Greenhouse gas emissions from wetlands
Greenhouse gas emissions from wetlands of concern consist primarily of methane and nitrous oxide emissions. Wetlands are the largest natural source of atmospheric methane in the world, and are therefore a major area of concern with respect to climate change.[1][2][3] Wetlands account for approximately 20–30% of atmospheric methane through emissions from soils and plants, and contribute an approximate average of 161 Tg of methane to the atmosphere per year.[4]
Wetlands are characterized by
Some wetlands are a significant source of methane emissions[6][7] and some are also emitters of nitrous oxide.[8][9] Nitrous oxide is a greenhouse gas with a global warming potential 300 times that of carbon dioxide and is the dominant ozone-depleting substance emitted in the 21st century.[10] Wetlands can also act as a sink for greenhouse gases.[11]
Emissions by type of wetland
Part of a series on the |
Carbon cycle |
---|
Characteristics of wetland classes can assist to inform on magnitude of methane emissions. However, wetland classes have displayed high variability in methane emissions spatially and temporally.[12]
.Amounts
Depending on their characteristics, some wetlands are a significant source of methane emissions[7] and some are also emitters of nitrous oxide.[8][9]
Methane
Wetlands account for approximately 20–30% of atmospheric methane through emissions from soils and plants.[15]
Nitrous oxide fluxes
Nitrous oxide is a greenhouse gas with a global warming potential 300 times that of carbon dioxide and is the dominant ozone-depleting substance emitted in the 21st century.[10] Excess nutrients mainly from anthropogenic sources have been shown to significantly increase the N2O fluxes from wetland soils through denitrification and nitrification processes (see table below).[16][8][17] A study in the intertidal region of a New England salt marsh showed that excess levels of nutrients might increase N2O emissions rather than sequester them.[16]
Data on nitrous oxide fluxes from wetlands in the southern hemisphere are lacking, as are ecosystem-based studies including the role of dominant organisms that alter sediment biogeochemistry. Aquatic invertebrates produce ecologically-relevant nitrous oxide emissions due to ingestion of denitrifying bacteria that live within the subtidal sediment and water column[18] and thus may also be influencing nitrous oxide production within some wetlands.
Wetland type | Location | N2O fluxa
(µmol N2O m−2 h−1) |
Reference |
---|---|---|---|
Mangrove | Shenzhen and Hong Kong | 0.14 to 23.83 | [20] |
Mangrove | Muthupet, South India | 0.41 to 0.77 | [21] |
Mangrove | Bhitarkanika , East India
|
0.20 to 4.73 | [22] |
Mangrove | Pichavaram, South India | 0.89 to 1.89 | [22] |
Mangrove | Queensland, Australia | −0.045 to 0.32 | [23] |
Mangrove | South East Queensland, Australia | 0.091 to 1.48 | [24] |
Mangrove | Southwest coast, Puerto Rico | 0.12 to 7.8 | [25] |
Mangrove | Isla Magueyes, Puerto Rico | 0.05 to 1.4 | [25] |
Salt marsh | Chesapeake Bay, US | 0.005 to 0.12 | [26] |
Salt marsh | Maryland, US | 0.1 | [27] |
Salt marsh | North East China | 0.1 to 0.16 | [28] |
Salt marsh | Biebrza, Poland | −0.07 to 0.06 | [29] |
Salt marsh | Netherlands | 0.82 to 1.64 | [30] |
Salt marsh | Baltic Sea | −0.13 | [31] |
Salt marsh | Massachusetts, US | −2.14 to 1.27 | [32] |
aThe flux rates are shown as hourly rates per unit area. A positive flux implies flux from soil into air; a negative flux implies flux from air into the soil.[33] Negative N2O fluxes are common and are caused by consumption by the soil.[34]
Pathways of methane emission
Wetlands counteract the sinking action that normally occurs with soil because of the high water table. The level of the water table represents the boundary between anaerobic methane production and aerobic methane consumption. When the water table is low, the methane generated within the wetland soil has to come up through the soil and get past a deeper layer of
Once produced, methane can reach the atmosphere via three main pathways: molecular
Fermentation
Fermentation is a process used by certain kinds of
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
Diffusion
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.[37] For example, in peatlands, the mass amount of dead, but not decaying, organic matter results in relatively slow diffusion of methane through the soil.[38] 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.
Aerenchyma
Plant aerenchyma refers to the vessel-like transport tubes within the tissues of certain kinds of plants. Plants with aerenchyma 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.[38] 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
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
Controlling factors
The magnitude of
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 oxidized by methanotrophic bacteria before reaching the atmosphere.[12] 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.
Water table
The first controlling factor to consider is the level of the water table. 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.[12] 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.[12]
Temperature
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.[41]
Substrate composition
The composition of soil and substrate availability 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. Plant leachates such as phenolic compounds from Sphagnum can also interact with soil characteristics to influence methane production and consumption.[42] 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.[12] Soil pH and composition must still be compared to the effects of water table and temperature.
Net ecosystem production
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.[12]
Causes for additional emissions
Human development of wetlands
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.[12] 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.[12] 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.[citation needed]
References
- ^ Houghton, J. T., et al. (Eds.) (2001) Projections of future climate change, Climate Change 2001: The Scientific Basis, Contribution of Working Group I to the Third Assessment Report of the Intergovernmental Panel on Climate Change, 881 pp.
- S2CID 134078252.
- S2CID 14228726.
- ISSN 1866-3508.
- .
- S2CID 244711959.
- ^ S2CID 198421761.
- ^ .
- ^ PMID 22451101.
- ^ S2CID 2100618.
- ISBN 978-981-16-4482-5.
- ^ a b c d e f g h Bubier, Jill L. and Moore, Tim R. (1994). "An ecological perspective on methane emissions from northern wetlands." Trends in Ecology & Evolution. 9 (12): 460-464. doi:10.1016/0169-5347(94)90309-3.
- ^ US EPA, OW (2015-04-09). "Classification and Types of Wetlands". US EPA. Retrieved 2021-05-15.
- PMID 26172606.
- )
- ^ .
- PMID 29876073.
- PMID 19255427.
- ^ Moseman-Valtierra, S. (2012). "Chapter 1: Reconsidering the climatic roles of marshes: Are they sinks or sources of greenhouse gases?". In Abreu, D. C.; Borbón, S. L. (eds.). Marshes: Ecology, Management and Conservation. New York, NY: Nova Science.
- ^ PMID 20381125.
- ^ Krithika, K.; Purvaja, R.; Ramesh, R. (2008). "Fluxes of methane and nitrous oxide from an Indian mangrove". Current Science. 94: 218–224.
- ^ .
- .
- .
- ^ S2CID 86450737.
- S2CID 9109080.
- ^ Weller, D. E.; Cornell, D. L.; Jordan, T. E. (1994). "Denitrification in riparian forests receiving agricultural discharges". Global Wetlands: Old World and New: 117–131.
- S2CID 16763038.
- S2CID 94635551.
- PMID 12931872.
- ^ Liikanen, A. (2009). "Methane and nitrous oxide fluxes in two coastal wetlands in the northeastern Gulf of Bothnia, Baltic Sea". Boreal Environment Research. 14 (3): 351–368.
- .
- ^ Agricultural & Environmental Data Archive
- ISSN 0038-0717.
- .
- . L20503.
- Bibcode:2008AGUFM.B33B0424T.
- ^ a b Couwenberg, John. Greifswald University. "Methane emissions from peat soils." http://www.imcg.net/media/download_gallery/climate/couwenberg_2009b.pdf
- S2CID 14228726.
- ^ Glaser, P.H., J.P. Chanton, P. Morin, D.O. Rosenberry, D.I. Siegel, O. Ruud, L.I. Chasar, A.S. Reeve. 2004. "Surface deformations as indicators of deep ebullition fluxes in a large northern peatland."
- S2CID 9305759.
- ISSN 0038-0717.