Spatial and temporal variability in methane emissions from rice paddies: Implications for assessing regional methane budgets

Growing concern over anthropogenic global climate change has intensified the need to develop accurate budgets of atmospheric methane and other greenhouse gases. Globally, flooded rice cultivation represents a major source of atmospheric methane that is expected to grow with human population. However, current estimates of global methane flux from rice paddies vary by roughly 50%. Understanding the sources of this large variability is critical for developing management strategies for atmospheric methane. Using data collected each growing season from Texas, USA, rice paddies over a 9-year period we examined the spatial and temporal sources of methane flux variability. Using standard deviation of the mean methane flux as a measured of variability, we found that accounting for rice plant height and grain yield reduced spatial variability from 25.2 to 17.7% of the mean. Temporal variability over the entire 9-year data set was 49% of the mean, 71% of which was explained by variations in average rice plant height and total nitrogen fertilizer application. The magnitude of temporal and spatial variability suggests that reliance on single-field studies for determination of global methane budgets may be questionable.     

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.     

Scientific basis for establishing country CH4 emission estimates for rice-based agriculture: A Korea (south) case study

Methane emission inventory, based on several experimental results regarding the effect of water management, application of organic matter, cultural method, and rice type with field growth duration on methane emission factors was established for Korea specific conditions. Annual methane emission was estimated to be 410 Gg CH₄in 1990 and 345 Gg CH₄ in 1999. This annual emission estimated as a Korean specific modification is an average of 17.6 and 32.9%, lower than the OECD default value and maximum scaling of the IPCC default, respectively. This Korean modified average is 4.5 times higher than the minimum scaling of IPCC default.     

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.     

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.     

Methane emission fluxes from rice paddies and the methane productive potential in the soils

We estimated the productive potential of methane in paddy soils by anaerobically incubating soils in the laboratory. In addition, we determined the emission fluxes from the rice paddies through rice plants during the whole growth period, according to the methods suggested by Cicerone & Shetter (1981). The results showed that the total amounts of methane emission from rice paddies were very close to the productive potential of the soils and suggested that the large parts of methane emitted from rice paddies originated from the productive potential of methane. This revised version was published online in August 2006 with corrections to the Cover Date.     

Possible options for mitigating methane emission from rice cultivation

Studies focused on mitigating CH₄ emission from rice paddy fields are summarized and the possibilities and limits that the options might be applied to world's rice cultivation are discussed. The mitigation options are water management, soil amendments, organic matter management, different tillage, rotation, and cultivar selection. Altering water management, in particular promoting midseason aeration by short-term drainage, is one of the most promising strategies, although these practices may be limited to the rice paddy fields where the irrigation system is well prepared. Improving organic matter management by promoting aerobic degradation through composting or incorporating into soil during off-season drained period is another most promising candidate. There are several formidable obstacles to adopt the mitigation options into local rice farming, including limited applicability to different types of rice fields, increasing cost and labor, negative effects on rice yield and soil fertility, and time requirement for practical application. Further studies to verify the mitigation options should focus on the feasibility for local farmers. This revised version was published online in August 2006 with corrections to the Cover Date.     

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

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

Methane and nitrous oxide emissions: an introduction

Methane and nitrous oxide are important greenhouse gases. They contribute to global warming. To a large extent, emissions of methane and nitrous oxide are connected with the intensification of food production. Therefore, feeding a growing world population and at the same time controlling these emissions is a great challenge. Important anthropogenic sources of biogenic methane are wet rice fields, cattle, animal waste, landfills and biomass burning. Important anthropogenic sources of biogenic nitrous oxide are land-use change, fertilizer production and use and manure application. The ultimate objective of the Framework Convention on Climate Change implies a stabilization of greenhouse gas concentrations in the atmosphere. As a small first step towards achieving this objective, the Convention requires the industrialized countries to bring their anthropogenic emissions of greenhouse gases by 2000 back to 1990 levels. It was also agreed that all parties would make national inventories of anthropogenic greenhouse gas emissions and programmes for control (UN, 1992).In this context, in February 1993 an international workshop was held in Amersfoort in the Netherlands to discuss methods in national emission inventories for methane and nitrous oxide, and options for control (Van Amstel, 1993). A selection of the papers presented in Amersfoort that focus on agricultural sources is published in this volume. This introductory chapter gives background information on biogenic sources and sinks of methane and nitrous oxide and options for their control. The goal of the Climate Convention is described as well as the IPCC effort to develop an internationally accepted methodology for the monitoring of greenhouse gas emissions and sinks. Finally, some preliminary results from country inventories are given. It is concluded that a common reporting framework and transparency of the inventories are important to obtain comparable results that can be used for complying with the requirements of the Climate Convention and for facilitating the international debate about appropriate response strategies.     

Methane Production, Oxidation, and Emission from Indian Rice Soils

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

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.     

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

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

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.     

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.     

Sources of methane emitted from paddy fields

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

A Process-Based Model for Methane Emissions from Irrigated Rice Fields: Experimental Basis and Assumptions

In this paper, we review the process-level studies that the authors have performed in rice fields of Texas since 1989 and the development of a semi-empirical model based on these studies. In this model, it is hypothesized that methanogenic substrates are primarily derived from rice plants ad added organic matter. Rates of methane (CH₄) production in flooded rice soils are determined by the availability of methanogenic substrates and the influence of climate, soil, and agronomic factors. Rice plant growth and added carbon control the fraction of CH₄ emitted. The amount of CH₄ transported from the soil to the atmosphere is determined by the rates of production and the emitted fraction. Model calibration against observations from a single rice-growing season in Texas, USA, without organic amendments and with continuous irrigation demonstrated that the seasonal variation of CH₄ emission is regulated by rice biomass and cultivar type. A further validation of the model against measurements from irrigated rice paddy soils in various regions of the world, including Italy, China, Indonesia, Philippines, and the United States, suggests that CH₄ emission can be predicted from rice net productivity, cultivar character, soil texture, temperature, and organic matter amendments.     

Methane Emission from Deepwater Rice Fields in Thailand

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

Effects of rice straw returning methods on N<Subscript>2</Subscript>O emission during wheat-growing season

A field experiment was conducted in Jurong of Nanjing, Jiangsu Province, China from 2006 to 2008 to investigate N₂O emission during the wheat-growing season as affected by various rice straw returning methods prior to wheat cultivation. The study was designed to have four treatments: no rice straw applied (CK), rice straw burnt in situ (RB), rice straw evenly incorporated into the topsoil (RI), rice straw evenly spread over the field as mulch (RM). Results showed that N₂O emission was decreased by 24–29% in Treatment RB and by 3–18% in Treatment RI, but increased by 15–39% in Treatment RM, compared with that in Treatment CK. The contents of soil total C and N at wheat harvest were significantly increased by 7–13% and by 8–12% in Treatment RI, respectively, compared with that in Treatment CK. The wheat grain yield in Treatment RI was 1.0–1.2 times that in the Treatment CK. Based on these results, the best management practice of returning rice straw to the soil prior to wheat cultivation is evenly incorporating rice straw into the topsoil, as the method tended to reduce N₂O emission during the wheat-growing season and increase wheat yield and soil fertility.     

The effect of nitrogen fertilizer use and other practices on methane emission from flooded rice

Methane (CH₄) flux measurements from rice paddy fields in the world and its controlling factors, especially fertilizer effects are summarized. The measurements at rice paddy fields in various locations of the world showed that there were large temporal variations of CH₄ flux and that the flux differed markedly with climate, characteristics of soil and paddy, application of organic matter and mineral fertilizer, and agricultural practices. From the data, it appears that identifying and controlling CH₄ flux factors have a potential to reduce CH₄ emission from rice cultivation. Potential mitigation options include: the form and amount of nitrogen and other chemical fertilizers, the method of fertilizer applications, the application of other chemical amendments, water management and cultivation practices.     

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.