Scientists had always believed that plant matter produced methane in regions well isolated from the typically rich oxygenated environments found on the Earth's surface. Then, in 2005, Frank Keppler from the Max-Planck-Institut für Kernphysik, Germany, and colleagues reported that the greenhouse gas could also be created by plants in oxygen-rich environments – a result that attracted much criticism and controversy. "We wanted to confirm that these emissions really exist," Ivan Vigano of Utrecht University told environmentalresearchweb.
The new work, published in the journal Biogeosciences, describes how light and temperature affect aerobic methane release from plant structural compounds and plant matter. The experiments clearly demonstrate that the emission depends on UV light and on temperature, though the researchers have also shown that small amounts of methane are emitted in the dark.
"Our results are proof that methane can be formed under aerobic conditions," said Vigano.
The team exposed several detached fresh and dry leaves, and basic organic plant compounds, to UV light and elevated temperatures. In each case, the researchers measured methane emissions that varied with light intensity, wavelength and the plant material investigated. The leaves produced methane almost instantaneously when irradiated with UV light, which indicates a direct photochemical process, say the scientists.
Emission rates varied from 5 ng CH4 g-1dw h-1 for perennial ryegrass to 4300 ng CH4 g-1/dw h-1 for Guzmania at room temperature. Plants like sweet vernal grass emit 200 ng CH4 g-1dw h-1 and maize just 26 ng CH4 g-1dw h-1 at room temperature.
"Methane is the simplest reduced organic compound and to observe such a compound in a non-reducing environment is unexpected but exciting at the same time," said Vigano. "It remains to be seen what consequences this has for the global methane budget."
He added that the result could be important for climate models that take methane into account but the impact will depend on how big the source really is. The biosphere is the largest reservoir of carbon with potentially large sources of methane from wetlands, rice paddies, marshes and similar environments. "This new source, which could involve the entire biosphere, may have to be implemented in climate models," he explained.
The researchers are now carrying out experiments with living plants and looking more closely at where the methane comes from within the plant structure. They will also go out into the field to perform their experiments.