Methane Production, Emission and Mitigation from Rice Fields
Author: Ram Kishor Fagodiya

  • 1. Introduction

    Methane (CH4) is discovered by Allessandro Volta in the year of 1778. It is colorless, odorless and important greenhouse gas (GHG). It has 12.4 years of atmospheric residence time and 25 times higher global warming potential (GWP) than that of carbon dioxide (CO2) and contributing 16% to total anthropogenic GHG emission in 2010 (IPCC, 2014). About half (50.6%) of total methane emission is from agriculture sector alone and out of which rice fields contribute about 20% (Ke et al., 2014). Globally rice is cultivated under four different water managements i.e. irrigated (51%); rainfed (27%); upland (11%) and deepwater (10%) (Wassmann et al. 2000). In Asia, about 80% of rice area is irrigated and under flooded condition. Generally, 5"10 cm standing water is the best environment for growth and development of rice crop. However, this stagnated water in paddy field caused anaerobic environment which favors CH4 production and emission from the paddy field.
  • 2. Methane production in a rice field

    CH4 in rice fields are generally produced from the successive breakdown of complex organic matter i.e. carbohydrates, protein, and lipids under anaerobic conditions. The organic matter in rice field can be added from the terrestrial and aquatic weeds, algal biomass, the litter of rice plants, rice stubbles, microbial biomass, aquatic animals and organic fertilizers. The successive breakdown of complex organic materials is generally achieved by a variety of microorganisms i.e. acidogens (acid producing); hydrogen producing, acetogens (acetate producing); and methanogens (methane producing). This process is carried out in four steps viz. hydrolysis, acidogenesis, acetogenesis, and methanogenesis. The details of each step are discussed below.
  • Hydrolysis : It is the process of conversion of complex organic compounds i.e. cellulose, lipids, Nucleic acids and proteins into simpler monomers i.e. monosaccharides, glycerol and fatty acids, nucleotides, and amino acids by different hydrolytic enzymes i.e. cellulases, amylases, lipases, and proteases. The process of hydrolysis may occur in both aerobic to the anaerobic environment.
  • Acidogenesis : It is a biological reaction in which the simpler monomers (product of hydrolysis) are converted into volatile fatty acids (acetate, propionate, butyrate, and lactate), ammonia, organic acids, alcohols, hydrogen, and carbon dioxide (Cairo and Paris 1988). These monomers are fermented by the acidogens bacteria which can either be strictly anaerobic or facultative aerobic.
  • Acetogenesis : It is a biological reaction in which the volatile fatty acids are converted into acetic acid, carbon dioxide, and hydrogen in the presence of the acetogens present in rice fields. Acetogens are obligatory anaerobic bacteria which generally, support to enhance the biological degradation of organic compounds by coupling the oxidation of H2 and the reduction of CO2 to acetate (Ragsdale and Pierce, 2008). Acetogens bacteria works effectively in the wide range of temperature i.e. 15-50 °C which is the common temperature range of rice fields across all geographical region globally.
  • Methanogenesis : It is a biological reaction in which the acetates are converted into methane and carbon dioxide. The hydrogen which produced during the Acetogenesis process is generally consumed here. The acetate, CO2, and H2 is the precursor for methanogenesis bacteria in rice soils (Mitra et al., 2012). Methanogens are strictly anaerobic obligate. All the methanogens gain energy by producing CH4 from substrates i.e. formate, ethanol, acetate and H2 and CO2. Methanogens are mostly mesophilic and can produce CH4 in the 20-40 °C temperature range. Mostly CH4 producing bacteria are generally use acetate as a C source while 10-30% methanogens use formate and H2/CO2. The Methanococcales, Methanomicrobiales, Methanobacteriales, and Methanomicrobiales are common orders of methanogens.
  • Methane emission from rice field Generally, CH4 is present in as either in the gas phase or as dissolved phase in the rice fields. However, the dominated phase is the gas phase and the dissolved CH4 is present in the low amount due to its low solubility in water at 35 °C. The overall regulation of total CH4 in soil is mainly governed by methanogens (methane producing), methanotrophs (methane consuming) and atmospheric soil CH 4 interactions. Generally, diffusion, ebullition, and plant-mediated CH4 transport are three possible mechanisms for CH4 emission from soil.
  • Diffusion : Diffusion is the process of movement of CH4 molecule from an area of high concentration to an area of low concentration. It is a purely physical process. Generally, it is the mobility of CH4 gas in the active layer. It is very slow process due to the low solubility of CH4 which account very less to total flux of methane emission. The diffusion of CH4 is highest in sandy soils and it is negligible in clay soil (Neue, 1993). In deep water, rice diffusion is active only in the upper water column (Neue, 1993).
  • Ebullition : It is the process of the transportation of methane from soils to atmosphere in the form of gas bubbles. Generally, ebullition process is faster than the diffusion process. It generally occurs when there is the high production of CH4 particularly, during the early growth period of rice, and when there is the high input of organic matter. The emission of CH4 from rice field through the ebullition process is a common mechanism, and it contributes significantly to the total emission CH4 flux (IPCC, 1996). Ebullition is the season dependent process and it contributes between 4 and 100% (seasonal dependency). This process can contribute 26-45% to total CH4 flux at panicle formation and 60-68% at the grain filling stages due to season dependency (Tokida et al. 2013).
  • Plant-mediated transport : It is the primary biological process of CH4 emission from rice field, through aerenchyma tissue. Aerenchyma tissue is generally having air vacuoles to which is helpful for rice plant for the adaptation in the flooded environment and its main purpose is the transportation of oxygen from leaves to root for respiration. Simultaneously, methane is also transported from rhizosphere to the atmosphere through aerenchyma tissue in rice. This process contributes about 80-90% to the total emission of CH 4 flux from the rice field (IPCC, 1996). Primarily CH4 is released through the micropores in the leaf sheath and released secondarily through the stomata in the leaf blade.
  • Factors affecting methane production CH4 production and emission from rice fields is a biological phenomenon, which depends on production and oxidation of CH4, which are controlled by the population of methanogens and methanotrophs. CH4 production generally depends on several interplaying factors like soil organic matter content, soil pH soil texture, soil water content, fertilizers and soil temperature. The emission of CH4 from rice fields are generally affected by seasonal and diurnal variation, elevated ozone and elevated CO2, and management practices such as rice cultivar, nutrient application, and water management.
  • Mitigation of methane emission from rice field The deep understanding of factors affecting the CH4 production and oxidation and emission from rice fields under anaerobic conditions are very helpful in formulations of CH 4 mitigation strategies. The different management of factors water management, soil organic carbon, rice cultivar and fertilizer has been well studied and documented. On basis of findings of different studies, the mitigation strategies for CH 4 from flooded rice fields are discussed below.
  • Water management : Water management one of the most important practices for mitigation of CH4 emission. The intermittent irrigation/flooding (apply irrigation on alternative basis throughout growing season), midseason short-term drainage of about 15-20 days before tillering stage of rice crop, alternate wetting and drying, controlled irrigation (duration and volume of irrigation water) and multiple drainages are effective approaches for mitigation of CH4 emission from the rice field.
  • Plantation methods : CH4 emission is mainly reported from flooded rice condition i.e. the conventional transplanting of rice (TPR). Direct seeded rice (DSR) was a common practice in India before green revolution. Two plantations methods i.e. DSR and system of rice intensification (SRI) are very effective in reduction of CH 4 emission. The DSR could reduce CH4 emissions significantly over conventional transplanting method. 82-98% and 61% reduction in cumulative CH4 emission have been reported in DSR and SRI, respectively over TPR.
  • Varietal selection of rice : The difference in cumulative CH4 emission with the different varieties of rice is well documented in the different part of the globe. This varietal difference is mainly attributed to plant structure, size, number of tillers, metabolism, root exudates and transport of CH4. The selection of suitable rice cultivars plays a big role in CH4 emissions regulations from rice fields.
Fertilization and nitrification inhibitors : CH 4 emission from rice field is also affected by the type of fertilizers, rate of application and methods of application. In general, the management of nitrogen in rice can reduce CH 4 emission by 30-50% as compared to the control. The ammonium-based N fertilizer has the potential for reducing overall CH 4 emission as compared to urea. The application of bio-fertilizers viz. azolla and mycorrhizae are helpful in an increase in crop yield laterally it also helps in reduction of CH 4 emission from rice fields.

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About Author / Additional Info:
I am working as Scientist, ARS at ICAR-CSSRI, Karnal, Haryana since last two years. I have done my MSc and PhD in the discipline of Environmental Science with specialization in climate change. I have worked on climate change adaptation and mitigation.