Methane-making plants in the Inner Mongolian steppe
Although a new study confirms previous findings that plants make methane, this ability may be limited to shrubs.
When, nearly 2 years ago, a study first suggested that plants emit methane, scientists received the news with a flurry of excitement, a dash of skepticism, and hasty speculations on plants' contribution to global warming. A new study published in ES&T (DOI: 10.1021/es071224l) is the first to confirm that plants do indeed make this potent greenhouse gas. But it also finds that the methane-making ability varies among types of plants and, at least in the grasslands of Inner Mongolia, is limited to woody shrubs.
Until recently, scientists thought they had all the sources of methane figured out. They knew that anaerobic bacteria living in swamps, rice paddies, and the stomachs of ruminating animals produce it. The rest is known to come from human activities like burning of fossil fuels, fires, and waste management.
In the original study, published in January 2006, Frank Keppler of the Max Planck Institute for Nuclear Physics (Germany) tested 30 temperate and tropical trees and grasses. He found them burping tiny amounts of methane, which added up to global emissions of 62–236 million metric tons per year (Mt/yr), larger than emissions from wetlands—the biggest known source of methane. Subsequent studies by other groups reduced the estimates to 20–60 Mt/yr.
In April 2007, Thomas Dueck of Wageningen University (The Netherlands) cast a shadow of doubt on the validity of Keppler's findings. Unlike Keppler, who used detached leaves, Dueck used intact plants and failed to find biologically significant levels of emissions. Using detached leaves is faulty methodology, says Dueck, because it doesn't rule out the possibility of background methane diffusing out of leaves.
In the current ES&T study, lead author Zhi-Ping Wang of the Chinese Academy of Sciences and his team took a "landscape approach" instead of randomly picking plants as done in previous studies. The team wanted to know whether methane emissions from plants could be important across a vast landscape such as that of Inner Mongolia, says coauthor Jay Gulledge of the Pew Center on Global Climate Change.
The researchers tested 34 upland plants—25 herbs and 9 shrubs—from the Inner Mongolian steppe. None of the herbs showed any sign of methane production, whereas seven of the shrubs emitted methane at levels that varied among the species. And unlike Keppler's work, this study found a carbon isotope fingerprint that explicitly marked the methane as of plant and not microbial origin. However, the total amount of the gas released was insufficient to make a dent in the grasslands' huge carbon-absorbing ability. The findings suggest that methane is likely to be "very important in some ecosystems and not in all others," says Gulledge.
"This is a nice piece of work," says Keppler, referring to the ES&T study. "It also shows how complex plants are—that is, release rates of methane from plants are varying tremendously between species."
And even the skeptical Dueck is impressed. He admits that he may have failed to detect methane in his experiments because he tested only herbs. And he is more convinced by Wang's methods than by Keppler's, because chopping up the leaves, as Wang did, would have released any background methane from them.
Although the study provides "some support" for Keppler's discovery, "the possibility of methane diffusion from plant tissues has not been entirely ruled out by this work," says Michael Keller, chief of sciences at the National Earth Observatory Network (NEON). The results "provide interesting circumstantial evidence but not absolute proof" that the source of the methane is indeed a chemical process in the plant.
Gulledge understands the skepticism. He says the biggest problem with his and Wang's findings, and with those of Keppler, is that the potential physiological mechanism behind their observations remains a mystery. "Until you explain the pathway, you can't be entirely sure what's going on here," he says. Keppler's group is trying to solve this mechanistic mystery. "We have found specific carbon moieties in plants as precursors that are able to form methane under aerobic conditions," Keppler adds, referring to some of his unpublished results.
As for estimating global methane contributions from plants, it is still too early to extrapolate. For now, the fact that plants can indeed synthesize methane in the presence of oxygen is "just very, very huge, from a biological perspective," says Gulledge. "From an environmental perspective, it's probably a less major discovery."
