R.S. Lantin

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 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an increasing trend in emission rates reaching maximum toward grain ripening, and a second flush after water is withdrawn prior to harvesting.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.

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.

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.

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 production capacities of different rice soils derived from inherent and exogenous substrates

Methane production rates were determined at weekly intervals during anaerobic incubation of eleven Philippine rice soils. The average production rates at 25 °C varied in a large range from 0.03 to 13.6 μg CH4 g(d.w. soil)-1d-1. The development of methane production rates derived from inherent substrate allowed a grouping of soils in three classes: those with instantaneous development, those with a delay of approximately two weeks, and those with a suppression of methane production of more than eight weeks. Incubation at 30 and 35 °C increased production capacities of all soils, but the grouping of soils was still maintained. The Arrhenius equation provided a good fit for temperature effects on methane production capacities except for those soils with suppressed production. Acetate amendment strongly enhanced methane production rates and disintegrated the grouping. However, the efficiencies in converting acetate to methane differed among soils. Depending on the soil, 16.5–66.7% of the added acetate was utilized within five weeks incubation at 25 °C.Correlation analyses of methane production (over eight weeks) and physico-chemical soil parameters yielded significant correlations for the concentrations of organic carbon (R2 = 0.42) and organic nitrogen (R2 = 0.52). Correlation indices could substantially be enhanced by using the enriched fraction of organic carbon (R2 = 0.94) and organic nitrogen (R2 = 0.77), i.e. the differential between topsoil and subsoil concentrations of the respective compounds. The enriched organic material in the topsoil corresponds to the biologically active fraction and thus represents a good indicator of methane production derived from inherent substrate. The best indicators of the conversion rate of acetate in different soils were pH-value (R2 = 0.56) and organic carbon content (R2 = 0.52).Apparently, soil properties affect methane production through various pathways. Inherent organic substrate represents a considerable source of methane in some soils and is negligible in others. Likewise, soils also differ regarding the response to exogenous substrate. Both mechanisms yield in a distinct spatial variability of methane production in rice soils.

A sampling technique for the determination of dissolved methane in soil solution

A sampling technique was developed to sample floodwater and soil solution from wetland ricefields for the determination of dissolved CH4. The method was compared with the soil core method used in the measurement of entrapped CH4. This was done to assess if dissolved CH4 determination could be an alternative to soil-entrapped CH4 measurements since the latter is time-consuming, laborious and destructive in nature. The dynamics of both dissolved CH4 and entrapped CH4 follow the same seasonal pattern. They have the same degree of spatial and temporal variability. However, the sampling procedure developed for the determination of dissolved CH4 is relatively simple, easy and convenient compared to that for soil-entrapped CH4 measurements. It also allows in-situ solution sampling at different soil depths. Therefore, it is recommended that dissolved CH4 measurements can be an alternative to soil-entrapped CH4 determinations.

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.

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.

Release of entrapped methane from wetland rice fields upon soil drying

Methane emissions from Philippine rice paddies, fertilized with either urea or green manure, were monitored for several weeks after harvesting the dry and the wet season crops of 1992. The fields were still flooded during harvest but irrigation was stopped after harvest and the fields were allowed to evaporatively dry while CH4 emissions were monitored with a closed chamber technique. In all plots we observed a sudden, strong increase of CH4 emissions to the atmosphere for 2 to 4 days just after the soil fell dry. As soil drying continued, the soils began to crack and CH4 emissions decreased to nil. The release of CH4 during soil drying was observed for fields on three different soil types and both for urea or organically manured rice fields in both seasons. The absolute amounts of CH4 emitted during soil drying differed greatly depending on fertilizer treatment. However, the ratio between the amount of CH4 released upon soil drying and CH4 emitted during the growing season was quite constant (0.10 ±0.04). This suggests that about 10% of the CH4 emitted during a full rice crop cycle is released during drying of the fields and thus needs to be included in estimates of the total CH4 emission from rice agriculture.