4种树木.培养时间对叶片排放CH4速率有显著影响,温度胁迫对树木排放CH4的影响受植物活性的控制.在低温或高温条件下,树木干叶均不能排放CH4.高温胁迫对树木叶片排放CH4有重要影响,全球变暖可能增加植物的CH4排放.     

中国地区稻田CH4和生物质燃烧CO排放对对流层氧化性的影响

利用稻田甲烷排放模型估计中国地区稻田CH4排放,得到排放总量为9.26Tg,排放有明显的季节和空间变化。运用中尺度气象模式MM5以及光化学模式CALGRID,研究上述稻田CH4排放以及Streets等估计的生物质燃烧CO排放对对流层化学的影响。结果表明,这些排放对对流层低层CH4、CO、OH、HO2、O3浓度均有影响。稻田CH4排放在主要水稻种植区附近使得CH4浓度明显增加、最大增量达到66.97μg.m-3。由于大气中存在活性更强的VOC和CO,稻田CH4排放对自由基和O3全国平均浓度的影响不大,使O3增加0.24%、使HO2增加0.4%、使OH减小0.06%。生物质燃烧CO排放使主要排放区附近CO浓度增加大于60μg.m-3,使OH全国平均浓度减小2.2%,使HO2和O3浓度分别增加2.8%和0.9%。生物质燃烧CO排放对中国地区近地面O3浓度影响强于稻田CH4排放。两者的综合影响使得全国对流层低层O3浓度平均增加1.2%,局部地区最大增加量达4.12μg.m-3。上述自然源排放的影响有明显的空间变化,与下垫面类型有关;还有强烈的季节变化,通常夏季影响最为显著。     

灌溉条件下藏北紫花针茅光合特性及其对温度和CO2浓度的短期响应

利用LI-6400便携式光合作用测定仪, 测定不同灌溉措施下紫花针茅(Stipa purpurea)的光合特性对CO₂浓度和温度的响应, 探讨了土壤水分、温度和CO₂浓度升高对藏北高寒草地紫花针茅光合作用的影响。结果表明: 1)紫花针茅各项光合特性参数对CO₂浓度、温度和土壤水分的变化响应显著, 并表现出明显的交互作用; 2) CO₂浓度升高促进光合速率, 但CO₂浓度过高时光合速率反而下降; 温度升高抑制光合速率, 土壤水分增加对高温条件下的光合作用具有补偿作用; 土壤水分增加促进紫花针茅光合速率的升高; 3)随着CO₂浓度的升高, 胞间CO₂浓度逐渐增大, 蒸腾速率降低, 水分利用效率升高, 气孔导度逐渐减小, 且温度升高加剧气孔导度下降的程度。各光合参数在不同温度水平和土壤水分下表现不同: 气孔导度在20 ℃时达到最大值, 且土壤水分增加利于气孔导度的增大; 温度上升抑制了胞间CO₂浓度, 且在土壤水分充足的条件下更显著; 蒸腾速率随着温度的上升而加快, 蒸腾速率与土壤水分的正相关关系明显; 叶片饱和水汽压亏缺与温度成正比, 充足的土壤水分会适当降低饱和水汽压亏缺; 水分利用效率随着温度上升和土壤水分增多而减小。不同土壤水分条件下光合参数对温度的响应结果表明, 土壤水分的增加对较高温度下光合及其生理参数与温度的关系具有一定的补偿作用。     

川中丘陵区旱地小麦生态系统CO2、N2O和CH4排放特征

利用静态箱/气相色谱法观测川中丘陵旱地小麦生态系统CO2、N2O和CH4的排放通量.结果表明,旱地小麦CO2、N2O和CH4排放具有明显的日变化和季节变化特征.在小麦的整个生长季中,常规、无氮、空白、裸地等实验处理的CO2、N2O和CH4的排放通量具有显著差异.CO2排放通量表现为常规＞空白＞裸地,CO2日最高排放值在每日1300～1500时出现,最低排放值出现在凌晨300～600,且作物的不同生长季CO2排放通量具有一定的差异.在小麦生长季中,N2O通量排放呈递增趋势,N2O排放通量表现为常规＞空白＞裸地.旱地小麦CH4通量排放和温度、湿度等环境因子不具有显著的相关性,排放无规律.     

CH4 emission with differences in atmospheric CO2 enrichment and rice cultivars in a Japanese paddy soil

A pot experiment was conducted to investigate CH₄ emissions from a sandy paddy soil as influenced by rice cultivars and atmospheric CO₂ elevation. The experiment with two CO₂ levels, 370 μL L⁻¹ (ambient) and 570 μL L⁻¹ (elevated), was performed in a climatron, located at the National Institute for Agro‐Environmental Sciences, Tsukuba, Japan. Four rice cultivars were tested in this experiment, including IR65598, IR72, Dular and Koshihikari. Tiller number, root length and grain yield were clearly larger under elevated CO₂ than under ambient CO₂. IR72 and Dular showed significantly higher tiller number, root length and grain yield than Koshihikari and IR65598. Average daily CH₄ fluxes under elevated CO₂ were significantly larger by 10.9–23.8% than those under ambient CO₂, and varied with the cultivars in the sequence Dular ≧ IR72>IR65598 ≧ Koshihikari. Dissolved organic C (DOC) content in the soil was obviously higher under elevated CO₂ than under ambient CO₂ and differed among the cultivars, in the sequence IR72>Dular>Koshihikari>IR65598. The differences in average daily CH₄ fluxes between CO₂ levels and among the cultivars were related to different root exudation as DOC content, root length and tiller number. This study indicated that Koshihikari should be a potential cultivar for mitigating CH₄ emission and simultaneously keeping stable grain yield, because this cultivar emitted lowest CH₄ emission and produced medium grain yield.