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:     

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.     

Factors and processes controlling methane emissions from rice fields

Understanding the major controlling factors of methane emissions from ricefields is critical for estimates of source strengths. This paper reports results on the relationship of different plant characteristics and methane fluxes in ricefields. Methane fluxes in ricefields show distinct diel and seasonal variations. Diel variations are mainly controlled by soil solution temperature and the partial pressure of methane. One or two distinct seasonal maxima are observed in irrigated ricefields. The first is governed by methane production from soil and added organic matter and a second at heading is plant derived. During ripening and maturity, root exudation, root porosity and root oxidation power may control methane emission rates. Rice plants play an important role in methane flux. The aerenchyma conduct methane from the bulk soil into the atmosphere. The amount of carbon utilized in methane formation varied among cultivars. A strong positive effect of rice root exudates on methane production imply that cultivar selections for lower methane emissions should not only be based on the gas transport capabilities but also on the quality and quantity of root exudates. Soils show a wide range of methane production potential but no simple correlation between any stable soil property and methane production is evident. Various cultural practices affect methane emissions. Defined aeration periods reduce methane emissions. Soil entrapped methane is released to the atmosphere as a result of soil disturbances. Mineral fertilizers influence methane production and sulfate containing fertilizer decrease methane production. The methane release per m² from different rice ecosystems follow the order: deepwater rice>irrigated rice>rainfed rice. Abatement strategies may only be accepted if the methane source strength of ricefields is reliably discriminated and if mitigation technologies are in accordance with increased rice production and productivity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Evaluation of methane oxidation in rice plant-soil system

Mechanisms of methane oxidation in the plant-soil system of rice were studied in a pot experiment using two cultivars (PSBRc-30 and IR72) at two growth stages (flowering and heading). Methane emission was measured by chambers, while methane oxidation was determined through propylene amendment as an alternative substrate to be propylene oxide (PPO) and acetylene as an inhibitor for methane oxidizing (methanotrophic) bacteria. Cell numbers (methanotrophic and methanogenic bacteria) were determined by the most probable number method. The cultivar PSBRc-30 consistently showed higher methane emission rates than IR72. Methane flux clearly decreased from flowering to heading stages in both cultivars. This observation was largely reflected by trends in the mechanisms involved: either methanogenic cell numbers or activities decreased with plant age while methanotrophic cell numbers or activities generally showed an increasing trend. The methanogenic population was in the order of 10⁵ g^–1 dry soil, while the population of methanotrophs ranged from 10⁴ to nearly 10⁶ g^–1 dry soil. Methanotrophic activity followed the order; root (1.7–2.8 nL PPO g^–1 DM h^–1) > shoot (0.7–2.0) > soil (0–0.4) when the consumption of alternative substrate was related to dry matter. Derived from the estimated amounts of soil and plant biomass in the pot experiment, however, the soil generally accounted for more than 90% of the total methane oxidation. Within the plant segments, methane oxidation activities in the root exceeded those of the shoot by factor of approximately 10.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Effects of Elevated CO2 and Temperature on Methane Production and Emission from Submerged Soil Microcosm

Incubation experiments were conducted under controlled laboratory conditions to study the interactive effects of elevated carbon dioxide (CO₂) and temperature on the production and emission of methane (CH₄) from a submerged rice soil microcosm. Soil samples (unamended soil; soil + straw; soil + straw + N fertilizer) were placed in four growth chambers specifically designed for a combination of two levels of temperature (25 °C or 35 °C) and two levels of CO₂ concentration (400 or 800 mol mol^–1) with light intensity of about 3000 Lx for 16 h d^–1. At 7, 15, 30, and 45 d after incubation, CH₄ flux, CH₄ dissolved in floodwater, subsurface soil-entrapped CH₄, and CH₄ production potential of the subsurface soil were determined. The results are summarized as follows: 1) The amendment with rice straw led to a severalfold increase in CH₄ emission rates, especially at 35 °C. However, the CH₄ flux tended to decrease considerably after 15 d of incubation under elevated CO₂. 2) The amount of entrapped CH₄ in subsurface soil and the CH₄ production potential of the subsurface soil were appreciably larger in the soil samples incubated under elevated CO₂ and temperature during the early incubation period. However, after 15 d, they were similar in the soil samples incubated under elevated or ambient CO₂ levels. These results clearly indicated that elevated CO₂ and temperature accelerated CH₄ formation by the addition of rice straw, while elevated CO₂ reduced CH₄ emission at both temperatures.     

Feasible suppression technique of methane emission from paddy soil by iron amendment

A revolving furnace slag (RFS), which is a by-product of the steel industry, and a spent disposable portable body warmer (PBW), which harnessed the heat of iron oxidation reaction, were used as iron materials. Portions of 4 kg of Coarse and Medium Textured Gley soil were placed into plastic pots (3 L). RFS was added to the pots at the rate of 0 (control), 10, 20, 40, 100 ton ha^–1, while PBW was added at 10 ton ha^–1 only. Methane flux from the potted soil with rice plants and Eh were measured during cropping seasons in 1999 and 2000. In the 1999 experiment, the RFS treatments showed lower Eh values compared with the control, especially at the early period of cultivation, although the RFS was applied to maintain the soil oxidative. The rapid decrease in Eh under high application of RFS may be due to the high pH of the RFS (pH (RFS:H₂O = 1:2.5) was 12.2). However, total methane emission during the cultivation period significantly decreased, about 10%, when 10–40 ton ha^–1 of RFS and 10 ton ha^–1 of PBW were applied. The grain yield was significantly increased, about 30%, when 40 or 100 ton ha^–1 of RFS was applied. This was also partly due to the release of inorganic nutrients from RFS and also from soil. The latter, due to effect of the alkaline RFS on soil. In the 2000 experiment, the pots with soils from 1999 were used without further application of iron materials. The influence of high application of RFS on soil Eh disappeared, compared with 1999. Total methane emission significantly decreased, about 35%, at 20 ton ha^–1 of RFS. However, the increase of grain yield caused by RFS in 1999 was diminished, compared with 1999. Production activity of both methane and carbon dioxide at the RFS treatments were decreased, while methane oxidizing activity was increased. The decrease in total methane emission may be attributed to not only inhibition of methane production but also enhanced methane oxidation. In conclusion, methane emission from paddy soil could be suppressed, over two cropping seasons by single application of RFS without loosing grain yield.     

Urease and nitrification inhibitors to reduce emissions of CH4 and N2O in rice production

Strategies used to reduce emissions of N₂O and CH₄ in rice production normally include irrigation management and fertilization. To date, little information has been published on the measures that can simultaneously reduce both emissions. Effects of application of a urease inhibitor, hydroquinone (HQ), and a nitrification inhibitor, dicyandiamide (DCD) together with urea (U) on N₂O and CH₄ emission from rice growing were studied in pot experiments. These fertilization treatments were carried out in the presence and absence of wheat straw, applied to the soil surface. Without wheat straw addition, in all treatments with inhibitor(s) the emission of N₂O and CH₄ was significantly reduced, as compared with the treatment whereby only urea was applied (control). Especially for the U+HQ+DCD treatment, the total emission of N₂O and CH₄ was about 1/3 and 1/2 of that in the control, respectively. In the presence of wheat straw, the total N₂O emission from the U+HQ+DCD treatment was about 1/2 of that from the control. The total CH₄ emission was less influenced. Wheat straw addition, however, induced a substantial increase in emissions of N₂O and CH₄. Hence, simultaneous application of organic materials with a high C/N ratio and N-fertilizer (e.g. urea) is not a suitable method to reduce the N₂O and CH₄ emission. Application of HQ+DCD together with urea seemed to improve the rice growth and to reduce both emissions. The NO₃ ^–-N content of the rice plants and denitrification of (NO₃ ^–+NO₂ ^–)-N might contribute to the N₂O emission from flooded rice fields.     

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.     

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.     

Effect of rice cultivars and fertilizer management on methane emission in a rice paddy in Beijing

Experiments were conducted during April-Oct. 1994 in a Beijing rice field. Four types of rice varieties have been tested. Large cultivar differences in methane emission flux have been found. Variety 93812 emitted about fivefold more CH₄ than did the Qiuguang variety. An organic amendment plus (NH₄)₂SO₄as the base fertilizer and (NH₄)₂SO₄as the topdressing applied in different amounts and growth stages, compared with no topdressing, reduced methane emission about 58% and increased rice yield about 31.7%. Emission peaks of CH₄ in the tillering stage and reproductive stage were suppressed. A comprehensive strategy could meet both the goal for sustainable rice productivity and methane reduction. Such a strategy includes: 1. Selection of cultivars which have reduced root exudate and litter but increased root mass most of which growing in the oxidized soil layer, cultivars also need an effective number of tillers for optimum yield but with less CH₄transportation ability; 2. Application of organic manure combined with chemical fertilizers, that reduce CH₄ emissions. Fertilizers such as SO₄ ² -or other inhibitors can be maintained for a long period in soil; 3. Adoption of scientific irrigation mode such as flooding-drainage- intermittent irrigation ,that can both increase the rice yield and decrease the CH₄ emission, etc.. This revised version was published online in August 2006 with corrections to the Cover Date.     

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.