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.     

Atmospheric methane and nitrous oxide: sources, sinks and strategies for reducing agricultural emissions

I discuss production, emission and oxidation of CH₄ in rice paddy fields and N₂O in fertilized soils. The quantity of CH₄ emitted from rice paddy fields depends upon several important factors including soil factors, nutrient management, water regimes, cultivation practices and others. Important factors for N₂O emitted from fertilized soils are soil water content, temperature, nitrate or ammonium concentration, available organic carbon for denitrification and pH. I provide an estimate of mitigation potential in agricultural systems based on this estimate and the management technology. This revised version was published online in August 2006 with corrections to the Cover Date.     

Methane emission from rice paddy fields in all of Japanese prefecture

Field measurements of CH₄ emission from rice paddy field during cultivation periods were performed at all of 47 Japanese prefectures under the project of ‘Research for evaluation of CH₄ and N₂O emissions from agricultural land, and improvement methods of soil, water and fertilizer management’ conducted by Agricultural Production Bureau, the Ministry of Agriculture, Forestry and Fisheries. Although this project was carried out at 159 fields, the data of 132 fields were used for this report because other 27 fields had not enough data to be suitable for the statistics analyses. The measurements at rice paddy fields in various locations in Japan showed that there were large temporal variations of CH₄ flux and that the fluxes differed markedly with climate, characteristics of soil and paddy, application of organic matter and mineral fertilizer, and agricultural management practices. These data mainly indicated that CH₄ emission from Gley soils was greater than those from other soil types such as Andosols, Upland soils, fine-textured Lowland soils, medium and coarse-textured Lowlands soils and gravelly Lowland soils, and that water and organic matter managements influenced CH₄ emission. It is suggested that midsummer drainage treatment suppressed while the application of fresh organic matter such as rice straw and wheat straw enhanced CH₄ emission, respectively. This revised version was published online in August 2006 with corrections to the Cover Date.     

Effects of Organic and N Fertilizers on Methane Production Potential in a Chinese Rice Soil and its Microbiological Aspect

An incubation experiment to determine the effects of organic and chemical N fertilizers on methane (CH₄) production potential in a Chinese flooded rice soil was conducted. Organic matter, added as rice straw and organic manure, increased CH₄ production rate significantly. Chemical N fertilizers such as ammonium bicarbonate (AB), modified ammonium bicarbonate (MAB), and urea (U) did not show a clear effect when they were applied with rice straw. Field results may be very different because of the involvement of rice plants. Organic manure showed different promoting effects on CH₄ production rate. Pig manure stimulated the production rate most, followed by chicken and cattle manure. This difference in organic manure was not related to either total C added to the system or to C/N. The study on bacteria groups related to CH₄ production indicated that the different effects of organic matter may be closely related to content of easily decomposable organic matter. A significant linear relationship between CH₄ production and the logarithm of the number of zymogenic bacteria was found with an r value of 0.96. This finding suggests that the number of zymogenic bacteria may be used as an index to predict CH₄ production potential in flooded rice fields and other wetlands.     

Properties of rice soils affecting methane production potentials: 2. Differences in topsoil and subsoil

Methane (CH₄) is one of the important greenhouse gases accounting for 15% of the total enhanced greenhouse effect. A laboratory experiment was conducted with nine soils from the Philippines and two soils from India to determine the CH₄ production potential of topsoil and subsoil, and to assess the role of different fractions of soil organic C in influencing CH₄ production potential. CH₄ production potentials of topsoils varied in a wide range from 20 g g^–1 soil (Urdaneta soil) to 837 g g^–1 soil (Pila soil) over 100 d of incubation. In contrast, CH₄ production potentials of subsoils were low (< 2 g g^–1 soil over 100 d of incubation). The topsoil was the main source of CH₄ in the flooded rice soils contributing 99.95% to the total CH₄ production while the subsoil contributed negligibly (0.05%). CH₄ production potentials of the topsoils showed significant correlation with cation exchange capacity (CEC), total N and available K contents of soils. For the subsoils, CH₄ production potentials had a significant correlation with available P and clay contents of the soils. Considering the differences in all the soil properties and the CH₄ production potentials between topsoils and subsoils, a significant relationship of CH₄ production potential with CEC, available K and enriched C (extra C content of topsoil compared to that of subsoil) was obtained. Two carbon fractions, water soluble C (H₂O-C) and carbon mineralised under anaerobic conditions (AnMC) affected total CH₄ production indirectly rather than directly.     

Sources of methane emitted from paddy fields

Soil organic matter, roots (photosynthates) and applied organic materials (rice straw etc.) are the main sources of methane (CH₄) emitted from paddy fields. The potential CH₄ production in Japanese paddy fields were estimated from chemical properties of paddy soils of respective soil series, their acreage and thermal regimes during the rice growing period. The estimated amounts of potential CH₄ production were from 24 to 54 kg-C ha^-1 among 7 Districts in Japan, which are around one fifth of the amounts of CH₄ emission observed from paddy fields in the world. ¹³CO₂ uptake pot experiments were carried out three times from Aug. 8 to Sept. 25 to the treatment without rice straw applications in 1993 and four times from June 30 to Sept. 13 to the treatments with and without rice straw applications in 1994 to estimate the contribution of photosynthesized carbon to CH₄ emission. The contribution percentages of photosynthesized carbon to the total CH₄ emitted to the atmosphere were calculated to be 22% and 29-39% for the entire growth period in the treatments with and without rice straw applications, respectively. The relationship between the amount of CH₄ emission to the atmosphere from submerged paddy soils with rice plants and the application level (0-8 g kg^-1) of rice straw in soil was investigated in a pot experiment. The increase (Y) in cumulative amounts of CH₄ with the increase in the application level of rice straw was formulated with a logistic curve: Y=k[a/(1 +be^-cx)]; x, application level of rice straw; k, a coefficient for relative CH₄ emission. Since the seasonal variations in coefficients a, b and c in the equation were also formulated as the function of the sum of effective temperature (E, Σ (T-15); T, daily average temperature), Y from any paddy soil by any level of rice straw application was known to be estimated by the equation: Y=k[a(E)/(1 +b(E)e^-c(E)x)]. This revised version was published online in August 2006 with corrections to the Cover Date.     

Methane Production, Oxidation, and Emission from Indian Rice Soils

Experiments were conducted to investigate methane (CH₄) production, oxidation, and emission from flooded rice soils. Incorporation of green manure (Sesbania rostrata) into rice fields led to a several-fold increase in CH₄ emission. A stimulatory effect of organic sources on CH₄ production in soil samples was noticed even under nonflooded conditions. Addition of rice straw at 1% (w/w) to nonflooded soil samples held at –1.5 MPa effected a 230-fold increase in CH₄ production over that in corresponding unamended soil samples at 35 d, as compared with a threefold increase in rice straw-amended soil over that in unamended soil under flooded conditions. In a study involving two experimental field sites differing in water regimes but planted to the same rice cultivar (cv Gayatri) and fertilized with prilled urea at 60 kg N ha^–1, the field plots with deep submergence of around 30 cm (site I) emitted distinctly more CH₄ than did the plots with continuous water depth of 3–6 cm (site II). Likewise, in another incubation study, CH₄ production in flooded soil samples increased with a progressive increase in standing water column from 5 mm to 20 mm. Application of carbamate insecticide, carbofuran, at 2 kg ai ha^–1 to rice fields retarded CH₄ emission through enhanced CH₄ oxidation. Hexachlorocyclohexane was found to inhibit CH₄ emission. The results suggest the need for extensive research efforts to develop technologies with dual objectives of environmental protection and crop productivity.     

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.     

Methane Emission from Rice Fields at Cuttack, India

Methane (CH₄) emission from rice fields at Cuttack (State of Orissa, eastern India) has been recorded using an automatic measurement system (closed chamber method) from 1995–1998. Experiments were laid out to test the impact of water regime, organic amendment, inorganic amendment and rice cultivars. Organic amendments in conjunction with chemical N (urea) effected higher CH₄ flux over that of chemical N alone. Application of Sesbania, Azolla and compost resulted in 132, 65 and 68 kg CH₄ ha^–1 in the wet season of 1996 when pure urea application resulted in 42 kg CH₄ ha^–1. Intermittent irrigation reduced emissions by 15% as compared to continuous flooding in the dry season of 1996. In the wet season of 1995, four cultivars were tested under rainfed conditions resulting in a range of emissions from 20 to 44 kg CH₄ ha^–1. Application of nitrification inhibitor dicyandiamide (DCD) inhibited while Nimin stimulated CH₄ flux from flooded rice compared to that of urea N alone. Wide variation in CH₄ production and oxidation potentials was observed in rice soils tested. Methane oxidation decreased with soil depth, fertilizer-N and nitrification inhibitors while organic amendment stimulated it. The results indicate that CH₄ emission from the representative rainfed ecosystem at the experimental site averaged to 32 kg CH₄ ha^–1 yr^–1.     

Control of microbial methane production in wetland rice fields

Methane emission rates are a function of production, transport and oxidation of CH₄ in the rice field. Production of CH₄ is the prerequisite for any flux. The most important variables that control CH₄ production include soil type, rice variety, temperature, soil redox potential, water management and fertilization with organic carbon or nitrogen. The effects of these variables have empirically been assessed on a macroscopic scale. However, the actual mechanisms by which these variables affect the microbial CH₄ production on a microscopic scale are little understood. The purpose of the present contribution is to review existing knowledge of microbiological data and microscopic processes that are relevant for the control of CH₄ production. These include the flow of carbon and electrons during the anaerobic degradation process, thermodynamic constraints of reactions in-situ and changes in the composition of the microbial community.     

Scientific basis for establishing country greenhouse gas estimates for rice-based agriculture: An Indian case study

A comprehensive scientific assessment of CH₄ budget estimation for Indian rice paddies, based on a decade of measurements in India, is presented. Indian paddy cultivation areas contain soils that have low to medium levels of soil organic carbon. The average seasonally integrated CH₄ flux (E _sif) values calculated from these measurements were 15.3 ± 2.6 g m^–2 for continuously flooded (CF), 6.9 ± 4.3 g m^–2 for intermittently flooded (IF) single aeration (SA) and 2.2 ± 1.5 g m^–2 for IF multiple aeration (MA) rice ecosystems. For CF and IF (MA) rice ecosystems having high soil organic carbon, without organic amendments, the CH₄ flux (E _sif) may be increased by 1.7 times relative to low soil organic carbon, whereas it may enhance by 5.3 for CF if amended organically. Organic amendment and high soil organic carbon paddy areas do not alter the methane budget estimates for India (3.6±1.4 TgY^–1) much, due to their small paddy harvested area. Methane estimated using average emission factors (E _sif) for all paddy water regimes, which include harvested areas having soils with high organic carbon and organic amendments, may give a budget of 5 TgY^–1 for India.     

Properties of rice soils affecting methane production potentials: 1. Temporal patterns and diagnostic procedures

Thirty-one rice soils from different locations in the Philippines were incubated anaerobically for 100 d to determine methane (CH₄) production potentials and to establish relationships between physico-chemical properties of soil and CH₄ production potential. These soils showed pronounced variations in pattern and magnitude of CH₄ production. Total CH₄ production over 100 d incubation ranged from 163 to 837 g CH₄ g^–1 soil. Total N, soil texture (clay and sand fractions mainly) and cation exchange capacity (CEC) of the soils had significant effect on CH₄ production potential. Available K and active Fe content also affected the CH₄ production potentials of various soils. An assessment of CH₄ production with high accuracy could be obtained from soil redox potential (Eh) development during incubation; the difference between initial and equilibrium Eh allowed a computation of CH₄ production with more than 70% reliability. The CH₄ production potentials obtained over long incubation periods could be assessed, with reasonable accuracy, by a relatively short incubation experiments and fewer measurements of CH₄ production. Only three samplings of CH₄ production rate within a short incubation period of 37 d facilitated a prediction of total CH₄ production over 100 d incubation using the following algorithm:P _0-100=99.21+10.79X₄+11.69X₁₆+45.79X₃₇ (R²=0.91; P<0.01),where P _0-100 is the total CH₄ production during 100 d of incubation and X _n is CH₄ production rate at n days of incubation. Longer incubation periods (86 d) were required to achieve a reliability of more than 95%.     

Spatial analysis of methane emissions from paddy soils in China and the potential for emissions reduction

China is a major source of anthropogenic methane (CH₄) emissions because it is the world's largest producer of rice grain, nearly all of which is grown in irrigated paddies. This study sought to reduce the uncertainty in estimates of CH₄ emissions from rice cultivation in China by improving the quantification of the effects of management practices (intermittent drainage and fertilizer inputs) on emissions. These results were spatially extrapolated with digital maps of type of rice using new estimates of organic matter and fertilizer inputs, as well as estimates of soil drainage. The estimated total annual CH₄ emissions from rice agriculture in China in 1990 were 9.9 ± 3.0 × 10¹² g. If intermittent drainage practices were adopted on 33% of the poorly drained soils used for rice cultivation in southern China, the estimated emissions would be 8.9 ± 2.7 × 10¹² g CH₄ yr^-1. Reducing projected organic matter inputs by 50% as a sensitivity analysis to reflect the trend for reduced use of organic fertilizer, resulted in emissions of 9.6 ± 2.9 × 10¹² g CH₄ yr^-1, with 8.7 ± 6 × 10¹² yr^-1 emitted with 33% adoption of intermittent drainage on poorly drained paddies. Although intermittent drainage has been shown to reduce emissions by 50%, the area of rice that is relatively easy to drain and re-flood is not very large. The use of intermittent drainage with better drained paddies is limited because of problems with re-flooding and it is also limited with very poorly drained paddies that are difficult to drain. The 10% emission reduction predicted with 33% adoption of intermittent drainage practices, while not large, is conservative and may be possible to realize. These CH₄ emissions results are relative estimates because the uncertainty remains large due to a lack of emissions measurements from paddies in more regions and a lack of detailed information about organic fertilizer application rates. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.     

Importance of water regime during the non-rice growing period in winter in regional variation of CH4 emissions from rice fields during following rice growing period in China

Rice fields are either continuously flooded or drained in China in the winter (non-rice growth season). Due to great spatial variation of precipitation and temperature, there is a spatial variation of soil moisture in the fields under drained conditions during the winter season. The effect of water regime in winter on CH₄ emissions during the following rice growing period and their regional variation were investigated. Soil moisture in the winter was simulated by DNDC model with daily precipitation and temperature as model inputs. Under the same management during the rice growing period, CH₄ emissions was higher from rice fields flooded, compared to those from fields drained during winter. CH₄ emission from rice fields correlated significantly with simulated soil moisture and with mean precipitation of the preceding winter season. Spatial variation of precipitation in winter and corresponding variations of soil moisture regimes control the regional and annual variation of CH₄ emissions from rice fields in China. Keeping soils drained as much as possible during winter seems to be a feasible option to reduce CH₄ emissions during the following rice growing seasons.     

Combining Upscaling and Downscaling of Methane Emissions from Rice Fields: Methodologies and Preliminary Results

The uncertainty in the methane (CH₄) source strength of rice fields is among the highest of all sources in the global CH₄ budget. Methods to estimate the source strength of rice fields can be divided into two scaling categories: bottom-up (upscaling) and top-down (downscaling). A brief review of upscaling and downscaling methodologies is presented. The combination of upscaling and downscaling methodologies is proposed as a potential method to reduce the uncertainty in the regional CH₄ source strength of rice fields. Some preliminary results based on upscaling and downscaling are presented and the limitations of the approaches are discussed. The first case study focuses on upscaling by using a field-scale model in combination with spatial databases to calculate CH₄ emissions for the island of Java. The reliability of upscaling results is limited by the uncertainty in model input parameters such as soil properties and organic carbon management. Because controlling variables such as harvested rice area may change on relatively short time scales, a land use change model (CLUE) was used to quantify the potential land use changes on Java in the period 1994–2010. The predicted changes were evaluated using the CH₄ emission model. Temporal scaling by coupling land use change models and emission models is necessary to answer policy-related questions on future greenhouse gas emissions. In a downscaling case study, we investigate if inverse modeling can constrain the emissions from rice fields by testing a standard CH₄ from rice scenario and a low CH₄ from rice scenario (80 and 30 Tg CH₄ yr^–1, respectively). The results of this study are not yet conclusive; to obtain fine-resolution CH₄ emission estimates over the Southeast Asian continent, the monitoring network atmospheric mixturing ratios need to be extended and located closer to the continental sources.     

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.     

Effect of drainage in the fallow season on reduction of CH<Subscript>4</Subscript> production and emission from permanently flooded rice fields

Field and incubation experiments were conducted during 2007–2009 to study the effect of drainage in the fallow season on CH₄ production and emission from permanently flooded rice fields. It was found that drainage in the fallow season significantly affected the temporal variations of CH₄ production and emission from permanently flooded rice fields. CH₄ production and emission from permanently flooded rice fields (Treatment FF) mainly occurred during the rice season, where they were found to be much lower in the late fallow season. No CH₄ flux was detected from drained fields (Treatment DF) in the fallow season. Compared with Treatment FF, Treatment DF was delayed not only its onset of CH₄ production and emission, but also appearance of the highest peak of CH₄ production during the rice season. A significant positive relationship was observed between CH₄ production rates of paddy soil and corresponding CH₄ fluxes (P < 0.01). CH₄ production in rice roots was the highest in rate at the rice booting stage, but was obviously lower at the rice tillering, grain filling and ripening stages, and the highest value reached at the same time as the peak of CH₄ production occurred in the paddy soil. Drainage in the fallow season significantly decreased CH₄ production and emission from Treatment FF. Compared with Treatment FF, Treatment DF was about 42–61% lower in mean CH₄ production rate in the paddy soil during the rice season, and was reduced by approximately 56% in mean CH₄ production rate in rice roots. Accordingly, Treatment DF was 20.6–30.2 g CH₄ m⁻², 39–52% lower than Treatment FF in total CH₄ emission during the rice season, and 44–57% lower in annual total CH₄ emission. Rice yield in Treatment DF tended to be 4–7% lower than that in Treatment FF.     

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.