Tropical forests spew methane into the air, unwittingly abetting human-produced climate change. So say geochemist Frank Keppler of the Max Planck Institute for Chemistry in Mainz, Germany, and atmospheric physicist Thomas Röckmann of the Institute for Marine and Atmospheric Research Utrecht in the Netherlands. But new research, using more sensitive measurement techniques and better growing conditions, failed to turn up any evidence of significant methane emission.
The new study, conducted by botanist Tom Dueck and his colleagues at Plant Research International in Wageningen, the Netherlands, found only negligible methane emissions coming from plants. "[The concentrations] were so low and variable that they did not significantly differ from zero," Dueck says. "We thought that if Keppler's claim was true, we could affirm his findings with better methods. If not, we'd have a scientific dispute."
In Keppler's study, the background levels of methane in the atmosphere, nearly 2000 parts per billion (ppb), dwarfed the 37 ppb of methane emitted by vernal grass, the plant species he found to produce the most methane. To clearly delineate what methane came from where, Dueck's team grew plants in an atmosphere with carbon 13, an isotope of carbon with an extra neutron at the center of each atom. Because the carbon was thus labeled, any methane (a compound of carbon and hydrogen—CH4) that the plants produced would also carry this unique marker. Measuring small amounts of methane containing carbon 13 in a controlled setting is less difficult because background atmospheric levels of the isotope are much lower, 22 ppb.
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Keppler says that using carbon 13 to grow plants could have significant and unknown impacts on the normal function of the plants. He also argues that Dueck's studies lack "sufficient scientific rigor" to prove that normal plants, which contain the more common isotope carbon 12, do not emit methane.
Scientists, however, have been cultivating simple carbon 13–labeled plants like algae since the late 1980s as well as more complex plants like kale for the past few years without evidence of significant physiological changes. Botanist Thomas Sharkey of the University of Wisconsin–Madison found that carbon 13 plants he grew in his lab "behave exactly the same" as normal plants.
Dueck believes that Keppler's experiment, and thus his findings, may have been flawed. The reason, he says: the sealed jars Keppler used to grow the plants did not allow enough air to circulate around the leaves, which "might result in abnormal reactions from the plants." Keppler also applied stresses like light and heat to the plants, which could further interact with lack of air circulation and possibly cause the vegetation to "cook," a process that could plausibly produce methane due to the organic breakdown of the plant matter, according to Sharkey.
Dueck and his colleagues grew their plants under steady-state conditions with enough air circulation in the chambers for gas exchange to occur between the vegetation and the surrounding environment. Sharkey considers these flow-through methods, which have been used by botanists for the past decade or so, to be "far superior and very convincing" and much more similar to plants growing in a natural environment.
The lack of a known physiological process or metabolic pathway that would cause plants to create methane also raises questions about Keppler's results. But such a biological basis would help explain the high methane concentrations that are often found above tropical forests. Keppler's findings, if confirmed, would indicate that nearly 10 to 40 percent of yearly methane emissions stem from plants.
Dueck says that he and his team are scheduled to meet with Thomas Röckmann, Keppler's co-author, in a few weeks to discuss the findings and to try to "clear up these differences." In the meantime, the question of whether plants emit copious amounts of methane remains a mystery.