Background
Although the atmospheric concentration of methane - the second most important trace-gas contributor to global warming next to CO2 - has more than doubled since pre-industrial times, its rate of increase has slowed considerably over the last few decades (Steele et al., 1992).  Prinn et al. (1992) suggest that one of the major causes of the slowdown is the increasing magnitude of methane 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).  Hence, as Tamai et al. note, "this biological sink plays an important role in modulating global warming."

The methane soil sink appears to be ubiquitous.  Methane uptake, for example, 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 appearing to be the most efficient (Le Mer and Roger, 2001).

What was done
The authors studied soil methane uptake rates in three Japanese cypress (Chamaecyparis obtusa) plantations of 30- to 40-year-old trees.

What was learned
Through all seasons of the year, methane was absorbed by the soils of all three sites, being positively correlated with temperature, as has 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 also - and even more strongly - positively correlated with the soil organic matter C/N ratio.

What it means
In light of the authors' 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 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].

Based on these observations, plus the fact that both atmospheric CO2 concentration and temperature have risen over the past few decades, it would appear that these phenomena have already conspired to dramatically slow the prior rapid rise in the atmosphere's methane concentration to the point where it may soon cease to rise any further [see our Editorial of 8 January 2003].

References
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.

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.

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.

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.

Steele, L.P., Dlugokencky, E.J., Lang, P.M., Tans, P.P. Martin, R.C. and Massarie, K.A.  1992.  Slowing down of the global accumulation of atmospheric methane during the 1980s.  Nature 358: 313-316.

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.

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.

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.