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.     

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.     

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.     

Methane and nitrous oxide emissions from irrigated lowland rice paddies after wheat straw application and midseason aeration

Straw application and midseason drainage play role in controlling methane (CH₄) and nitrous oxide (N₂O) emissions from rice paddy fields, but little information is available on their integrative effect on CH₄ and N₂O emissions. A two-year field experiment was conducted to study the combined effect of timing and duration of midseason aeration and wheat straw incorporation on mitigation of global warming potential (GWP) of CH₄ and N₂O emissions from irrigated lowland rice paddy fields. Results showed that incorporation of wheat straw increased CH₄ by a factor of 5–9 under various water regimes, but simultaneously decreased N₂O emission by 19–42 % during the rice growing season. Without straw incorporation, prolonged aeration significantly reduced the net 100-year GWP of CH₄ and N₂O emissions by 6 %, but also decreased rice production when compared with normal aeration. With straw incorporation, the lowest GWP was found by early aeration, which reduced GWP by 7 and 20 % in 2007 and 2008, respectively. Estimation of net GWPs of CH₄ and N₂O emissions indicated that early midseason drainage with straw incorporation offered the potential to mitigate CH₄ and N₂O emissions from irrigated lowland rice paddies in China.     

Land use legacies and nitrogen fertilization affect methane emissions in the early years of rice field development

Methane (CH₄) emissions are critical to greenhouse gas (GHG) management in agriculture, especially in areas growing rice (Oryza sativa). However, studies on CH₄ emissions and the nitrogen (N) fertilization effect in new rice fields in subtropical regions are still scarce. In this study, we designed a split-plot field experiment in Jiangxi Province, southern China, to examine whether land-use legacies and N fertilization would influence CH₄ emissions. Using static chambers and gas chromatography, we measured CH₄ fluxes in a newly developed rice paddy and a 10-year-old rice paddy. We also measured climatic factors and soil chemical and physical properties to match the flux measurements. The results showed that annual CH₄ emissions in the new rice plots were significantly lower than in the old rice plots regardless of N fertilization. Annual CH₄ emissions increased with the land-use years of rice paddies, following the order of 1 year < 2 years < 3 years < 10 years. N fertilization significantly decreased CH₄ emissions by 36.9% in the first year after the new rice plots were developed, whereas it had no significant effects on CH₄ emissions in the old rice plots or the new rice plots in the second and third years. The results suggest that land-use legacies have significant effects on CH₄ emissions and may influence the N fertilization effect on CH₄ emissions in rice fields in subtropical regions. The findings suggest that land-use legacies should be considered in managing and estimating GHG emissions in rice-growing regions.     

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 emissions from a rice agroecosystem in South China: Effects of water regime, straw incorporation and nitrogen fertilizer

To quantitatively assess the effects of agricultural practices on methane (CH₄) emissions from rice fields, a two-year (2005/2006) field experiment with 2³ factorial designs was conducted to assess the effects of three driving factors on CH₄ emissions in South China: continuously flooded (W0) and mid-season and final drainages (W2), straw (S1) and nitrogen fertilizer (N1) applications and their controls (S0, N0). Results showed that averaged across all the treatments about 75?% of the seasonal total CH₄ occurred between the rice transplanting and booting stage, while constituted only 33?% of the seasonal total rice biomass during the same period. Averaged across the treatments in 2006, CH₄ emissions were substantially decreased by mid-season drainage up to 60?% (15.6 vs. 39.0?g?m^?2). The decreased CH₄ emissions represented almost all of the decrease in the total global warming potentials. Without straw incorporation CH₄ emissions substantially decreased up to 59?% (15.9 vs. 38.7?g?m^?2). The stimulating effects of straw were significantly greater for W0 than W2 treatment, being also greater in the 2005 than in the 2006 season. A significant inter-annual difference in CH₄ emissions was found when averaged across straw incorporation and N fertilizer applications for the W2 treatment (42.8 and 15.4?g?m^?2 in 2005 and 2006, respectively). Moreover, N fertilization has no significant effect on CH₄ emissions in this study. Our results demonstrate that although straw effects varied greatly with specific management, both straw managements and water regimes are equally important driving factors and thus being the most promising measures attenuating CH₄ emissions while achieving sustainable rice production.     

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.     

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.     

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.     

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. 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.     

Comparing a process-based agro-ecosystem model to the IPCC methodology for developing a national inventory of N2O emissions from arable lands in China

Nations are now obligated to assess their greenhouse gas emissions under the protocols of Article 4 of the United Nations Framework Convention on Climate Change. The IPCC has developed `spreadsheet-format' methodologies for countries to estimate national greenhouse gas emissions by economic sector. Each activity has a magnitude and emission rate and their product is summed over all included activities to generate a national total (IPCC, 1997). For N₂O emissions from cropland soils, field studies have shown that there are important factors that influence N₂O emissions at specific field sites that are not considered in the IPCC methodology. We used DNDC, a process-oriented agroecosystem model, to develop an unofficial national inventory of direct N₂O emissions from cropland in China. We assembled county-scale data on soil properties, daily weather, crop areas, N-fertilizer use, livestock populations (for manure inputs to cropland), and agricultural management for the 2500 counties in mainland China. Total 1990 cropland area was 0.95 million km². Total N-fertilizer use in China in 1990 was 16.6 Tg N. The average fertilization rate was 175 kg N ha⁻¹ cropland. One-year simulations with DNDC were run for each crop type in each county to generate estimates of direct N₂O emissions from soils. National totals were the sum of results for all crop simulations across all counties. Baseline simulations estimated that total N₂O emission from arable land in China in 1990 was 0.31 Tg N₂O-N yr⁻¹. We also ran simulations with zero N-fertilizer input; the difference between the zero-fertilizer and the baseline run is an estimate of fertilizer-induced N₂O emissions. The fertilizer-induced emission was 0.13 Tg N₂O-N yr⁻¹, about 0.8% of total N-fertilizer use (lower than the mean but within the IPCC range of 1.25±1.0%). We compared these results to our estimates of county-scale IPCC methodology emissions. Total emissions were similar but geographical patterns were quite different. This revised version was published online in August 2006 with corrections to the Cover Date.     

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 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.     

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 and carbon dioxide emissions and nitrogen turnover during liquid manure storage

Animal slurry stored in-house and outside is a significant source of atmospheric methane (CH₄). The CH₄ source strength of stored slurry is greatly affected by temperature. To improve emission calculations on a global scale there is a need for knowledge about the relationship between production of CH₄ in slurry and temperature. In this study, the filling of slurry channels was reproduced in the laboratory by gradually filling 1 m-high PVC vessels during 9 days followed by incubation for 100–200 days. A preliminary test showed that little CH₄ was produced from animal slurry during 10 days of incubation at 20°C, if no inoculum (slurry incubated anaerobically at the test temperature for 1.5–2 months) was present. However, the addition of 7.6% inoculum supported an immediate production of CH₄. Vessels amended with inoculum and gradually filled with cattle or pig slurry were then incubated at 10, 15 and 20°C. Methane production from stored pig and cattle slurry was not significant at temperatures below 15°C, where CO₂ was the main product of decomposition processes. In contrast, the anaerobic production of CH₄ was high and significant relative to the production of CO₂ at 20°C. Peak emissions of CH₄ averaging 0.012 and 0.02 g C h⁻¹ kg⁻¹ volatile solids (VS) were reached within about 10 days at 10 and 15°C, respectively. At 20°C, the emission of CH₄ from pig slurry was about 0.01 g C h⁻¹ kg⁻¹ for 10 days, and thereafter emissions increased to about 0.10 g C h⁻¹ kg⁻¹ VS. For cattle slurry a peak emission of 0.08 g C h⁻¹ kg⁻¹ VS was measured after 180 days. Degradation of organic nitrogen (N) in cattle slurry was related to the reduction of organic material as reflected in CO₂ and CH₄ emission. The mineralization of organic N during storage represented 10–80% of organic N in cattle slurry, and 40–80% of the organic N in pig slurry.     

Methane and nitrous oxide emissions from rice paddy soil as influenced by timing of application of hydroquinone and dicyandiamide

A pot trial and a field experiment were conducted to study the effect of timing of application of nitrification inhibitor dicyandiamide (DCD) on N₂O and CH₄ emissions from rice paddy soil. Four treatments including Treatment CK₁, DCD-1 (application of DCD with basal fertilizer), DCD-2 (DCD with tillering fertilizer) and DCD-3 (DCD with panicle initiation fertilizer), were designed and implemented in pot experiment. Total N₂O and CH₄ emissions from DCD-treated soils were decreased profoundly when compared with that from urea alone (P < 0.05). Application of DCD together with basal fertilizer, tillering fertilizer and panicle initiation fertilizer reduced N₂O emission by 8, 30 and 2%, respectively, while those for CH₄ were 21, 8 and 1%. The field experiment with four treatments was carried out subsequently, and a kind of urease inhibitor hydroquinone (HQ) was also incorporated with DCD simultaneously. The combined use of HQ and DCD with basal fertilizer, tillering fertilizer and panicle initiation fertilizer decreased N₂O emissions by 24, 56 and 17%, respectively, while those for CH₄ were 35, 19 and 12%. N₂O emission was effectively reduced by the inhibitor(s) applied with tillering fertilizer before midseason aeration, while CH₄ emission was effectively decreased by the combined use of inhibitor(s) with basal fertilizer before rice transplanting. Furthermore, an increase in rice yield and a reduction of total global warming potential (GWP) of CH₄ and N₂O could be achieved by using inhibitor(s) in rice paddy field.     

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.     

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.