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Methane (Extractions from the Atmosphere) -- Summary
Methane (CH4) is an important greenhouse gas, contributing roughly 20% of total non-H2O radiative forcing (Menyailo and Hungate, 2003).  Its atmospheric concentration is determined by the difference between how much CH4 goes into the air (emissions) and how much comes out of it (extractions) over the same time period.  We discuss the former of these topics under the heading Methane (Agricultural Emissions) in our Subject Index.  Here we concentrate on the later topic.

According to Prinn et al. (1992), one of the major means by which methane is removed from the atmosphere is via oxidation by methanotrophic bacteria in the aerobic zones of soils, the magnitude of which phenomenon is believed to be equivalent to the annual input of methane to the atmosphere (Watson et al., 1992).  This soil sink for methane appears to be ubiquitous, as methane uptake has been observed in soils of tundra (Whalen and Reeburgh, 1990), boreal forests (Whalen et al., 1992), temperate forests (Steudler et al., 1989; Yavitt et al., 1990), grasslands (Mosier et al., 1997), arable lands (Jensen and Olsen, 1998), tropical forests (Keller, 1986; Singh et al., 1997), and deserts (Striegl et al., 1992), with forest soils - especially boreal and temperate forest upland soils (Wahlen and Reeburgh, 1996) - appearing to be the most efficient in this regard (Le Mer and Roger, 2001).

In an attempt to learn more about this subject, Tamai et al. (2003) studied methane uptake rates by the soils of three Japanese cypress plantations composed of 30- to 40-year-old trees.  Through all seasons of the year, they found that methane was absorbed by the soils of all three sites, being positively correlated with temperature, as has also been observed in several other studies (Peterjohn et al., 1994; Dobbie and Smith, 1996); Prieme and Christensen, 1997; Saari et al., 1998).  Methane absorption was additionally - and even more strongly - positively correlated with the C/N ratio of the cypress plantations' soil organic matter.  Based on these results, it can be appreciated that CO2-induced global warming, if real, would produce two biologically-mediated negative feedbacks to counter the increase in temperature: (1) a warming-induced increase in methane uptake from the atmosphere that is experienced by essentially all soils, and (2) an increase in soil methane uptake from the atmosphere that is produced by the increase in plant-litter C/N ratio that typically results from atmospheric CO2 enrichment [see the various sub-headings under Decomposition in our Subject Index].

Another study that deals with this topic is that of Menyailo and Hungate (2003), who assessed the influence of six boreal forest species -- spruce, birch, Scots pine, aspen, larch and Arolla pine -- on soil CH4 consumption in the Siberian artificial afforestation experiment, in which the six common boreal tree species had been grown under common garden conditions for the past 30 years under the watchful eye of the staff of the Laboratory of Soil Science of the Institute of Forest, Siberian Branch of the Russian Academy of Sciences (Menyailo et al., 2002).  They determined, in their words, that "soils under hardwood species (aspen and birch) consumed CH4 at higher rates than soils under coniferous species and grassland."  Under low soil moisture conditions, for example, the soils under the two hardwood species consumed 35% more CH4 than the soils under the four conifers; while under high soil moisture conditions they consumed 65% more.  As for the implications of these findings, Pastor and Post (1988) have suggested, in the words of Menyailo and Hungate, that "changes in temperature and precipitation resulting from increasing atmospheric CO2 concentrations will cause a northward migration of the hardwood-conifer forest border in North America."  Consequently, if such a shifting of species does indeed occur, it will likely lead to an increase in methane consumption by soils and a reduction in methane-induced global warming potential, thereby providing yet another biologically-mediated negative feedback factor that has yet to be incorporated into models of global climate change.

Last of all, we note that increases in the air's CO2 concentration will likely lead to a net reduction in vegetative isoprene emissions, which, as explained in our Subject Index under Isoprene, should also lead to a significant removal of methane from the atmosphere.  Hence, as the air's CO2 content -- and possibly its temperature -- continues to rise, we can expect to see a significant increase in the rate of methane removal from earth's atmosphere, which should help to reduce the potential for further global warming.

Dobbie, K.E. and Smith, K.A.  1996.  Comparison of CH4 oxidation rates in woodland, arable and set aside soils.  Soil Biology & Biochemistry 28: 1357-1365.

Jensen, S. and Olsen, R.A.  1998.  Atmospheric methane consumption in adjacent arable and forest soil systems.  Soil Biology & Biochemistry 30: 1187-1193.

Keller, M.  1986.  Emissions of N2O, CH4, and CO2 from tropical forest soils.  Journal of Geophysical Research 91: 11,791-11,802.

Le Mer, J. and Roger, P.  2001.  Production, oxidation, emission and consumption of methane by soils: a review.  European Journal of Soil Biology 37: 25-50.

Menyailo, O.V. and Hungate, B.A.  2003.  Interactive effects of tree species and soil moisture on methane consumption.  Soil Biology & Biochemistry 35: 625-628.

Menyailo, O.V., Hungate, B.A. and Zech, W.  2002.  Tree species mediated soil chemical changes in a Siberian artificial afforestation experiment.  Plant and Soil 242: 171-182.

Mosier, A.R., Parton, W.J., Valentine, D.W., Ojima, D.S., Schimel, D.S. and Heinemeyer, O.  1997.  CH4 and N2O fluxes in the Colorado shortgrass steppe.  2. Long-term impact of land use change.  Global Biogeochemical Cycles 11: 29-42.

Pastor, J. and Post, W.M.  1988.  Response of northern forests to CO2-induced climate change.  Nature 334: 55-58.

Peterjohn, W.T., Melillo, J.M., Steudler, P.A. and Newkirk, K.M.  1994.  Responses of trace gas fluxes and N availability to experimentally elevated soil temperatures.  Ecological Applications 4: 617-625.

Prieme, A. and Christensen, S.  1997.  Seasonal and spatial variation of methane oxidation in a Danish spruce forest.  Soil Biology & Biochemistry 29: 1165-1172.

Prinn, R., Cunnold, D., Simmonds, P., Alyea, F., Boldi, R., Crawford, A., Fraser, P., Gutzler, D., Hartley, D., Rosen, R. and Rasmussen, R.  1992.  Global average concentration and trend for hydroxyl radicals deduced from ALE/GAGE trichloroethane (methyl chloroform) data for 1978-1990.  Journal of Geophysical Research 97: 2445-2461.

Saari, A., Heiskanen, J., Martikainen, P.J.  1998.  Effect of the organic horizon on methane oxidation and uptake in soil of a boreal Scots pine forest.  FEMS Microbiology Ecology 26: 245-255.

Singh, J.S., Singh, S., Raghubanshi, A.S., Singh, S., Kashyap, A.K. and Reddy, V.S.  1997.  Effect of soil nitrogen, carbon and moisture on methane uptake by dry tropical forest soils.  Plant and Soil 196: 115-121.

Steudler, P.A., Bowden, R.D., Meillo, J.M. and Aber, J.D.  1989.  Influence of nitrogen fertilization on CH4 uptake in temperate forest soils.  Nature 341: 314-316.

Striegl, R.G., McConnaughey, T.A., Thorstensen, D.C., Weeks, E.P. and Woodward, J.C.  1992.  Consumption of atmospheric methane by desert soils.  Nature 357: 145-147.

Tamai, N., Takenaka, C., Ishizuka, S. and Tezuka, T.  2003.  Methane flux and regulatory variables in soils of three equal-aged Japanese cypress (Chamaecyparis obtusa) forests in central Japan.  Soil Biology & Biochemistry 35: 633-641.

Watson, R.T., Meira Filho, L.G., Sanhueza, E. and Janetos, A.  1992.  Sources and sinks.  In: Houghton, J.T., Callander, B.A. and Varney, S.K. (Eds.), Climate Change 1992: The Supplementary Report to The IPCC Scientific Assessment, Cambridge University Press, Cambridge, UK, pp. 25-46.

Whalen, S.C. and Reeburgh, W.S.  1990.  Consumption of atmospheric methane by tundra soils.  Nature 346: 160-162.

Wahlen, S.C. and Reeburgh, W.S.  1996.  Moisture and temperature sensitivity of CH4 oxidation in boreal soils.  Soil Biology & Biochemistry 28: 1271-1281.

Whalen, S.C., Reeburgh, W.S. and Barber, V.A.  1992.  Oxidation of methane in boreal forest soils: a comparison of seven measures.  Biogeochemistry 16: 181-211.

Yavitt, J.B., Downey, D.M., Lang, D.E. and Sextone, A.J.  1990.  CH4 consumption in two temperate forest soils.  Biogeochemistry 9: 39-52.