"We somehow forgot about this methane source." This is the latest news on global warming. Scientists have now realized that plants are also big emitters of methane, which is a greenhouse gas having the second greatest effect on climate, after carbon dioxide. The concentration of methane in the atmosphere has almost tripled in the last 150 years.
Although methane is best known as natural gas, 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. Nowadays, methane in the atmosphere in fact is largely of biogenic origin.
However, until now, it has been assumed that biogenic methane is formed anaerobically, that is, via micro-organisms and in the absence of oxygen. In this way, acetate or hydrogen and carbon dioxide are transformed into methane; they themselves are created in the anaerobic decomposition of organic materials. The largest sources
of methane are wetlands and rice fields, as well as the digestion of ruminants and termites, waste disposal sites, and the gas produced by sewage treatment plants. According to previous estimates, these sources make up two-thirds of the 600 million tones worldwide annual methane production.
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 rye grass. In this investigation, it turned out that living plants let out some 10 to 1000 times more methane than dead plant material. The researchers then were 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, 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.
This discovery also explains some previously puzzling evidences. For example, a research group from the University of Heidelberg has recently measured via satellite the methane concentrations over tropical forests. They have found it unexpectedly high. Now it is clear why: the forests themselves are producing it!
It is estimated that about 10 to 30 percent of present annual methane production comes from plants.
But why would such a seemingly obvious discovery 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," says 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."
In order to determine the quantity of emissions the researchers from Heidelberg conducted very careful experiments, most of them 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 the established opinion - they made a discovery that will require textbooks to have their passages about methane production rewritten.
A related and exciting question is which role the biosphere has played in methane production in the history of the earth, and what kind of influence rising global temperatures and carbon dioxide concentration have on the production of methane from plants. Answers to these questions are important for understanding the feedback mechanisms between climate change and greenhouse gas production.
Photo Credit: Max Planck Institute for Nuclear Physics
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