Crop Management Affecting Methane Emissions from Irrigated and Rainfed Rice in Central Java (Indonesia)

Methane (CH₄) emissions were determined from 1993 to 1998 using an automated closed chamber technique in irrigated and rainfed rice. In Jakenan (Central Java), the two consecutive crops encompass a gradient from low to heavy rainfall (wet season crop) and from heavy to low rainfall (dry season crop), respectively. Rainfed rice was characterized by very low emission at the onset of the wet season and the end of the dry season. Persistent flooding in irrigated fields resulted in relatively high emission rates throughout the two seasons. Average emission in rainfed rice varied between 19 and 123 mg CH₄ m^–2 d^–1, whereas averages in irrigated rice ranged from 71 to 217 mg CH₄ m^–2 d^–1. The impact of organic manure was relatively small in rainfed rice. In the wet season, farmyard manure (FYM) was completely decomposed before CH₄ emission was initiated; rice straw resulted in 40% increase in emission rates during this cropping season. In the dry season, intensive flooding in the early stage promoted high emissions from organically fertilized plots; seasonal emissions of FYM and rice straw increased by 72% and 37%, respectively, as compared with mineral fertilizer. Four different rice cultivars were tested in irrigated rice. Average emission rates differed from season to season, but the total emissions showed a consistent ranking in wet and dry season, depending on season length. The early-maturing Dodokan had the lowest emissions (101 and 52 kg CH₄ ha^–1) and the late-maturing Cisadane had the highest emissions (142 and 116 kg CH₄ ha^–1). The high-yielding varieties IR64 and Memberamo had moderately high emission rates. These findings provide important clues for developing specific mitigation strategies for irrigated and rainfed rice.     

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.     

Characterization of Methane Emissions from Rice Fields in Asia. I. Comparison among Field Sites in Five Countries

The Interregional Research Program on Methane Emissions from Rice Fields established a network of eight measuring stations in five Asian countries. These stations covered different environments and encompassed varying practices in crop management. All stations were equipped with a closed chamber system designed for frequent sampling and long-term measurements of emission rates. Even under identical treatment--e.g., continuous flooding and no organic fertilizers--average emission rates varied from 15 to 200 kg CH₄ ha^–1 season^–1. Low temperatures limited CH₄ emissions in temperate and subtropical stations such as northern China and northern India. Differences observed under given climates, (e.g., within the tropics) indicated the importance of soil properties in regulating the CH₄ emission potential. However, local variations in crop management superseded the impact of soil- and climate-related factors. This resulted in uniformly high emission rates of about 300 kg CH₄ ha^–1 season^–1 for the irrigated rice stations in the Philippines (Maligaya) and China (Beijing and Hangzhou). The station in northern India (Delhi) was characterized by exceptionally low emission rates of less than 20 kg CH₄ ha^–1 season^–1 under local practice. These findings also suggest opportunities for reducing CH₄ emission through a deliberate modification of cultural practice for most irrigated rice fields.     

Methane Emission from Deepwater Rice Fields in Thailand

Field experiments were conducted in the Prachinburi Rice Research Center (Thailand) from 1994 to 1998. The major objective was to study methane (CH₄) emission from deepwater rice as affected by different crop management. Irrigated rice was investigated in adjacent plots, mainly for comparison purposes. The 4-yr average in CH₄ emission from deepwater rice with straw ash (burned straw) treatment was 46 mg m^–2d^–1 and total emission was 98 kg ha^–1 yr^–1. For irrigated rice, the average emission rate and total emission for the straw ash treatment was 79 mg m^–2 d^–1 and 74 kg ha^–1 yr^–1, respectively. Low emission rates may partially be related to acid sulfate soil of the experimental site. Without organic amendment, the seasonal pattern of CH₄ emission from deepwater rice was correlated with an increase in biomass of rice plants. Emission rates from deepwater rice depend on the production of biomass and the straw management as well. Methane emission was greatest with straw incorporation, followed by straw compost incorporation, zero-tillage with straw mulching, and least with straw ash incorporation. The seasonal pattern of CH₄ ebullition in deepwater rice was consistent with seasonal emission, and total ebullition corresponded to 50% of total emission. Dissolved CH₄ concentrations in the surface soil (0–5 cm) were similar to those in the subsoil (5–15 cm), and the seasonal fluctuation of dissolved CH₄ was also consistent with the seasonal CH₄ emission. Increase in plant density and biomass of irrigated rice grown by pregerminated seed broadcasting enhanced CH₄ emission as compared with transplanting.     

The Effects of Cultural Practices on Methane Emission from Rice Fields

A field experiment was conducted in a clayey soil to determine the effects of cultural practices on methane (CH₄) emissions from rice fields. The factors evaluated were a) direct seeding on dry vs wet soil, b) age of transplanted seedlings (8 d old and 30 d old), and c) fall vs spring plowing. Methane emissions were measured weekly throughout the rice-growing season using a closed static chamber technique. Transplanted 8-d-old seedlings showed the highest emission of 42.4 g CH₄ m^–2 season^–1, followed by transplanted 30-d-seedlings (40.3 g CH₄ m^–2 season^–1), and direct seeding on wet soil (37.1 g CH₄ m^–2 season^–1). Direct seeding on dry soil registered the least emission of 26.9 g CH₄ m^–2 season^–1. Thus transplanting 30-d-old seedlings, direct seeding on wet soil, and direct seeding on dry soil reduced CH₄ emission by 5%, 13%, and 37%, respectively, when compared with transplanting 8-d-old seedlings. Methane emission under spring plowing was 42.0 g CH₄ m^–2 season^–1 and that under fall plowing was 31.3 g CH₄ m^–2 seasons^–1. The 26% lower emission in the field plowed in spring was caused by degradation of organic matter over the winter.     

Using a Crop/Soil Simulation Model and GIS Techniques to Assess Methane Emissions from Rice Fields in Asia. IV. Upscaling to National Levels

The process-based crop/soil model MERES (Methane Emissions from Rice EcoSystems) was used together with daily weather data, spatial soil data, and rice-growing statistics to estimate the annual methane (CH₄) emissions from China, India, Indonesia, Philippines, and Thailand under various crop management scenarios. Four crop management scenarios were considered: (a) a 'baseline' scenario assuming no addition of organic amendments or field drainage during the growing season, (b) addition of 3,000 kg DM ha^–1 of green manure at the start of the season but no field drainage, (c) no organic amendments but drainage of the field for a 14-d period in the middle of the season and again at the end of the season, and (d) addition of 3,000 kg DM ha^–1 of green manure and field drainage in the middle and end of the season. For each scenario, simulations were made at each location for irrigated and rainfed rice ecosystems in the main rice-growing season, and for irrigated rice in the second (or 'dry') season. Overall annual emissions (Tg CH₄ yr^–1) for a province/district were calculated by multiplying the rates of CH₄ emission (kg CH₄ ha^–1 yr^–1) by the area of rice grown in each ecosystem and in each season obtained from the Huke and Huke (1997) database of rice production. Using the baseline scenario, annual CH₄ emissions for China, India, Indonesia, Philippines, and Thailand were calculated to be 3.73, 2.14, 1.65, 0.14, and 0.18 Tg CH₄ yr^–1, respectively. Addition of 3,000 kg DM ha^–1 green manure at the start of the season increased emissions by an average of 128% across the five countries, with a range of 74–259%. Drainage of the field in the middle and at the end of the season reduced emissions by an average of 13% across the five countries, with a range of –10% to –39%. The combination of organic amendments and field drainage resulted in an increase in emissions by an average of 86% across the five countries, with a range of 15–176%. The sum of CH₄ emissions from these five countries, comprising about 70% of the global rice area, ranged from 6.49 to 17.42 Tg CH₄ yr^–1, depending on the crop management scenario.     

Characterization of Methane Emissions from Rice Fields in Asia. III. Mitigation Options and Future Research Needs

Methane (CH₄) emissions from rice fields were determined using automated measurement systems in China, India, Indonesia, Thailand, and the Philippines. Mitigation options were assessed separately for different baseline practices of irrigated rice, rainfed, and deepwater rice. Irrigated rice is the largest source of CH₄ and also offers the most options to modify crop management for reducing these emissions. Optimizing irrigation patterns by additional drainage periods in the field or an early timing of midseason drainage accounted for 7–80% of CH₄ emissions of the respective baseline practice. In baseline practices with high organic amendments, use of compost (58–63%), biogas residues (10–16%), and direct wet seeding (16–22%) should be considered mitigation options. In baseline practices using prilled urea as sole N source, use of ammonium sulfate could reduce CH₄ emission by 10–67%. In all rice ecosystems, CH₄ emissions can be reduced by fallow incorporation (11%) and mulching (11%) of rice straw as well as addition of phosphogypsum (9–73%). However, in rainfed and deepwater rice, mitigation options are very limited in both number and potential gains. The assessment of these crop management options includes their total factor productivity and possible adverse effects. Due to higher nitrous oxide (N₂O) emissions, changes in water regime are only recommended for rice systems with high baseline emissions of CH₄. Key objectives of future research are identifying and characterizing high-emitting rice systems, developing site-specific technology packages, ascertaining synergies with productivity, and accounting for N₂O emissions.     

Methane Emissions from Irrigated Rice Fields in Northern India (New Delhi)

Methane (CH₄) emission fluxes from rice fields as affected by water regime, organic amendment, and rice cultivar were measured at the Indian Agricultural Research Institute, New Delhi, using manual and automatic sampling techniques of the closed chamber method. Measurements were conducted during four consecutive cropping seasons (July to October) from 1994 to 1997. Emission rates were very low (between 16 and 40 kg CH₄ m^–2 season^–1) when the field was flooded permanently. These low emissions were indirectly caused by the high percolation rates of the soil; frequent water replenishment resulted in constant inflow of oxygen in the soil. The local practice of intermittent flooding, which encompasses short periods without standing water in the field, further reduced emission rates. Over the course of four seasons, the total CH₄ emission from intermittently irrigated fields was found to be 22% lower as compared with continuous flooding. The CH₄ flux was invariably affected by rice cultivar. The experiments conducted during 1995 with one cultivar developed by IRRI (IR72) and two local cultivars (Pusa 169 and Pusa Basmati) showed that the average CH₄ flux from the intermittently irrigated plots without any organic amendment ranged between 10.2 and 14.2 mg m^–2 d^–1. The impact of organic manure was tested in 1996 and 1997 with varieties IR72 and Pusa 169. Application of organic manure (FYM + wheat straw) in combination with urea (1:1 N basis) enhanced CH₄ emission by 12–20% as compared with fields treated with urea only. The site in New Delhi represents one example of very low CH₄ emissions from rice fields. Emissions from other sites in northern India may be higher than those in New Delhi, but they are still lower than in other rice-growing regions in India. The practice of intermittent irrigation--in combination with low organic inputs--is commonly found in northern India and will virtually impede further mitigation of CH₄ emissions in significant quantities. In turn, the results of this study may provide clues to reduce emissions in other parts of India with higher baseline emissions.     

Methane Emissions and Mitigation Options in Irrigated Rice Fields in Southeast China

Methane (CH₄) emissions from rice fields were monitored in Hangzhou, China, from 1995 to 1998 by an automatic measurement system based on the "closed chamber technique." The impacts of water management, organic inputs, and cultivars on CH₄ emission were evaluated. Under the local crop management system, seasonal emissions ranging from 53 to 557 kg CH₄ ha^–1 were observed with an average value of 182 kg CH₄ ha^–1. Methane emission patterns differed among rice seasons and were generally governed by temperature changes. Emissions showed an increasing trend in early rice and a decreasing trend in late rice. In a single rice field, CH₄ emissions increased during the first half of the growing period and decreased during the second half. Drainage was a major modifier of seasonal CH₄ emission pattern. The local practice of midseason drainage reduced CH₄ emissions by 44% as compared with continuous flooding; CH₄ emissions could further be reduced by intermittent irrigation, yielding a 30% reduction as compared with midseason drainage. The incorporation of organic amendments promoted CH₄ emission, but the amount of emission varied with the type of organic material and application method. Methane emission from fields where biogas residue was applied was 10–16% lower than those given the same quantity (based on N content) of pig manure. Rice straw applied before the winter fallow period reduced CH₄ emission by 11% as compared with that obtained from fields to which the same amount of rice straw was applied during field preparation. Broadcasting of straw instead of incorporation into the soil showed less emission (by 12%). Cultivar selection influenced CH₄ emission, but the differences were smaller than those among organic treatments and water regimes. Modifications in water regime and organic inputs were identified as promising mitigation options in southeast China.     

A Four-Year Record of Methane Emissions from Irrigated Rice Fields in the Beijing Region of China

Methane (CH₄) emissions from irrigated rice fields were measured using an automatic sampling-measuring system with a closed chamber method in 1995–98. Average emission rates ranged from 11 to 364 mg m^–2 d^–1 depending on season, water regime, and fertilizer application. Crop management typical for this region (i.e., midseason drainage and organic/mineral fertilizer application) resulted in emission of 279 and 139 mg CH₄ m^–2 d^–1 in 1995 and 1997, respectively. This roughly corresponds to emissions observed in other rice-growing areas of China. Emissions were very intense during the tillering stage, which accounted for 85% of total annual emission, but these were suppressed by low temperature in the late stage of the season. The local irrigation practice of drying at mid-season reduced emission rates by 23%, as compared with continuous flooding. Further reduction of CH₄ emissions could be attained by (1) alternate flooding/drying, (2) shifting the drainage period to an earlier stage, or (3) splitting drainage into two phases (of which one is in an earlier stage). Emission rates were extremely sensitive to organic amendments: seasonal emissions from fields treated with pig manure were 15–35 times higher than those treated with ammonium sulfate in the corresponding season. On the basis of identical carbon inputs, CH₄ emission potential varied among organic amendments. Rice straw had higher emissions than cattle manure but lower emissions than pig manure. Use of cultivar Zhongzhuo (modern japonica) reduced CH₄ emission by 56% and 50%, in 1995 and 1997, respectively, as compared with Jingyou (japonica hybrid) and Zhonghua (tall japonica). The results give evidence that CH₄ emissions from rice fields in northern China can be reduced by a package of crop management options without affecting yields.     

Methane Emission from Irrigated and Intensively Managed Rice Fields in Central Luzon (Philippines)

Methane (CH₄) emissions were measured with an automated system in Central Luzon, the major rice producing area of the Philippines. Emission records covered nine consecutive seasons from 1994 to 1998 and showed a distinct seasonal pattern: an early flush of CH₄ before transplanting, an increasing trend in emission rates reaching maximum toward grain ripening, and a second flush after water is withdrawn prior to harvesting. The local practice of crop management, which consists of continuous flooding and urea application, resulted in 79–184 mg CH₄ m^–2 d^–1 in the dry season (DS) and 269–503 mg CH₄ m^–2 d^–1 in the wet season (WS). The higher emission in the WS may be attributed to more labile carbon accumulation during the dry fallow period before the WS cropping as shown by higher % organic C. Incorporation of sulfate into the soil reduced CH₄ emission rates. The use of ammonium sulfate as N fertilizer in place of urea resulted in a 25–36% reduction in CH₄ emissions. Phosphogypsum reduced CH₄ emissions by 72% when applied in combination with urea fertilizer. Midseason drainage reduced CH₄ emission by 43%, which can be explained by the influx of oxygen into the soil. The practice of direct seeding instead of transplanting resulted in a 16–54% reduction in CH₄ emission, but the mechanisms for the reducing effect are not clear. Addition of rice straw compost increased CH₄ emission by only 23–30% as compared with the 162–250% increase in emissions with the use of fresh rice straw. Chicken manure combined with urea did not increase CH₄ emission. Fresh rice straw has wider C/N (25 to 45) while rice straw compost has C/N = 6 to 10 and chicken manure has C/N = 5 to 8. Modifications in inorganic and organic fertilizer management and water regime did not adversely affect grain yield and are therefore potential mitigation options. Direct seeding has a lower yield potential than transplanting but is getting increasingly popular among farmers due to labor savings. Combined with a package of technologies, CH₄ emission can best be reduced by (1) the practice of midseason drainage instead of continuous flooding, (2) the use of sulfate-containing fertilizers such as ammonium sulfate and phosphogypsum combined with urea; (3) direct seeding crop establishment; and (4) use of low C/N organic fertilizer such as chicken manure and rice straw compost.     

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.     

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 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.     

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.     

Refinement in methodologies for methane budget estimation from rice paddies

To reduce the involved uncertainties in the methane budget estimation from rice paddy fields, the methodologies of methane budget estimation have been revised mainly on the basis of measurements undertaken in the Methane Asia Campaign (MAC-98). Studies from other continuous measurements of methane emission from rice paddy fields over last few years in other Asian countries were also used. The Asian Development Bank (ADB) sponsored Methane Asia Campaign (MAC-98) in which India, China, Indonesia, Philippines, Vietnam and Thailand participated during 1998–99.The resulting CH₄ measurements have shown that apart from water management, soil organic carbon also plays a significant role in determination of methane emission factors from rice paddy fields. The available data from participating countries reveal that paddy soils can be broadly classified into low soil organic carbon (<0.7%C) and high soil organic carbon (>0.7% C) classes which show average methane emission factors of 12 (5–29) and 36 (22–57) g m^–2 respectively for continuously flooded (CF) fields without organic amendments compared to the IPCC–96 emission factor of 20 g m^–2. Similarly for irrigated paddy fields with intermittently flooded multiple aeration (IF-MA) without organic amendments, the MAC-98 gives average emission factors of 2 (0.06–3) and 6 (0.6–24) g m^–2, respectively, for low and high organic carbon soils compared to IPCC–96 emission factor of 4 (0–10) g m^–2. Incorporation of soil organic carbon along with classification based on water management and organic amendments in the estimation of CH₄ emissions from rice paddy fields yields more characteristic emission factors for low and high organic carbon soils and is, therefore, capable of reducing uncertainties.     

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.     

Estimation of Regional Methane Emission from Rice Fields Using Simple Atmospheric Diffusion Models

Two atmospheric diffusion models, the box model ad the ATDL (Atmospheric Turbulent and Diffusion Laboratory) model, were used to calculate regional methane (CH₄) emissions of rice fields in the Beijing area. Compared with conventional closed chamber measurements, the box model overestimated CH₄ emission because of meteorological conditions--the ground inverse layer was not favorable for the application of the model during the rice-growing season. The ATDL model, on the other hand, handled this unfavorable meteorological condition and gave reasonable CH₄ emission estimates (about 6.1–8.5 mg m^–2 h^–1) close to conventional measurements (about 0.3–14.3 mg m^–2 h^–1) in June, a period generally characterized by significant CH₄ emission from rice fields. In September, CH₄ emission as measured with closed chambers was negligible (about 0–0.3 mg m^–2 h^–1), but the ATDL model still calculated it to be about 2.8–5.3 mg m^–2 h^–1, albeit at a low level and considerably below the June emission level. This discrepancy cannot be explained at present and needs further stuy. Most likely causes are measurement artifacts and/or the presence of minor local CH₄ sources (ditches, field depressions) in the study area. The application of atmospheric diffusion models for regional CH₄ emission estimation depends greatly on meteorological conditions. Moreover, the models tend to give much more reliable results during periods of rather high CH₄ emission. This coincides with the time that such regional CH₄ emission estimates are most valuable. The atmospheric diffusion models complement the closed chamber method by providing integrated CH₄ emission estimates from 1–100-km² rice areas. Detailed information about agricultural management of rice fields and other potential CH₄ sources within the study region are necessary to better understand the integrated regional emission estimates.     

Effect of timing and duration of midseason aeration on CH<Subscript>4</Subscript> and N<Subscript>2</Subscript>O emissions from irrigated lowland rice paddies in China

Midseason aeration (MSA) of rice paddy fields functions to mitigate CH₄ emission by a large margin, while simultaneously promoting N₂O emission. Alternation of timing and duration of MSA would affect CH₄ and N₂O emissions from intermittently irrigated rice paddies. A pot trial and a field experiment were conducted to study the effect of timing and duration of MSA on CH₄ and N₂O emissions from irrigated lowland rice paddy soils in China. Four different water regimes, i.e., early aeration, normal aeration (the same as the local practice in timing and duration of aeration), delayed aeration, and prolonged aeration, were adopted separately and compared with respect to global warming potential (GWP) of CH₄ and N₂O emissions and rice yields as well. Total emission of CH₄ from the rice fields ranged from 28.6 to 64.1 kg CH₄ ha⁻¹, while that of N₂O did from 1.71 to 6.30 kg N₂O–N ha⁻¹ during the study periods. Compared with the local practice, early aeration reduced CH₄ emission by 13.3–16.2% and increased N₂O emission by 19.1–68.8%, while delayed aeration reduced N₂O emission by 6.8–26.0% and increased CH₄ emission by 22.1–47.3%. The lowest GWP of CH₄ and N₂O emissions occurred in prolonged aeration treatment, however, rice grain yield was reduced by 15.3% in this condition when compared with normal practice. It was found in the experiments that midseason aeration starting around D 30 after rice transplanting, just like the local practice, would optimize rice yields while simultaneously limiting GWPs of CH₄ and N₂O emissions from irrigated lowland rice fields in China.     

Effect of soil water contents in the non-rice growth season on CH4 emission during the following rice-growing period

A pot experiment was carried out to investigate the effect of soil water content in the non-rice growth season (winter season) on CH₄ emission during the following rice-growing period. The results showed that CH₄ fluxes increased significantly with the increase of soil water content in the winter season, except air-dry water condition. The mean CH₄ fluxes of treatments with soil water contents in the winter of 3.89–5.37% (air-dry), 25–35%, 50–60%, 75–85% and 107% (flooded) of field water capacity (FWC) were 13.04, 4.04, 8.61, 13.26 and 20.47 mg m^–2 h^–1, respectively. Antecedent soil water contents also markedly affected temporal variation patterns of CH₄ fluxes and soil redox potential (E_h) during the rice-growing period. The higher soil water contents in the winter season were, the quicker soil E_h decreased, and the earlier CH₄ emission occurred after rice transplanting, except air-dry water condition. Though the seasonal mean CH₄ flux was significantly correlated with the seasonal mean soil E_h, the seasonal variation of CH₄ fluxes was not always significantly correlated with soil E_h. For the treatment flooded in the fallow season, there was no significant correlation between CH₄ flux and soil E_h, but there was significant correlation between CH₄ flux and soil temperature during rice growth season. In contrast, for the other four treatments, it was soil E_h, not soil temperature that significantly affected the temporal variation of CH₄ emissions. Soil water contents in the fallow season significantly influenced concentrations of soil labile organic carbon (including undecomposed plant debris), active Fe and Mn immediately before rice transplanting. The mean CH₄ fluxes during rice-growing period were significantly correlated with soil labile organic carbon contents (positively) and contents of soil active Fe and Mn (negatively).