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Aerobic Methane Emissions from Terrestrial Plants
Beerling, D.J., Gardiner, T., Leggett, G., McLeod, A. and Quick, W.P. 2008. Missing methane emissions from leaves of terrestrial plants. Global Change Biology 14: 1821-1826.

It was recently claimed by Keppler et al. (2006) that earth's terrestrial plants may emit the powerful greenhouse gas methane under aerobic conditions by an as-yet-unknown physiological process to a degree that may substantially contribute to the annual global methane budget of the atmosphere. However, in an attempt to confirm these singular observations with independent studies of three of the same species of plants, as well as three different species, Dueck et al. (2007) were unable to do so using a significantly different methodology. Now, Beerling et al. have attempted the feat via yet another means of measurement.

What was done
The five UK researchers raised the C4 plant Zea mays and the C3 plant Nicotiana tabacum from seed for six weeks at an ambient CO2 concentration of 400 ppm and an ambient methane concentration of 1800 ppb, after which their leaves were studied in "a custom-built flowthrough cuvette with a sufficiently large area to allow the detection of methane emissions" via "a process gas chromatograph linked to a high-precision, high-accuracy flame ionization detector," all of which was done in a controlled-environment room.

What was learned
Beerling et al. report that "well-illuminated actively photosynthesizing Z. mays leaves did not, in our experimental system, emit substantial quantities of methane during repeated three-hour high irradiance episodes," adding that "neither did we detect methane emissions from actively respiring leaves during repeated three-hour dark periods." They additionally state that "measurements with leaves of the C3 species N. tabacum also failed to detect substantial aerobic methane emissions in the light when photosynthesizing with regular stomatal conductances, and in the dark when respiring."

What it means
Up to this point in time, the revolutionary results of Keppler et al. have failed to be replicated, using two different sets of sensitive measurement techniques.

Dueck, T.A., de Visser, R., Poorter, H., Persijn, S., Gorissen, A., de Visser, W., Schapendonk, A., Verhagen, J., Snel, J., Harren, F.J.M., Ngai, A.K.Y., Verstappen, F., Bouwmeester, H., Voesenek, L.A.C. and van der Werf, A. 2007. No evidence for substantial aerobic methane emission by terrestrial plants: a 13C-labelling approach. New Phytologist 175: 29-35.

Keppler, F., Hamilton, J.T.G., Brass, M. and Rockmann, T. 2006. Methane emissions from terrestrial plants under aerobic conditions. Nature 439: 187-191.

Reviewed 17 September 2008