Puzzle remains over source of Amazonian methane


Courtesy of European Commission, Environment DG

Remote sensing observations have shown high methane concentrations in the troposphere above tropical forests in Venezuela and Brazil and scientists claim to have measured significant amounts of methane being emitted aerobically from the leaves of plants during photosynthesis. Recent experiments have been unable to reproduce these controversial results.

Detailed knowledge of the global atmospheric methane budget is of prime importance for climate change research, since it is one of the three principal greenhouse gases and confirmation of aerobic emissions from plants would have profound implications for our atmospheric models.

Several experiments in either closed chambers, or chambers with regulated air circulation, have failed to find measurable increases in methane concentrations over several days. A recent paper reports new, detailed experiments that again find no evidence for methane emissions from two important subtropical plant species which have been identified as potential emitters. The new study has taken real-time measurements of methane and carbon dioxide concentrations over the leaves of plants at 10-minute intervals over 10-12 hour periods.

Experiments used either maize (an important tropical crop species) or tobacco plants, maintained under artificial lamps emitting a known spectrum of light and subject to controlled three-hour periods of light and darkness. Researchers using gas chromatography found no increase in methane concentrations in the closed atmosphere around the actively-photosynthesising leaves while irradiated, nor during dark periods of respiration. Measurements showed that leaf pores were sufficiently open during the experiments to allow the escape of any metabolic methane, had it been produced. Observations also indicated that the plants were not taking up atmospheric methane, either under light or dark conditions

The authors do not dismiss the few studies which have observed methane emissions, suggesting several reasons why their results may differ: temperature and UV light significantly affect the release of methane from detached leaves (leaf litter) and other structural components of plants. Since UV light was not produced by the lamps used in this experiment, scientists may need to study emissions from plants following prolonged exposure to the full solar spectrum.

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