Differences Among Rice Cultivars in Root Exudation, Methane Oxidation, and Populations of Methanogenic and Methanotrophic Bacteria in Relation to Methane Emission

Greenhouse experiments were conducted under subtropical conditions to understand the mechanism of rice cultivar differences in methane (CH₄) emission. Three rice cultivars were studied. Differences in CH₄ emission rates among the three rice cultivars became evident in the middle and late growth stages. Rice root exudates per plant measured as total released C were significantly different among rice cultivars. The effect of root exudates on CH₄ production in soil slurry differed accordingly. The amount of root exudates was not significantly different among rice cultivars when computed on a dry matter basis, indicating that it is positively correlated to root dry matter production. The root CH₄-oxidizing activity differed among rice cultivars. IR65598 had a higher oxidative activity than IR72 and Chiyonishiki. Root air space was not significantly different among rice cultivars at the late growth stage, indicating that it is probably not a factor contributing to cultivar differences in CH₄ emission. The population level of methanogenic bacteria differed significantly in soil grown to different rice cultivars, but not in roots, at booting stage and ripening stage. Methanotrophic bacteria population differed significantly in roots among rice cultivars at ripening. Rice cultivars with few unproductive tillers, small root system, high root oxidative activity, and high harvest index are ideal for mitigating CH₄ emission in rice fields.     

A Process-Based Model for Methane Emissions from Irrigated Rice Fields: Experimental Basis and Assumptions

In this paper, we review the process-level studies that the authors have performed in rice fields of Texas since 1989 and the development of a semi-empirical model based on these studies. In this model, it is hypothesized that methanogenic substrates are primarily derived from rice plants ad added organic matter. Rates of methane (CH₄) production in flooded rice soils are determined by the availability of methanogenic substrates and the influence of climate, soil, and agronomic factors. Rice plant growth and added carbon control the fraction of CH₄ emitted. The amount of CH₄ transported from the soil to the atmosphere is determined by the rates of production and the emitted fraction. Model calibration against observations from a single rice-growing season in Texas, USA, without organic amendments and with continuous irrigation demonstrated that the seasonal variation of CH₄ emission is regulated by rice biomass and cultivar type. 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 CH₄ emission can be predicted from rice net productivity, cultivar character, soil texture, temperature, and organic matter amendments.     

Modeling Rice Plant-Mediated Methane Emission

Late-season methane (CH₄) emissions from flooded ricefields appear to be fueled by root exudation and death and to be transmitted to the atmosphere largely through the plant. We present a general transport-reaction model which accommodates these phenomena, together with a simplified ("cartoon") version intended to reproduce the salient features of most plant-dominated CH₄-emitting systems. Our cartoon model is capable of reproducing measured concentration profiles and fluxes. Sensitivity annalysis suggests that cultivars with high specific root transmissivity may, other things being equal, reduce rather than enhance net emission. Simulations assuming exponential growth of the root system followed by Gaussian die-back resemble measured flux trajectories and also point to great variability in the fraction of CH₄ oxidized before it reaches the atmosphere. Air entry on drainage reduces simulated CH₄ fluxes and the fractions of those fluxes mediated by plants. It also increases the fraction of CH₄ oxidized.     

CH4 emission and oxidation in Chinese rice paddies

In the paper, the characteristics of CH₄ emission from the rice paddies, its temporary and spatial variations as well as factors regulating CH₄ emission and oxidation are reviewed with an emphasis on CH₄ emission from rice paddies in China. The observed four types of diel variation and two type of seasonal variation can be explained by the variations of methane production in the soil and the transport efficiencies of the three transport routs. The inter-annual variation of CH₄ emission from rice fields is significant, but the process causing this change is very complicated and unclear based on the available data at present. The large special variation, more than 10 times difference, of the total season methane emissions observed in various rice fields in China, is largely attributed to soil type difference although both soil physics and chemistry are important. Rice growing activities regulate the diel and seasonal variation patterns of the methane emissions. Drainage of flooded water may significantly reduce the emission. Organic fertilizer may enhance the emission, while some of the chemical fertilizers may reduce the emission. Local climate conditions, average temperature and annual rainfall, may be responsible for part of the observed year to year differences of the total season emission. Estimates of total emissions of CH₄ from Chinese rice fields, based on field measurement and model calculation, are 9.7–12.7 Tg/year and 8.17–10.52 Tg/year respectively, for the year of 1994. Oxidation of CH₄ reduces the emission of CH₄ produced in the soil of rice field to the atmosphere. The most likely sites for CH₄ oxidation in rice fields are the water–soil interface and the rhizosphere. When the flood water dries up in irrigated fields, the oxidation of CH₄ in the soil is more important and can partially explain the lower emission rates during the last period before harvest in most experiments. The magnitude of oxidation in the rhizosphere is not well known. Good correlation between methane reduction and O₂ mixing ratio in the soil has been found in most soil types. Methane oxidation rate is mainly controlled by the gas transport resistance in the soil. The oxidation rate increases with the increase of temperature in the temperature range of 5–36 °C.     

N2O and CH4 emissions from fertilized agricultural soils in southwest France

Emissions of nitrogen compounds from heavily fertilized and irrigated maize fields have been studied in the Southwest of France, over an annual cultivation cycle. Strong nitrous oxide emissions from denitrification were observed after application of nitrogen fertilizer. Flux intensity appears to be stimulated by rain or irrigation. Emission algorithms, taking into account both nitrogen input and soil water content were established on the basis of the experimental data set. They allowed us to estimate annual nitrogen loss in the form of nitrous oxide modulated by rainfall. Production of methane is observed at the level of the water table under anoxic conditions. Nevertheless, the net flux between soil and atmosphere is negative for most of the time. When methane is produced, fluxes were very low due to methane oxidation in the soil surface layer.     

Methane emissions from rice paddies: a process study summary

Irrigated rice cultivation is one of the largest sources (approximately 15–20% of the annual total) of atmospheric methane, a potent greenhouse gas. This review examines the results of work performed over the past six years in which we have investigated the processes leading to the emission of methane from irrigated rice cultivation. These studies describe the daily and seasonal effects on methane production and emission of different planting dates, water management, organic amendments, soil texture, and cultivar choice. Because rice agriculture is one of the few sources of methane where emission reduction through management is considered possible, it promises to be a critical focus of mitigation efforts. We have identified several potential management practices for rice cultivation that may stabilize or reduce the emission of methane even in the face of future increased grain production necessary to meet the demands of an expanding world population. This revised version was published online in August 2006 with corrections to the Cover Date.     

The effect of agricultural practices on methane and nitrous oxide emissions from rice field and pot experiments

The authors of this paper measured the methane and nitrous oxide fluxes emissions from rice field with different rice varieties and the two fluxes from pot experiments with different soil water regime and fertilizer treatment. The experiment results showed that: (1) The CH₄ emission rates were different among different varieties; (2) There was a trade-off between CH₄ and N₂O emissions from rice field with some agricultural practices; (3) We must consider the mitigation options comprehensively to mitigate CH₄ and N₂O emissions from rice fields. This revised version was published online in August 2006 with corrections to the Cover Date.     

Factors controlling diel patterns of methane emission via rice

Methane emissions from flooded rice grown under greenhouse conditions were monitored using a closed chamber technique. The three rice cultivars showed similar diel emission patterns though the amplitudes differed. Variation in emissions (maximum emission rate) from the different cultivars ranged from 0.164–0.241 mg/pot/h at tillering stage, 0.714–2.334 mg/pot/h at heading stage, 0.399–1.393 mg/pot/h at ripening stage. The methane emissions increased in the morning at accelerating rates, reached a maximum in the early afternoon, then decreased rapidly to constant rates during the night. The diel emission pattern was modeled using a Gaussian equation for daytime, and a constant for nocturnal emissions. Applying an Arrhenius equation, more than 90% of the diel variation of methane emissions could be predicted from soil temperature fluctuations. The predictions improved by using a diffusion model based on soil temperature and dissolved methane concentrations in soil solution. Soil temperature and methane concentration in soil solution are the two major factors controlling diel methane emissions.     

Using a Crop/Soil Simulation Model and GIS Techniques to Assess Methane Emissions from Rice Fields in Asia. II. Model Validation and Sensitivity Analysis

The MERES (Methane Emissions from Rice EcoSystems) simulation model was tested using experimental data from IRRI and Maligaya in the Philippines and from Hangzhou in China. There was good agreement between simulated and observed values of total aboveground biomass, root weight, grain yield, and seasonal methane (CH₄) emissions. The importance of the contribution of the rice crop to CH₄ emissions was highlighted. Rhizodeposition (root exudation and root death) was predicted to contribute about 380 kg C ha^–1 of methanogenic substrate over the season, representing 37% of the total methanogenic substrate from all sources when no organic amendments were added. A further 225 kg C ha^–1 (22%) was predicted to come from previous crop residues, giving a total of around 60% originating from the rice crop, with the remaining 41% coming from the humic fraction of the soil organic matter (SOM). Sensitivity analysis suggested that the parameter representing transmissivity to gaseous transfer per unit root length (_r) was important in determining seasonal CH₄ emissions. As this transmissivity increased, more O₂ was able to diffuse to the rhizosphere, so that CH₄ production by methanogens was reduced and more CH₄ was oxidized by methanotrophs. These effects outweighed the opposing influence of increased rate of transport of CH₄ through the plant, so that the overall effect was to reduce the amount of CH₄ emitted over the season. Varying the root-shoot ratio of the crop was predicted to have little effect on seasonal emissions, the increased rates of rhizodeposition being counteracted by the increased rates of O₂ diffusion to the rhizosphere. Increasing the length of a midseason drainage period reduced CH₄ emissions significantly, but periods longer than 6–7 d also decreased rice yields. Organic amendments with low C/N were predicted to be more beneficial, both in terms of enhancing crop yields and reducing CH₄ emissions, even when the same amount of C was applied. This was due to higher rates of immobilization of C into microbial biomass, removing it temporarily as a methanogenic substrate.     

Mechanisms of Crop Management Impact on Methane Emissions from Rice Fields in Los Baños, Philippines

This article comprises 4 yr of field experiments on methane (CH₄) emissions from rice fields conducted at Los Baños, Philippines. The experimental layout allowed automated measurements of CH₄ emissions as affected by water regime, soil amendments (mineral and organic), and cultivars. In addition to emission records over 24 h, ebullition and dissolved CH₄ in soil solution were recorded in weekly intervals. Emission rates varied in a very wide range from 5 to 634 kg CH₄ ha^–1, depending on season and crop management. In the 1994 and 1996 experiments, field drying at midtillering reduced CH₄ emissions by 15–80% as compared with continuous flooding, without a significant effect on grain yield. The net impact of midtillering drainage was diminished when (i) rainfall was strong during the drainage period and (ii) emissions were suppressed by very low levels of organic substrate in the soil. Five cultivars were tested in the 1995 dry and wet season. The cultivar IR72 gave higher CH₄ emissions than the other cultivars including the new plant type (IR65597) with an enhanced yield potential. Incorporation of rice straw into the soil resulted in an early peak of CH₄ emission rates. About 66% of the total seasonal emission from rice straw-treated plots was emitted during the vegetative stage. Methane fluxes generated from the application of straw were 34 times higher than those generated with the use of urea. Application of green manure (Sesbania rostrata) gave only threefold increase in emission as compared with urea-treated plots. Application of ammonium sulfate significantly reduced seasonal emission as compared with urea application. Correlation between emissions and combined dissolved CH₄ concentrations (from 0 to 20 cm) gave a significant R² of 0.95 (urea + rice straw), and 0.93 (urea + Sesbania), whereas correlation with dissolved CH₄ in the inorganically fertilized soils was inconsistent. A highly significant correlation (R² =0.93) existed between emission and ebullition from plots treated with rice straw. These findings may stimulate further development of diagnostic tools for easy and reliable determination of CH₄ emission potentials under different crop management practices.     

Field Validation of DNDC Model for Methane and Nitrous Oxide Emissions from Rice-based Production Systems of India

The DNDC (DeNitrification and DeComposition) model was tested against experimental data on CH₄ and N₂O emissions from rice fields at different geographical locations in India. There was a good agreement between the simulated and observed values of CH₄ and N₂O emissions. The difference between observed and simulated CH₄ emissions in all sites ranged from −11.6 to 62.5 kg C ha⁻¹ season⁻¹. Most discrepancies between simulated and observed seasonal fluxes were less than 20% of the field estimate of the seasonal flux. The relative deviation between observed and simulated cumulative N₂O emissions ranged from −237.8 to 28.6%. However, some discrepancies existed between observed and simulated seasonal patterns of CH₄ and N₂O emissions. The model simulated zero N₂O emissions from continuously flooded rice fields and poorly simulated CH₄ emissions from Allahabad site. For all other simulated cases, the model satisfactorily simulated the seasonal variations in greenhouse gas emission from paddy fields with different land management. The model also simulated the C and N balances in all the sites, including other gas fluxes, viz. CO₂, NO, NO₂, N₂ and NH₃ emissions. Sensitivity tests for CH₄ indicate that soil texture and pH significantly influenced the CH₄ emission. Changes in organic C content had a moderate influence on CH₄ emission on these sites. Introducing the mid-season drainage reduced CH₄ emissions significantly. Process-based biogeochemical modeling, as with DNDC, can help in identifying strategies for optimizing resource use, increasing productivity, closing yield gaps and reducing adverse environmental impacts.     

Dynamics of methane emission,active soil organic carbon and their relationships in wetland integrated rice-duck systems in Southern China

A randomized field experiment with three replicates was conducted in the subtropical region of China to investigate the effects of integrated rice-duck system (RD) on methane (CH₄) emission, active soil organic carbon fractions and their relationships in 2007 and 2008, compared with conventional rice system (CK). Methane emissions were measured at 7–9 days intervals using a closed static chamber technique, and two fractions of active soil organic carbon, namely, dissolved organic carbon (DOC) and microbial biomass carbon (MBC), were analyzed simultaneously. Soil DOC and MBC in RD and CK had similarly distinct seasonal variation patterns within the 2 years. During this time DOC and MBC concentrations were low at the early growth stage, increased during panicle differentiation and heading period, and dropped during grain filling period of rice. CH₄ emission fluxes from RD and CK followed a similar seasonal variation pattern both in 2007 and 2008. Two peaks of CH₄ emission were observed, the first at the tillering stage, second at panicle differentiation and heading stage. The CH₄ cumulative emission was reduced in RD by 19.3 and 19.6% in 2007 and 2008, respectively, compared with CK. Seasonal variation pattern of CH₄ emission was regulated by soil DOC, MBC and soil temperature, all of which were significantly positively correlated with methane emissions. Improvement in soil redox status was the predominant reason for significant reduction of CH₄ emission in RD. These results clearly indicate that integrated rice-duck system could be an effective mode of rice farming for decrease in methane emission in southern China.     

New estimates of methane emissions from Chinese rice paddies

In this paper, a new method had been developed, which is suitable to estimate methane emissions from rice fields in China at present. The method is developed based upon the methane models developed in China, different from those recommended by OECD/IPCC. The influences of climate conditions, field water management, organic fertilizers and soil types on methane emission from rice fields are considered. The methane model has been tested with field measurement data. Methane emissions from Chinese rice fields are estimated to be 9.67–12.66 Tg/yr in 1990. These values are lower than previous estimates and are more nearly to the measured data, because of the improved method extrapolations of field measurements.     

Impact of agriculture on soil consumption of atmospheric CH4 and a comparison of CH4 and N2O flux in subarctic, temperate and tropical grasslands

Increasing concentrations of methane (CH₄) in the atmosphere are projected to account for about 25% of the net radiative forcing. Biospheric emissions of CH₄ to the atmosphere total approximately 400 Tg C y^-1. An estimated 300 Tg of CH₄-C y^-1 is oxidized in the atmosphere by hydroxyl radicals while about 40 Tg y^-1 remains in the atmosphere. Approximately 40 Tg y^-1 of the atmospheric burden is oxidized in aerobic soils. Research efforts during the past several years have focused on quantifying CH₄ sources while relatively less effort has been directed toward quantifying and understanding the soil sink for atmospheric CH₄. Recent research has demonstrated that land use change, including agricultural use of native forest and grassland systems has decreased the soil sink for atmospheric methane. Some agricultural systems consume atmospheric CH₄ at rates less than 10% of those found in comparable undisturbed soils. While it has been necessary to change land use practices over the past centuries to meet the required production of food and fiber, we need to recognize and account for impacts of land use change on the biogeochemical nutrient cycles in the biosphere. Changes that have ensued in these cycles have and will impact the atmospheric concentrations of CH₄ and N₂O. Since CH₄ and N₂O production and consumption are accomplished by a variety of soil microorganisms, the influence of changing agricultural, forest, and, demographic patterns has been large. Existing management and technological practices may already exist to limit the effect of land use change and agriculture on trace gas fluxes. It is therefore important to understand how management and land use affect trace gas fluxes and to observe the effect of new technology on them. This paper describes the role of aerobic soils in the global CH₄ budget and the impact of agriculture on this soil CH₄ sink. Examples from field studies made across subarctic, temperate and tropical climate gradients in grasslands are used to demonstrate the influence of nutrient cycle perturbations on the soil consumption of atmospheric CH₄ and in increased N₂O emissions. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effect of Land Management in Winter Crop Season on CH4 Emission During the Following Flooded and Rice-Growing Period

A greenhouse pot experiment was carried out to study the effect of land management during the winter crop season on methane (CH₄) emissions during the following flooded and rice-growing period. Three land management patterns, including water management, cropping system, and rice straw application time were evaluated. Land management in the winter crop season significantly influenced CH₄ fluxes during the following flooded and rice-growing period. Methane flux from plots planted to alfalfa (ALE) in the winter crop season was significantly higher than those obtained with treatments involving winter wheat (WWE) or dry fallow (DFE). Mean CH₄ fluxes of treatments ALE, WWE, and DFE were 28.6, 4.7, and 4.1 mg CH₄ m^–2 h^–1 in 1996 and 38.2, 5.6, and 3.2 mg CH₄ m^–2 h^–1 in 1997, respectively. The corresponding values noted with continuously flooded fallow (FFE) treatment were 6.1 and 5.2 times higher than that of the dry fallow treatment in 1996 and 1997, respectively. Applying rice straw just before flooding the soil (DFL) significantly enhanced CH₄ flux by 386% in 1996 and by 1,017% in 1997 compared with rice straw application before alfalfa seed sowing (DFE). Land management in the winter crop season also affected temporal variation patterns of CH₄ fluxes and soil Eh after flooding. A great deal of CH₄ was emitted to the atmosphere during the period from flooding to the early stage of the rice-growing season; and CH₄ fluxes were still relatively high in the middle and late stages of the rice-growing period for treatments ALE, DFL, and FFE. However, for treatments DFE and WWE, almost no CH₄ emission was observed until the middle stage, and CH₄ fluxes in the middle and late stages of the rice-growing period were also very small. Soil Eh of treatments ALE and DFL decreased quickly to a low value suitable for CH₄ production. Once Eh below –150 mV was established, the small changes in Eh did not correlate to changes in CH₄ emissions. The soil Eh of treatments DFE and WWE did not decrease to a negative value until the middle stage of the rice-growing period, and it correlated significantly with the simultaneously measured CH₄ fluxes during the flooded and rice-growing period.     

Characterization of Methane Emissions from Rice Fields in Asia. II. Differences among Irrigated, Rainfed, and Deepwater Rice

Methane (CH₄) emission rates were recorded automatically using the closed chamber technique in major rice-growing areas of Southeast Asia. The three experimental sites covered different ecosystems of wetland rice--irrigated, rainfed, and deepwater rice--using only mineral fertilizers (for this comparison). In Jakenan (Indonesia), the local water regime in rainfed rice encompassed a gradual increase (wet season) and a gradual decrease (dry season) in floodwater levels. Emission rates accumulated to 52 and 91 kg CH₄ ha^–1 season^–1 corresponding to approximately 40% of emissions from irrigated rice in each season. Distinct drainage periods within the season can drastically reduce CH₄ emissions to less than 30 kg CH₄ ha^–1 season^–1 as shown in Los Baños (Philippines). The reduction effect of this water regime as compared with irrigated rice varied from 20% to 80% from season to season. Methane fluxes from deepwater rice in Prachinburi (Thailand) were lower than from irrigated rice but accumulated to equally high seasonal values, i.e., about 99 kg CH₄ ha^–1 season^–1, due to longer seasons and assured periods of flooding. Rice ecosystems with continuous flooding were characterized by anaerobic conditions in the soil. These conditions commonly found in irrigated and deepwater rice favored CH₄ emissions. Temporary aeration of flooded rice soils, which is generic in rainfed rice, reduced emission rates due to low CH₄ production and high CH₄ oxidation. Based on these findings and the global distribution of rice area, irrigated rice accounts globally for 70–80% of CH₄ from the global rice area. Rainfed rice (about 15%) and deepwater rice (about 10%) have much lower shares. In turn, irrigated rice represents the most promising target for mitigation strategies. Proper water management could reduce CH₄ emission without affecting yields.     

Methane emission patterns from rice fields planted to several rice cultivars for nine seasons

The presented data set comprises a series of field experiments conducted in the period from 1993 to 1999 at the International Rice Research Institute, Philippines. Methane emissions from different rice cultivars were compared during nine seasons using an automated measuring system. The list of cultivars in this experiment consists of high yielding semi-dwarf cultivars (IR72, IR52, PSBRc20, PSBRc14), traditional tall cultivars (Dular, Intan), hybrid (Magat) as well as plant types with high yield potential that are currently under development (IR65597, IR65600). Seasonal averages in emission rates ranged from 20 to 89 mg CH₄ m^–2 d^–1 under inorganic fertilization and from 129 to 413 mg CH₄ m^–2 d^–1 following organic amendments. However, differences were generally small within a given season and stayed below significance level for the bulk of the inter-cultivar comparisons. Each experiment included IR72 to allow computation of cultivar-specific emission indices in relation to this reference. These indices ranged from 0.57 (PSBRc14) to 1.8 (Magat), but did not reveal consistent ranking for rice genotypes. The similarity in methane emissions was corroborated in a field screening of 19 cultivars using dissolved CH₄ in soil solution as a proxy for relative emission rates. Irrespective of cultivars, higher plant density (10*20 cm spacing vs. 20*20 cm spacing of plant hills) stimulated methane production in the soil, but did not result in higher emission rates. This finding was attributed to higher oxygen influx into the soil and subsequent stimulation of methane oxidation when plants hills were more abundant. Over multi-seasonal periods, differences observed between cultivars were inconsistent indicating complex interactions with the environment. These results stress the need for more mechanistic understanding on cultivar effects to exploit the mitigation potential of cultivar selection in rice systems.formerly at IRRI Corresponding author; e-mail:     

Simultaneous Records of Methane and Nitrous Oxide Emissions in Rice-Based Cropping Systems under Rainfed Conditions

Rainfed rice (Oryza sativa L.)-based cropping systems are characterized by alternate wetting and drying cycles as monsoonal rains come and go. The potential for accumulation and denitrification of NO₃ ^– is high in these systems as is the production and emission of CH₄ during the monsoon rice season. Simultaneous measurements of CH₄ and N₂O emissions using automated closed chamber methods have been reported in irrigated rice fields but not in rainfed rice systems. In this field study at the International Rice Research Institute, Philippines, simultaneous and continuous measurements of CH₄ and N₂O were made from the 1994 wet season to the 1996 dry season. During the rice-growing seasons, CH₄ fluxes were observed, with the highest emissions being in organic residue-amended plots. Nitrous oxide fluxes, on the other hand, were generally nonexistent, except after fertilization events where low N₂O fluxes were observed. Slow-release N fertilizer further reduced the already low N₂O emissions compared with prilled urea in the first rice season. During the dry seasons, when the field was planted to the upland crops cowpea [Vigna unguiculata (L.) Walp] and wheat (Triticum aestivum L.), positive CH₄ fluxes were low and insignificant except after the imposition of a permanent flood where high CH₄ fluxes appeared. Evidences of CH₄ uptake were apparent in the first dry season, especially in cowpea plots, indicating that rainfed lowland rice soils can act as sink for CH₄ during the upland crop cycle. Large N₂O fluxes were observed shortly after rainfall events due to denitrification of accumulated NO₃ ^–. Cumulative CH₄ and N₂O fluxes observed during this study in rainfed conditions were lower compared with previous studies on irrigated rice fields.     

Methane Transport Capacity of Rice Plants. II. Variations Among Different Rice Cultivars and Relationship with Morphological Characteristics

Of the total methane (CH4) emitted from a rice field during the growing season 60–90% is emitted through the rice plants. We determined the methane transport capacity (MTC) of rice plants at different physiological growth stages using an automatic measuring system under greenhouse conditions. A total of 12 cultivars (10 inbred varieties and 2 hybrids) were studied in sets of two experiments and was distinguished into three groups according to the patterns of MTC development. MTC is generally increasing from seedling stage to panicle initiation (PI), but differs in the development from PI to maturity. While the hybrid showed a gradual increase in MTC, the inbred cultivars showed either minor changes in MTC or a drastic decrease from flowering to maturity. Among tall cultivars, Dular showed the highest MTC, followed by B40; the lowest MTC was found in Intan. High-yielding dwarf cultivars showed MTC in the descending order of IR72 > IR52 > IR64 > PSBRc 20. New plant type cultivars showed very low MTC with IR65600 exhibiting the smallest MTC at PI, flowering, and maturity. Hybrids (Magat and APHR 2) showed the largest MTC that continued to increased with plant growth. The MTC patterns were attributed to growth parameters and the development of morphological characteristics of the aerenchyma. These results suggest that in tall, dwarf, and NPT cultivars, increase in root or aboveground biomass during initial growth determines a corresponding increase in MTC. Once aerenchyma has fully developed, further increase in plant biomass would not influence MTC. However, in the case of hybrids, a positive relationship of MTC with root + shoot biomass (r = 0.672, p 0.05) and a total plant biomass including grain (r = 0.849, p 0.01) indicate continuous development of aerenchyma with plant growth, resulting in enhanced MTC. In all cultivars, tiller number, but not height, was linearly related to MTC, indicating that the number of outlets/channels rather than plant size/biomass determines the transport of CH₄. These results clearly demonstrate that rice cultivars differ significantly in MTC. Therefore, the use of high-yielding cultivars with low MTC (for example, PSBRc 20, IR65598, and IR65600) could be an economically feasible, environmentally sound, and promising approach to mitigate CH₄ emissions from rice fields.