Plants - The Forgotten Methane Source

PhysOrg reports today that researchers from the Max Planck Institute for Nuclear Physics have made a surprising discovery: plants naturally release methane, a greenhouse gas. This discovery goes against all previous assumptions and adds a new wrinkly to our understanding of how gases which influence the climate are exchanged between the biosphere and atmosphere.

Equally surprising was the finding that the methane formation is not hindered by the presence of oxygen. This discovery is important not just for plant researchers but also for understanding the connection between global warming and increased greenhouse gas production.

Methane is the greenhouse gas which has the second greatest total effect on climate, after carbon dioxide (CO2), and has aproximately 20 times the heat absorbing climate change potential per molecule than CO2. The concentration of methane in the atmosphere has almost tripled in the last 150 years. As the article points out, only part of the methane uptake in the atmosphere is due to industrial activities connected to energy production and use. More important for the increase of methane in the atmosphere is the increase in so-called "biogenic" sources, e.g., rice cultivation or domestic ruminants related to the rise in the world's population. In fact, nowadays the methane in the atmosphere is largely of biogenic origin.

Until now, it has been assumed that biogenic methane is formed during the anaerobic decomposition of organic material in the absence of oxygen which occurs in land fills, sewage treatment plants, wetlands, in rice patties and in the digestive tracts of animals (i.e. cow farts!). According to previous estimates, these sources make up two-thirds of the 600 million tonnes worldwide annual methane production, as the article reports.

The article reports that scientists from the Max Planck Institute for Nuclear Physics have now discovered that plants themselves produce methane and emit it into the atmosphere, even in completely normal, oxygen-rich surroundings. The researchers made the surprising discovery during an investigation of which gases are emitted by dead and fresh leaves. Then, in the laboratory and in the wild, the scientists looked at the release of gases from living plants like maize and ryegrass. In this investigation, it turned out that living plants let out some 10 to 1000 times more methane than dead plant material undergoing aerobic (oxygen-exposed) decomposition. According to the article, the researchers were then able to show that the rate of methane production grew drastically when the plants were exposed to the sun.

Although the scientists have some first indications, PhysOrg reports that it is still unclear what processes are responsible for the formation of methane in plants. The researchers from Heidelberg assume that there is an unknown, hidden reaction mechanism, which current knowledge about plants cannot explain - in other words, a new area of research for biochemistry and plant physiology (obviously this is a discovery with exciting implications for the field of botany and plant physiology).

In terms of total amount of production worldwide, PhysOrg reports that the scientists' first guesses are between 60 and 240 million tonnes of methane per year. That equates to about 10 to 30 percent of present annual methane production. The largest portion of that - about two-thirds - originates from tropical areas, according to the report, because that is where the most biomass is located. The evidence of direct methane emissions from plants also explains the unexpectedly high methane concentrations over tropical forests, measured only recently via satellite by a research group from the University of Heidelberg.

The article also questions why such a seemingly obvious discovery would only come about now, 20 years after hundreds of scientists around the globe started investigating the global methane cycle? "Methane could not really be created that way," responds Dr. Frank Keppler. "Until now all the textbooks have said that biogenic methane can only be produced in the absence of oxygen. For that simple reason, nobody looked closely at this."

The fact is that, in order to determine the quantity of emissions, scientists indeed have to make very careful measurements, as the article discusses. The researchers from Heidelberg conducted most of their experiments in methane-free air, in order to factor out the high natural background of methane. Furthermore they used isotope analysis to show beyond doubt that this was an undiscovered process of methane production. By "looking closely" - despite established opinion - they made a discovery that will require textbooks to have their passages about methane production rewritten.

The article concludes by mentioning the the continued laboratory work and field and remote sensing studies that will be required to better quantify the strength of these methane emissions. The findings also raise the exciting question as to what role the biosphere has played in methane production throughout the earth's history, as well as what kind of influence rising global temperatures and carbon dioxide concentration have on the production of methane from plants. The answers to these questions obviously have important implications for our understanding of the feedback mechanisms between climate change and greenhouse gas production.

[An obvious hat tip to PhysOrg]