Published online 28 November 2007 | Nature | doi:10.1038/news.2007.307

News

Perhaps only woody plants, not grasses, emit the greenhouse gas.

Some sorts of ecosystems might release more methane than others. Pixtal

Nearly two years after the confusing discovery that terrestrial plants may be pumping out methane through an unknown mechanism, scientists are still producing puzzling results about this strange phenomenon. A study this week seems to confirm that methane is emitted in this way, but adds that it might be limited to woody plants — again for unknown reasons.

Methane (CH 4 ), the main component in natural gas, is a simple hydrocarbon and a potent greenhouse gas. It is most commonly released by organic matter decaying in oxygen-free environments, such as swamps and animal guts.

The original finding, published in January 2006, that green plants emit methane in the presence of atmospheric oxygen1 seemed to turn upside down textbook knowledge. It was clear that if global vegetation does release substantial amounts of the gas, one or more of the known methane sources must have been considerably overestimated (see The methane mystery).

But those initial measurements, by Frank Keppler of the Max Planck Institute for Nuclear Physics in Heidelberg Physics, Germany, have proven hard to verify. Using a different experimental approach, other researchers failed to find evidence for any significant ‘aerobic’ methane emissions by plants. In a study published in May2, Tom Dueck, then with Plant Research International in Wageningen, the Netherlands, and his colleagues concluded that the contribution of green plants to global methane emission is very small at most.

However, a Chinese–American team has now confirmed, with a very precise carbon-isotope method, that at least some plants do indeed emit measurable, if small, amounts of methane under aerobic conditions3. But the results suggest that the effect is not universal among plants.

Methane in a bottle

Zhi-Ping Wang of the Chinese Academy of Sciences' Institute of Botany in Nanxincun and his colleagues tested 44 different plant species which they had randomly collected around the Chinese Academy of Sciences' Ecosystem Research Station in Inner Mongolia. They put leaves or stems in gas-tight bottles, flushed the bottles with methane-free air (or alternatively with air containing small initial methane concentrations), and then let them rest in the dark at room temperature.

When 10-20 hours later they determined methane concentration, they found that 7 out of 9 woody species — most notably the shrub Achillea frigida — had emitted aerobically produced methane. Herbaceous plants and grasses had not, they report in Environmental Science and Technology3.

If correct, this finding would mean that mid-latitude grasslands such as the Inner Mongolian steppe are not a source for atmospheric methane, whereas ecosystems dominated by trees and shrubs might be more likely sources.

But the results are by no means definitive. Although Wang did not see emissions from herbaceous plants, Keppler did. And there is also some preliminary field evidence for the effect occurring over tropical grasses.

Confusion continues

“This is a difficult process to study,” says Jay Gulledge, a microbiologist at the University of Wyoming in Laramie and a co-author of the newest study. “It will take a lot more work to figure out when, where, and why it happens, and whether or not it is actually important to the global methane budget.”

ADVERTISEMENT

Keppler had estimated initially that terrestrial plants could account for up to 40% of total methane emissions, but the figure has since been revised downwards. If the effect is not universal among plants it would imply an even smaller contribution to the global methane budget.

As yet, all global estimates are extremely uncertain. But even if plants contribute little to the global methane budget, scientists are keen to understand the unknown chemistry behind the elusive effect.

Dueck, who had not looked at woody plants in his own study, says that the new results are interesting and instructive. But they do not clarify the mechanism by which the methane is produced. And it is even harder to see how stems might produce methane. “That only woody plants might emit the gas has got me completely puzzled,” he says.