Methane emission in paddy fields

 Rice fields generate large amounts of methane during plant growth. Methane in the Earth's atmosphere is a strong greenhouse gas with aglobal warming potential of 34 over a 100-year period. This means that a methane emission will have 34 times the impact on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect (100 times as strong as carbon dioxide) for a brief period (having a half-life of 7 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 86.

The concentration of methane in Earth's atmosphere has increased by about 150% since 1750, and it now accounts for 20% of the totalradiative forcing from all of the long-lived and globally mixed greenhouse gases. According to NOAA the atmospheric methane concentration is now above 1820ppb - largely due to the arctic methane release from melting methane clathrates. Atmospheric methane concentration has not been this high for over 420,000 years and correlates to 9 °C average Earth temperature increase.

Global methane cycle

A. Permafrost, glaciers, and ice cores – A source that slowly releases methane trapped in frozen environments as global temperatures rise.

B. Wetlands – Warm temperatures and moist environments are ideal for methane production. Most of the methane makes it past methane-consuming microorganisms.

C. Forest fire – Mass burning of organic matter releases methane into the atmosphere.

D. Rice paddies – The warmer and moister the rice field, the more methane is produced.

E. Animals – Microorganisms breaking down difficult to digest material in the guts of ruminant livestock and termites produce methane that is then released during defecation.

F. Plants – While methane can be consumed in soil before being released into the atmosphere, plants allow for direct travel of methane up through the roots and leaves and into the atmosphere.Plants may also be direct producers of methane.

G. Landfills – Decaying organic matter and anaerobic conditions cause landfills to be a significant source of methane.

H. Waste water treatment facilities – Anaerobic treatment of organic compounds in the water results in the production of methane.

Houweling et al. (1999) give the following values for methane emissions (Tg/a=teragrams per year):

Origin	CH 4 Emission
	Mass (Tg/a)	Type (%/a)	Total (%/a)
Natural Emissions
Wetlands (incl. Rice agriculture)	225	83	37
Termites	20	7	3
Ocean	15	6	3
Hydrates	10	4	2
Natural Total	270	100	45
Anthropogenic Emissions
Energy	110	33	18
Landfills	40	12	7
Ruminants (Livestock)	115	35	19
Waste treatment	25	8	4
Biomass burning	40	12	7
Anthropogenic Total	330	100	55
Sinks
Soils	-30	-5	-5
Tropospheric OH	-510	-88	-85
Stratospheric loss	-40	-7	-7
Sink Total	-580	-100	-97
Emissions + Sinks
Imbalance (trend)	+20	~2.78 Tg/(nmol/mol)	+7.19 (nmol/mol)/a

Any process that results in the production of methane and its release into the atmosphere can be considered a "source." The two main processes that are responsible for methane production occur as a result of microorganisms anaerobically converting organic compounds into methane.

Methanogenesis, the scientific term for methane production, occurs primarily in anaerobic conditions because of the lack of availability of other oxidants. In these conditions, microscopic organisms called archaea use acetate and hydrogen to break down essential resources in a process called fermentation.

Acetoclastic methanogenesis- certain archaea cleave acetate produced during anaerobic fermentation to yield methane and carbon dioxide.

H₃C-COOH → CH₄ + CO₂

Hydrogenotrophic methanogenesis- archaea oxidize hydrogen with carbon dioxide to yield methane and water.

4H₂ + CO₂ → CH₄ + 2H₂O

While acetoclastic methanogenesis and hydrogenotrophic methanogenesis are the two major source reactions for atmospheric methane, other minor biological methane source reactions also occur.

Natural sources of atmospheric methane

Most ecological emissions of methane relate directly to methanogen sgenerating methane in warm, moist soils as well as in the digestive tracts of certain animals.

Methanogens

Methanogens are methane producing microorganisms. In order to produce energy, they use an anaerobic process called methanogenesis. This process is used in lieu of aerobic, or with oxygen, processes because methanogens are unable to metabolise in the presence of even small concentrations of oxygen. When acetate is broken down in methanogenesis, the result is the release of methane into the surrounding environment.

Wetlands

Wetlands account for approximately 20 per cent of atmospheric methane through emissions from soils and plants. Wetlands counteract the sinking action that normally occurs with soil because of the high water table. When the water table is low, the methane generated within the wetland soil has to come up through the soil and get past multitudes of methanotrophic bacteria. When the water table is higher, then the methane produced in the soil can more easily diffuse through the water and escape into the atmosphere.

Animals

Ruminant animals, particularly cows and sheep, contain bacteria in their gastrointestinal systems that help to break down plant material. Some of these microorganisms use the acetate from the plant material to produce methane, and because these bacteria live in the stomachs and intestines of ruminants, whenever the animal “burps” or defecates, it emits methane as well. The amount of methane emitted by one cow is equivalent to the amount of methane that 2.5 acres of methanotrophic bacteria can consume.

Termites also contain methanogenic microorganisms in their gut. However, some of these microorganisms are so unique that they live nowhere else in the world except in the third gut of termites. These microorganisms also break down biotic components to produce ethanol, as well as methane by product. However, unlike ruminants who lose 20 percent of the energy from the plants they eat, termites only lose 2 percent of their energy in the process. Thus comparatively, termites do not have to eat as much food as ruminants to obtain the same amount of energy, and give off proportionally less methane.

Plants

Living plants (e.g. forests) have recently been identified as a potentially important source of methane, possibly being responsible for approximately 10 to 30 percent of atmospheric methane. calculated emissions of 62–236 Tg a⁻¹, and "this newly identified source may have important implications".
In wetlands, where rate of methane production are high, plants help methane travel into the atmosphere—acting like inverted lightning rods as they direct the gas up through the soil and into the air. They are also suspected to produce methane themselves, but because the plants would have to use aerobic conditions to produce methane, the process itself is still unidentified.

Ecological conversion

Conversion of forests and natural environments into agricultural plots increases the amount of nitrogen in the soil, which inhibits methane oxidation, weakening the ability of the methanotrophic bacteria in the soil to act as sinks. Additionally, by changing the level of the water table, humans can directly affect the soil’s ability to act as a source or sink. The relationship between water table levels and methane emission is explained in the wetlands section of natural sources.

Farm animals

A 2006 UN FAO report reported that livestock generate more greenhouse gases as measured in CO₂ equivalents than the entire transportation sector. Livestock accounts for 9 percent of anthropogenic CO2, 65 percent of anthropogenic nitrous oxide and 37 percent of anthropogenic methane. A senior UN official and co-author of the report, Henning Steinfeld, said "Livestock are one of the most significant contributors to today's most serious environmental problems."

Rice agriculture

Due to a continuously growing world population, rice agriculture has become one of the most powerful anthropogenic sources of methane. With warm weather and water-logged soil, rice paddies act like wetlands, but are generated by humans for the purpose of food production. Due to the swamp-like environment of rice fields, this crop alone is responsible for approximately 50-100 million metric tons of methane emission each year. This means that rice agriculture is responsible for approximately 15 to 20 percent of anthropogenic methane emissions. 

Landfills

Due to the large collections of organic matter and availability of anaerobic conditions, landfills are the third largest source of atmospheric methane. Even after a landfill is closed, the mass amount of decaying matter continues to emit methane for years. Although the methanotrophic bacteria in the surrounding soil does oxidize some of the methane, approximately 90 percent of the methane produced in landfills escapes through the landfill cover and into the atmosphere.

Waste water treatment

Waste water treatment facilities act to remove organic matter, solids, pathogens, and chemical hazards as a result of human contamination. Methane emission in waste treatment facilities occurs as a result of anaerobic treatments of organic compounds and anaerobic biodegradation of sludge.

Biomass burning

Incomplete burning of both living and dead organic matter results in the emission of methane. While natural wildfires can contribute to methane emissions, the bulk majority of biomass burning occurs as a result of humans- including everything from accidental burnings by civilians to deliberate burnings used to clear out land to biomass burnings occurring as a result of destroying waste.

Natural gas distribution

Methane is a primary component of natural gas, and thus during the production, processing, storage, transmission, and distribution of natural gas, a significant amount of methane is lost into the atmosphere.

Methane emissions occur in all sectors of the natural gas industry, from drilling and production, through gathering and processing and transmission, to distribution. These emissions occur through normal operation, routine maintenance, fugitive leaks, system upsets, and venting of equipment. In the oil industry, some underground crude contains natural gas that is entrained in the oil at high reservoir pressures. When oil is removed from the reservoir, associated natural gas is produced.

Methane sink 

Any process that consumes methane from the atmosphere can be considered a "sink" of atmospheric methane. The most prominent of these processes occur as a result of methane either being destroyed in the atmosphere or broken down in soil.

Reaction with the hydroxyl radical- The major removal mechanism of methane from the atmosphere involves radical chemistry; it reacts with the hydroxyl radical (·OH) in the troposphere or stratosphere to create theCH₃+ radical and water vapor. In addition to being the largest known sink for atmospheric methane, this reaction is one of the most important sources of water vapor in the upper atmosphere.

CH4 + ·OH→ ·CH3 +H2O

This reaction in the troposphere gives a methane lifetime of 9.6 years. Two more minor sinks are soil sinks (160 year lifetime) and stratospheric loss by reaction with ·OH, ·Cl and ·O¹D in the stratosphere (120 year lifetime), giving a net lifetime of 8.4 years. Oxidation of methane is the main source of water vapor in the upper stratosphere (beginning at pressure levels around 10 kPa).

The methyl radical formed in the above reaction will, during normal daytime conditions in the troposphere, usually react with another hydroxyl radical to form formaldehyde. Note that this is not strictly oxidative pyrolysis as described previously. Formaldehyde can react again with a hydroxyl radical to form carbon dioxide and more water vapor. Note that sidechains in these reactions may interact withnitrogen compounds that will likely produce ozone, thus supplanting radicals required in the initial reaction.

Methanotrophic bacteria in soils- Methanotrophic bacteria that reside within soil use methane as a source of carbon in methane oxidation. Methane oxidation allows methanotrophic bacteria to use methane as a source of energy, reacting methane with oxygen and as a result producing carbon dioxide and water.

CH₄ + 2O₂→ CO₂ + 2H₂O

Natural sinks of atmospheric methane

Most natural sinks occur as a result of chemical reactions in the atmosphere as well as oxidation by methane consuming bacteria in Earth’s soils.

Methanotrophs in soils

Soils act as a major sink for atmospheric methane through the methanotrophic bacteria that reside within them. This occurs with two different types of bacteria. “High capacity-low affinity” methanotrophic bacteria grow in areas of high methane concentration, such as waterlogged soils in wetlands and other moist environments. And in areas of low methane concentration, “low capacity-high affinity” methanotrophic bacteria make use of the methane in the atmosphere to grow, rather than relying on methane in their immediate environment.

Forest soils act as good sinks for atmospheric methane because soils are optimally moist for methanotroph activity, and the movement of gases between soil and atmosphere (soil diffusivity) is high. With a lower water table, any methane in the soil has to make it past the methanotrophic bacteria before it can reach the atmosphere.

Wetland soils, however, are often sources of atmospheric methane rather than sinks because the water table is much higher, and the methane can be diffused fairly easily into the air without have to compete with the soil’s methanotrophs.

Troposphere

The most effective sink of atmospheric methane is the hydroxyl radical in the troposphere, or the lowest portion of Earth’s atmosphere. As methane rises into the air, it reacts with the hydroxyl radical to create water vapor and carbon dioxide. The lifespan of methane in the atmosphere was estimated at 9.6 years as of 2001; however, increasing emissions of methane over time reduce the concentration of the hydroxyl radical in the atmosphere. With less OH˚ to react with, the lifespan of methane could also increase, resulting in greater concentrations of atmospheric methane.

Stratosphere

Even if it is not destroyed in the troposphere, methane can usually only last 12 years before it is eventually destroyed in Earth’s next atmospheric layer: the stratosphere. Destruction in the stratosphere occurs the same way that it does in the troposphere: methane is oxidized to produce carbon dioxide and water vapor.

Reaction with free chlorine

Methane also reacts with natural chlorine gas in the atmosphere to produce chloromethane and hydrochloric acid (HCl). This process is known as free radical halogenations.

CH₄ + Cl₂ → CH₃Cl + HCl

Data from 2007 suggested methane concentrations were beginning to rise again. This was confirmed in 2010 when a study showed methane levels were on the rise for the 3 years 2007 to 2009. After a decade of near-zero growth in methane levels, "globally averaged atmospheric methane increased by [approximately] 7 nmol/mol per year during 2007 and 2008. During the first half of 2009, globally averaged atmospheric CH₄ was [approximately] 7 nmol/mol greater than it was in 2008, suggesting that the increase will continue in 2009."

Methane emissions levels vary greatly depending on the local geography. For both natural and anthropogenic sources, higher temperatures and higher water levels result in the anaerobic environment that is necessary for methane production.

Natural methane cycles

Emissions of methane into the atmosphere are directly related to temperature and moisture. Thus, the natural environmental changes that occur during seasonal change act as a major control of methane emission. Additionally, even changes in temperature during the day can affect the amount of methane that is produced and consumed.

For example, plants that produce methane can emit as much as two to four times more methane during the day than during the night. This is directly related to the fact that plants tend to rely on solar energy to enact chemical processes.

Additionally, methane emissions are affected by the level of water sources. Seasonal flooding during the spring and summer naturally increases the amount of methane released into the air.

Methane management techniques

In order to reduce effects on methane oxidation in soil, several steps can be taken. Controlling the usage of nitrogen enhancing fertilizer and reducing the amount of nitrogen pollution into the air can both lower inhibition of methane oxidation—a major sink of atmospheric methane. Additionally, using drier growing conditions for crops such as rice and selecting strains of crops that produce more food per unit area can reduce the amount of land with ideal conditions for methanogenesis. Careful selection of areas of land conversion (for example, plowing down forests to create agricultural fields) can also reduce the destruction of major areas of methane oxidation.A portable methane detector has been developed which, mounted in a vehicle, can detect excess levels of methane in the ambient atmosphere and differentiate between natural methane from rotting vegetation or manure and gas leaks. As of 2013 the technology was being deployed by Pacific Gas & Electric.

Methane emission from wet land rice

As one of the most significant natural sources of atmospheric methane, wetlands remain a major area of concern with respect to climate change. Wetlands are characterized by water-logged soils and distinctive communities of plant and animal species that have evolved and adapted to the constant presence of water. Due to this high level of water saturation as well as warm weather, wetlands are one of the most significant natural sources of atmospheric methane.

Most methanogenesis, or methane production, occurs in oxygen poor environments. Because the microbes that live in warm, moist environments consume oxygen more rapidly than it can diffuse in from the atmosphere, wetlands are the ideal anaerobic, or oxygen poor, environments for fermentation.

Fermentation is a process used by certain kinds of microorganisms to break down essential nutrients. In a process called acetoclastic methanogenesis, microorganisms from the classification domainarchaea produce methane by fermenting acetate and H₂-CO₂ into methane and carbon dioxide.

H₃C-COOH → CH₄ + CO₂

Depending on the wetland and type of archaea, hydrogenotrophic methanogenesis, another process that yields methane, can also occur. This process occurs as a result of archaea oxidizing hydrogen with carbon dioxide to yield methane and water.

4H2 + CO₂ → CH₄ + 2H₂O

Natural progressions of wetlands

Many different kinds of wetlands exist, all characterized by unique compositions of plant life and water conditions. To list a few, marshes, swamps, bogs, fens, peat lands, muskegs, and pocosins are all examples of different kinds of wetlands. Because each type of wetland is unique, the same characteristics used to classify each wetland can also be used to characterize the amount of methane emitted from that particular wetland. Any waterlogged environment with moderate levels of decomposition create the anaerobic conditions needed for methanogenesis, but the amount of water and decomposition will affect the magnitude of methane emissions in a specific environment. For example, lower water tables result in lower levels of methane emission because methanotrophic bacteria require oxic conditions to oxidize methane into carbon dioxide and water. Higher water tables, however, result in higher levels of methane emission because there is less habitable area for methanotrophic bacteria to live, and thus the methane can more easily diffuse into the atmosphere without being broken down.

Often, the natural ecological progression of wetlands involves the development of one kind of wetland into one or several other kinds of wetlands. So over time, a wetland will naturally change the amount of methane emitted from its soil.

For example, Peatlands are wetlands that contain a large amount of peat, or partially decayed plant life. When peatlands are first developing, they often start out as fens, wetlands characterized by mineral rich soil. These flooded wetlands, with higher water tables, would naturally have higher emissions of methane. Eventually, the fens develop into bogs, acidic wetlands with accumulations of peat and lower water tables. With the lower water tables, methane emissions are more easily consumed by methanotrophic, or methane consuming, bacteria and never make it to the atmosphere. Over time, the peat lands develop and end up with accumulated pools of water, which once again increases emissions of methane.

Pathways of methane emission in wetlands

Primary productivity fuels methane emissions both directly and indirectly. Plants not only provide much of the carbon needed for methane producing processes in wetlands, but in addition, methane can utilize three different pathways provided by primary productivity to reach the atmosphere: diffusion through the profile, plant aerenchyma, and ebullition.

Diffusion through the profile refers to the movement of methane up through soil and bodies of water to reach the atmosphere. The importance of diffusion as a pathway varies per wetland based on the type of soil and vegetation. For example, in peat lands, the mass amount of dead, but not decaying, organic matter results in relatively slow diffusion of methane through the soil. Additionally, because methane can travel more quickly through soil than water, diffusion plays a much bigger role in wetlands with drier, more loosely compacted soil.

Plant aerenchyma refers to the vessel-like transport tubes within the tissues of certain kinds of plants. Plants with arenchyma possess porous tissue that allows for direct travel of gases to and from the plant roots. Methane can travel directly up from the soil into the atmosphere using this transport system. The direct “shunt” created by the aerenchyma allows for methane to bypass oxidation by oxygen that is also transported by the plants to their roots.

Ebullition refers to the sudden release of bubbles of methane into the air. These bubbles occur as a result of methane building up over time in the soil, forming pockets of methane gas. As these pockets of trapped methane grow in size, the level of the soil will slowly rise up as well. This phenomenon continues until so much pressure builds up that the bubble “pops,” transporting the methane up through the soil so quickly that it does not have time to be consumed by the methanotrophic organisms in the soil. With this release of gas, the level of soil then falls once more.

Ebullition in wetlands can be recorded by delicate sensors, called piezometers, that can detect the presence of pressure pockets within the soil. Hydraulic heads are also used to detect the subtle rising and falling of the soil as a result of pressure build up and release. Using piezometers and hydraulic heads, a study was done in northern United States peat lands to determine the significance of ebullition as a source of methane. Not only was it determined that ebullition is in fact a significant source of methane emissions in peat lands, but it was also observed that there was an increase in pressure after significant rainfall, suggesting that rainfall is directly related to methane emissions in wetlands.

Factors affecting methane emission from wetlands

The magnitude of methane emission from a wetland are usually measured using eddy covariance, gradient or chamber flux techniques, and depends upon several factors, including water table, comparative ratios of methanogenic bacteria to methanotrophic bacteria, transport mechanisms, temperature, substrate type, plant life, and climate. These factors work together to effect and control methane flux in wetlands.

Overall the main determinant of net flux of methane into the atmosphere is the ratio of methane produced by methanogenic bacteria that makes it to the surface relative to the amount of methane that is either consumed by methanotrophic bacteria or oxidized and lost before reaching the atmosphere. This ratio is in turn affected by the other controlling factors of methane in the environment. Additionally, pathways of methane emission affect how the methane travels into the atmosphere and thus have an equal effect on methane flux in wetlands.

The level of the water table is the first controlling factor to be considered. Not only does pool and water table location determine the areas where methane production or oxidation may take place, but it also determines how quickly methane can diffuse into the air. When traveling through water, the methane molecules run into the quickly moving water molecules and thus take a longer time to reach the surface. Travel through soil, however, is much easier and results in easier diffusion into the atmosphere. This theory of movement is supported by observations made in wetlands where significant fluxes of methane occurred after a drop in the water table due to drought. If the water table is at or above the surface, then methane transport begins to take place primarily through ebullition and vascular or pressurized plant mediated transport, with high levels of emission occurring during the day from plants that use pressurized ventilation.

        Temperature is also an important factor to consider as the environmental temperature—and temperature of the soil in particular—affects the metabolic rate of production or consumption by bacteria. Additionally, because methane fluxes occur annually with the seasons, evidence is provided that suggests that the temperature changing coupled with water table level work together to cause and control the seasonal cycles.

The composition of soil and substrate availability (Substrate composition) change the nutrients available for methanogenic and methanotrophic bacteria, and thus directly affects the rate of methane production and consumption. For example, wetlands soils with high levels of acetate or hydrogen and carbon dioxide are conducive to methane production. Additionally, the type of plant life and amount of plant decomposition affects the nutrients available to the bacteria as well as the acidity. A constant availability of cellulose and a soil pH of about 6.0 have been determined to provide optimum conditions for methane production and consumption; however, substrate quality can be overridden by other factors. Soil pH and composition must still be compared to the effects of water table and temperature.

Net ecosystem production (NEP) and climate changes are the all encompassing factors that have been shown to have a direct relationship with methane emissions from wetlands. In wetlands with high water tables, NEP has been shown to increase and decrease with methane emissions, most likely due to the fact that both NEP and methane emissions flux with substrate availability and soil composition. In wetlands with lower water tables, the movement of oxygen in and out of the soil can increase the oxidation of methane and the inhibition of methanogenesis, nulling the relationship between methane emission and NEP because methane production becomes dependent upon factors deep within the soil.

A changing climate affects many factors within the ecosystem, including water table, temperature, and plant composition within the wetland—all factors that affect methane emissions. However, climate change can also affect the amount of carbon dioxide in the surrounding atmosphere, which would in turn decrease the addition of methane into the atmosphere, as shown by an 80% decrease in methane flux in areas of doubled carbon dioxide levels.

Humans often drain wetlands in the name of development, housing, and agriculture. By draining wetlands, the water table is thus lowered, increasing consumption of methane by the methanotrophic bacteria in the soil. However, as a result of draining, water saturated ditches develop, which due to the warm, moist environment, end up emitting a large amount of methane. Therefore the actual effect on methane emission strongly ends up depending on several factors. If the drains are not spaced far enough apart, then saturated ditches will form, creating mini wetland environments. Additionally, if the water table is lowered significantly enough, then the wetland can actually be transformed from a source of methane into a sink that consumes methane. Finally, the actual composition of the original wetland changes how the surrounding environment is affected by the draining and human development.

 source:wiki