The dependence of methane transport in rice plants on the root zone temperature

Large diurnal and seasonal variations in methane flux from rice paddies have been found in many studies. Although these variations are considered to result from changes in methane formation rates in the soil and the transport capacity (e.g. biomass, physiological activities, and so on) of rice plants, the real reasons for such variations are as yet unclear. This study was conducted to clarify the effects of temperature on the rate of methane transport from the root zone to the atmosphere using hydroponically grown rice plants. Methane emission rates from the top of the rice plants whose roots were soaked in a solution with a high methane concentration were measured using a flow-through chamber method with the top or root of the rice plants being kept at various temperatures. The methane emission rates and methane concentrations in solution were analyzed using a diffusion model which assumes that the methane emission from a rice paddy is driven by molecular diffusion through rice plants by a concentration gradient. In the experiment where the temperature around the root was changed, the conductance for methane diffusion was typically 2.0-2.2 times larger when the solution temperature was changed from 15 to 30 °C. When the air temperature surrounding the top of the rice plant was changed, the change in conductance was much less. In addition, from measurements of methane flux and methane concentration in soil water in a lysimeter rice paddy during the 2 growing seasons of rice, it was found that the conductance for methane transport was correlated with the soil temperature at 5 cm depth. These results suggest that the temperature around the root greatly affects the methane transport process in rice plants, and that the process of passing through the root is important in determining the rate of methane transport through rice plants.     

Impact of gas transport through rice cultivars on methane emission from rice paddy fields

Two Italian rice (Oryza sativa var. japonica) cultivars, Lido and Roma, were tested in the field for methane production, oxidation and emission. In two consecutive years, fields planted with the rice cultivar Lido showed methane emissions 24–31% lower than fields planted with the cultivar Roma. This difference was observed irrespective of fertilizer treatment. In contrast to methane emissions, differences in methane production or oxidation were not observed between fields planted with the two cultivars. Plant-mediated transport of methane from the sediment to the atmosphere was the dominating pathway of methane emission. During the entire vegetation period, the contribution of this pathway to total methane emission amounted to c. 90%, whereas the contribution of gas bubble release and of diffusion through the water column to total methane emission was of minor significance. Results obtained from transport studies of tracer gas through the aerenchyma system of rice plants demonstrated that the root–shoot transition zone is the main site of resistance to plant-mediated gas exchange between the soil and the atmosphere. The cultivar Lido, showing relatively low methane emissions in the field, had a significantly lower gas transport capacity through the aerenchyma system than the cultivar Roma. Thus, the observed differences in methane emissions in the field between the cultivars Lido and Roma can be explained by different gas transport capacities. Apparently, these differences in gas transport capacities are a consequence of differences in morphology of the aerenchyma systems, especially in the root–shoot transition zone. It is, therefore, concluded that identification and use of high-yielding rice cultivars which have a low gas transport capacity represent an economically feasible, environmentally sound and promising approach to mitigating methane emissons from rice paddy fields.     

Importance of physical plant properties on methane transport through several rice cultivars

The transport rate of methane from a nutrient solution to the atmosphere via rice plants was controlled both by the methane concentration in the nutrient solution and by plant physiology. Measurements on 11 rice cultivars indicated that the conductance of the rice plant for methane increased as the plants developed. The conductance varied from 0.3±0.2 to 1.2±0.7 μmol min^-1 mM^-1 plant^-1 during the tillering stage, and from 0.6±0.3 to 2.5±0. 2 μmol min^-1 mM^-1 plant^-1 during the reproduction stage. Based on measurements of all plants, the conductance positively correlated with parameters of plant size. Conductance correlated best with stem inter-cellar volume at the tillering stage, and with root volume at the reproduction stage. For both stages taken together, the correlation between conductance and root volume was the most significant. The conductance could be approximated by a multiple linear regression using root volume and the length of root bundle. Higher conductance was found to generally associate with the plants with larger root volume, heavier root fresh weight. This revised version was published online in June 2006 with corrections to the Cover Date.     

Role of rice in mediating methane emission

Methane emitted at different plant conditions through the different organs of rice plants was studied using a closed chamber technique under the laboratory, phytotron, and greenhouse conditions in order to clarify and quantify the role of different organs of rice plant as methane emission sites. Rice plants grown in flooded soils emit methane to the atmosphere via the aerenchyma of leaves, nodes and panicles. Emission through the rice plants is controlled by diffusion. No methane is emitted via the transpiration stream. Leaves are the major release sites at the early growth stage while nodes become more important later. Cracks and porous structure were found in the nodes. Panicles generally contribute little to methane emission. Increasing water depth temporarily reduces methane emission while concentration gradients in rice plants readjust to unsubmerged emission sites. Methane emissions in rice plants cease only when the plants become totally submerged.     

Impact of phosphorus supply on root exudation, aerenchyma formation and methane emission of rice plants

This study evaluated the impact of P supply on rice plant development and the methane budget of rice fields by 2 different approaches: (1) root growth, exudation and aerenchyma formation were recorded in an experiment with hydroponic solution; (2) dissolved CH₄ concentration and CH₄ emission were investigated in a pot experiment. In both approaches, we used three different cultivars and three levels of P supply. In the experiment with solution culture (0.5 ppm, 5 ppm, and 10 ppm P), root exudation ranged between 0.5 to 36.7 mol C plant^–1 h^–1 and increased steadily with plant growth at given P level. Low P supply resulted in

Seasonal variation in methane flux from rice paddies associated with methane concentration in soil water,rice biomass and temperature,and its modelling

To attempt to develop physicochemical and physiological modelling for methane transport from the rhizosphere to the atmosphere through rice plants, methane flux, methane concentration in the soil water, and the biomass of rice were measured in lysimeter rice paddies (2.5 × 4 m, depth 2.0 m) once per week throughout the entire growing season in 1992 at Tsukuba, Japan. The addition of exogenous organic matter (rice straw) or soil amendments with the presence or absence of vegetation were also examined for their influence on methane emissions. The total methane emission over the growing season varied from 3.2 g CH₄ m^-2 y^-1 without the addition of rice straw to 49.7 g CH₄ m^-2 y^-1 with rice straw and microbiological amendment. In the unvegetated plot with the addition of rice straw, there was much ebullition of gas bubbles, particularly in the summer. The annual methane emission due to the ebullition of gas bubbles,from the unvegetated plot with the addition of rice straw was estimated to be almost the same as that from the vegetated site with the addition of rice straw. In the early growth stage, the methane flux can be analyzed by the diffusion model (Flux=Methane concentration × Conductance of rice body) using parameters for methane concentration in the soil water as a difference in concentration between the atmosphere and the rhizosphere, and for the biomass of rice as a conductance of rice body. On the other hand, although the diffusion model was inapplicable to a large extent from the middle to late growth stage, methane flux could be estimated by air temperature and concentration in the soil water. Thus, methane transport from the rhizosphere to the atmosphere through rice plants consisted of two phases: one was an explainable small part by diffusion in rice body; the other was a large part strongly, governed by air temperature. The existence of gas bubbles in the soil may be related to the transition between the two phases     

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.     

Diffusion-controlled transport of methane from soil to atmosphere as mediated by rice plants

Methane emission from rice grown in flooded soil was measured in pot experiments using headspaces with different gas composition. The emission rates varied with the atmospheric composition. Based on the kinetic theory of gases the binary diffusion coefficients for methane in various gases were calculated. The ratios of the measured emissions under a certain atmosphere relative to that in air were similar to the ratios of the binary diffusion coefficients showing that plant-mediated CH₄ transport is driven by diffusion. Small deviations from the theoretical ratios of emissions support the hypothesis that mass flow of gas to the submerged parts of the rice plant may depress the upward diffusive CH₄ flux. The results in combination with data from the literature suggest that the rate limiting step in plant-mediated methane transport is diffusion of CH₄ across the root/shoot junction.     

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.     

Effects of ammonium-based fertilisation on microbialprocesses involved in methane emission from soilsplanted with rice

The emission of the greenhouse gas CH₄ from ricepaddies is strongly influenced by management practicessuch as the input of ammonium-based fertilisers. Weassessed the impact of different levels (200 and 400kgN.ha^–1) of urea and (NH₄)₂HPO₄on the microbial processes involved in production andconsumption of CH₄ in rice field soil. We usedcompartmented microcosms which received fertilisertwice weekly. Potential CH₄ production rates weresubstantially higher in the rice rhizosphere than inunrooted soil, but were not affected by fertilisation.However, CH₄ emission was reduced by the additionof fertiliser and was negatively correlated with porewater NH ₄ ^plus concentration, probably as theconsequence of elevated CH₄ oxidation due tofertilisation. CH₄ oxidation as well as numbersof methanotrophs was distinctly stimulated by theaddition of fertiliser and by the presence of the riceplant. Without fertiliser addition,nitrogen-limitation of the methanotrophs will restrictthe consumption of CH₄. This may have a majorimpact on the global CH₄ budget, asnitrogen-limiting conditions will be the normalsituation in the rice rhizosphere. Elevated potentialnitrifying activities and numbers were only detectedin microcosms fertilised with urea. However, asubstantial part of the nitrification potential in therhizosphere of rice was attributed to the activity ofmethanotrophs, as was demonstrated using theinhibitors CH₃F and C₂H₂.     

Cultivar variation in methane efflux from tropical rice

Wide variation in CH4 flux was noticed among the ten rice cultivars grown under uniform field conditions. Cumulative CH4 flux ranged from 4.61 g m-2 to 20.25 g m-2. The rice cultivars could be ranked into three groups based on their CH4 flux potential. Rice cultivars could also be arranged based on their peak CH4 emission occurring either at vegetative, reproductive or at both growth stages. Of the several variables studied (root region redox potential, above- and underground biomass, grain and straw yield, duration of the crop, percent area occupied by the air space and root oxidase activity), only oxidase activity of the root tip exhibited a significant (negative) correlation with CH4 flux indicating an indirect effect of root oxidation potential on CH4 flux. Data presented in this study, demonstrate inherent variation in CH4 flux among different rice cultivars that can be used for developing future mitigation options.     

Influence of rice cultivar on methane emission from paddy fields

Influence of rice cultivars on CH₄ emissions from a paddy field was studied using four Japonica types, two Indica types, and two Japonica/Indica F₁ hybrids. In addition, the suppression of CH₄ emission by interrupting irrigation at the flowering stage was investigated. Patterns of seasonal variation in CH₄ emission rates were similar among the eight cultivars. Two of the Japonica types showed the maximum and minimum CH₄ emissions among the cultivars investigated. Neither the number of tillers, shoot length, shoot weight, and root weight correlated with the CH₄ emission rates at the tillering and reproductive growth stages. Following temporary interruption of irrigation at the flowering stage, CH₄ emission rates decreased drastically and remained at very low levels until the harvesting stage, indicating its great effectiveness for the suppression of CH₄ emission from rice paddies.     

Diurnal variation in methane efflux at different growth stages of tropical rice

Diurnal variation in CH₄ efflux from continuously flooded fields planted to rice (Oryza sativa L. cv. IR-36) was examined at different crop growth stages using a closed chamber method during the wet season. CH₄ emission showed a distinct diurnal pattern especially at tillering, panicle initiation and maturity stages of a field-grown rice crop, with maximum emission in the early afternoon (12.00 to 15.00) followed by a decline to a minimum around midnight. Among several variables (ambient temperature, flood water temperature, redox potential, soil pH, and root oxidase activity), a significant negative correlation existed between oxidase activity of the root base and diurnal fluctuations in CH₄ efflux at tillering stage. Evidence also suggested that redox status in the rhizosphere region and atmospheric, soil, and water temperatures influenced CH₄ emission from rice fields probably by their contrasting effects on CH₄ production and oxidation.     

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     

Inhibition of methane evolution by calcium sulfate addition to flooded rice

Calcium sulfate, a common soil amendment was applied at rates of 0, 1,000 and 2,000 kg ha^-1 to flooded rice plots treated with urea-N (128 kg ha^-1). Experimental plots were drill-seeded with Toro-2, a mid-season long-grain rice cultivar, and CH₄ emissions were measured over the first cropping season. Over the 70 d sampling season, the low and high rate of CaSO₄ reduced CH₄ evolution 29 and 46%, respectively, compared to control plots. No significant correlation between soil temperature (0, 5, 10 cm depths) and CH₄ emission was observed.     

The effect of clipping on methane emissions from Carex

The purpose of this study was to estimate theresistance to methane release of the above-groundportion of Carex, a wetland sedge, and todetermine the locus of methane release from the plant. Measurements conducted on plants clipped to differentheights above the water level revealed that themethane flux from clipped plants was on the order of97% to 111% of control (unclipped) values. Thegreatest increase was observed in the initial fluxmeasurement after the plants had been clipped to aheight of 10 cm. Subsequent measurements on the 10 cmhigh stubble were similar to control values. When theends of plants which had been clipped to 10 cm weresealed, the methane flux was reduced to 65% ofcontrol values. However, sealing had no effect on theflux from plants which were clipped at 15 cm andhigher, indicating that virtually all methane wasreleased on the lower 15 cm of the plants as theyemerged from the water. The results indicate that theabove-ground portions of Carex at our studysite offered only slight resistance to the passage ofmethane, and that the main sites limiting methaneemission are below-ground, at either theporewater-root or root-shoot boundary. We hypothesizethat the transitory increase in flux associated withclipping was due to the episodic release of methaneheld within the plant lacunae. The buildup ofCH₄ partial pressure within lacunal spacesovercomes the resistance to gas transport offered byaboveground parts.     

Effect of various anionic species on net methane production in flooded rice soils

In a laboratory incubation study, effect of various anions on net methane production in two rice soils (alluvial and acid sulphate) under flooded conditions was examined. Methane production was considerable in alluvial soil and almost negligible in acid sulphate soil, albeit with a higher density of viable methanogens, during 30-day incubation without salts. Sodium salts of hydroxide and phosphate further stimulated methane production in alluvial soil and marginally in acid sulphate soil. But, addition of sodium molybdate, a selective inhibitor of sulphate-reducing bacteria, increased the production of methane in acid sulphate soil. In contrast, nitrite, nitrate, sulphite and sulphate suppressed the production of methane in both soils. Acetate served as an excellent substrate for methanogenesis in alluvial soil, but not in acid sulphate soil. Succinate and citrate also stimulated methane production especially in alluvial soil, but after a longer lag. In acid sulphate soil, most of the added carbon in the form of sodium salts of carboxylic acids was converted to CO2 and not methane, which is consistent with their preferential use by the sulphate-reducing bacteria. In general, none of the amendments could increase production of methane in acid sulphate soil to the same level as in alluvial soil.     

Circadian methane oxidation in the root zone of rice plants

In the root zone of rice plants aerobic methanotrophic bacteria catalyze the oxidation of CH₄ to CO₂, thereby reducing CH₄ emissions from paddy soils to the atmosphere. However, methods for in situ quantification of microbial processes in paddy soils are scarce. Here we adapted the push–pull tracer-test (PPT) method to quantify CH₄ oxidation in the root zone of potted rice plants. During a PPT, a test solution containing CH₄ ± O₂ as reactant(s), Cl^? and Ar as nonreactive tracers, and BES as an inhibitor of CH₄ production was injected into the root zone at different times throughout the circadian cycle (daytime, early nighttime, late nighttime). After a 2-h incubation phase, the test solution/pore-water mixture was extracted from the same location and rates of CH₄ oxidation were calculated from the ratio of measured reactant and nonreactive tracer concentrations. In separate rice pots, O₂ concentrations in the vicinity of rice roots were measured throughout the circadian cycle using a fiber-optic sensor. Results indicated highly variable CH₄ oxidation rates following a circadian pattern. Mean rates at daytime and early nighttime varied from 62 up to 451 μmol l^?1 h^?1, whereas at late nighttime CH₄ oxidation rates were low, ranging from 13 to 37 μmol l^?1 h^?1. Similarly, daytime O₂ concentration in the vicinity of rice roots increased to up to 250% air saturation, while nighttime O₂ concentration dropped to below detection (<0.15% air saturation). Our results suggest a functional link between root-zone CH₄ oxidation and photosynthetic O₂ supply.     

A semi-empirical model of methane emission from flooded rice paddy soils

Reliable regional or global estimates of methane emissions from flooded rice paddy soils depend on an examination of methodologies by which the current high variability in the estimates might be reduced. One potential way to do this is the development of predictive models. With an understanding of the processes of methane production, oxidation and emission, a semi-empirical model, focused on the contributions of rice plants to the processes and also the influence of environmental factors, was developed to predict methane emission from flooded rice fields. A simplified version of the model was also derived to predict methane emission in a more practical manner. In this study, it was hypothesized that methanogenic substrates are primarily derived from rice plants and added organic matter. Rates of methane production in flooded rice soils are determined by the availability of methanogenic substrates and the influence of environmental factors. Rice growth and development control the fraction of methane emitted. The amount of methane transported from the soil to the atmosphere is determined by the rates of production and the emitted fraction. Model validation against observations from single rice growing seasons in Texas, USA demonstrated that the seasonal variation of methane emission is regulated by rice growth and development. A further validation of the model against measurements from irrigated rice paddy soils in various regions of the world, including Italy, China, Indonesia, Philippines and the United States, suggests that methane emission can be predicted from rice net productivity, cultivar character, soil texture and temperature, and organic matter amendments.     

猪粪与沼气渣对双季稻田甲烷排放的影响

随着环境温度的升高,稻田甲烷排放通量增加。早稻期间甲烷排放通量随着水稻生育期的增加而逐步加快,而晚稻甲烷排放主要集中在水稻生长的前中期,而且排放量很高。一天中甲烷排放具有很强的周期性,在6：00～8：00时,甲烷排放通量进入谷底,14：00时甲烷排放通量达到峰值。稻田甲烷排放通量与土壤5cm处的温度及土壤水溶解甲烷含量具有较高的相关性。猪粪和沼气渣的施用分别提高稻田甲烷排放量22．14％和4．40％。在早稻期间,施用猪粪和沼气渣分别提高土壤水溶解甲烷含量40．3％和11．9％,而晚稻期间仅分别提高23．9％和5．04％。