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Do Terrestrial Plants Emit Methane Under Normal Aerobic Conditions?
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.

The fifteen Dutch researchers say recent findings suggest that terrestrial plants may "emit methane under aerobic conditions by an as yet unknown physiological process (Keppler et al., 2006), and in this way may substantially contribute to the annual global methane budget (Bousquet et al., 2006)," resulting in "estimated values for methane emission by terrestrial plants varying between 10 and 260 Tg yr-1 (Houweling et al., 2006; Keppler et al., 2006; Kirschbaum et al., 2006)."

What was done
Dueck et al. conducted two separate experiments involving six plant species - Ocimum basilicum L. (basil), Triticum aestivum L. (wheat), Zea mays L. (maize), Salvia officinalis L. (sage), Lycopersicon esculentum Miller (tomato), and Oenothera biennis L. (common evening primrose) - the first three of which were also used by Keppler et al. (2006) in their study. The experiments were performed in "a unique hermetically sealed plant growth chamber with a volume of 3500 liters, specifically designed for atmospheric isotope labeling," where "plants were grown hydroponically to exclude any methane production derived from anaerobic soil pockets."

What was learned
When all was said and done, the researchers report there was no evidence for substantial aerobic methane emission by the terrestrial plants they studied, stating that "maximally," it was only "0.3% of the previously published studies." Indeed, they say that methane concentrations in continuous-flow gas cuvettes with plants "were not significantly higher than those of control cuvettes without plants," stating that under both the short- and long-term, they "did not find any evidence of a substantial emission of methane."

What it means
In the summary preceding their important paper, Dueck et al. write that "the results of a single publication [our italics] stating that terrestrial plants emit methane has sparked a discussion in several scientific journals," but that "an independent test has not yet been performed [our italics]." Clearly, this sad state of affairs was another unfortunate case of getting "the cart before the horse," as it were, much like what happened long ago with the entire CO2-climate debate, where the far-reaching politics of the issue quickly outstripped the insufficient scientific evidence that preceded it.

Bousquet, P., Ciasis, P., Miller, J.B., Dlugokencky, E.J., Hauglustaine, D.A., Prigent, C., van der Werf, G.R., Peylin, P., Brunke, E.-G., Carouge, C., Langenfelds, R.L., Lathiere, J., Papa, F., Ramonet, M., Schmidt, M., Steele, L.P., Tyler, S.C. and White, J. 2006. Contribution of anthropogenic and natural sources to atmospheric methane variability. Nature 443: 439-443.

Houweling, S., Rockmann, T., Aben, I., Keppler, F., Krol, M., Meirink, J.F., Dlugokencky, E.J. and Frankenberg, C. 2006. Atmospheric constraints on global emissions of methane from plants. Geophysical Research Letters 33: L15821.

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

Kirshbaum, M.U.F., Bruhn, D., Etheridge, D.M., Evans, J.R., Farquhar, G.D., Gifford, R.M., Ki, P. and Winters, A.J. 2006. Comment on the quantitative significance of aerobic methane release by plants. Functional Plant Biology 33: 521-530.

Reviewed 21 November 2007