METHANE is a tricky gas. In the 1970s, when the Viking landers added a nutrient soup made with the radioactive isotope carbon-14 to samples of Martian soil, they detected emissions of methane replete with that isotope. The experiment鈥檚 inventor, Gilbert Levin, claimed this as evidence of life 鈥� the gas was the work of microbes in the soil, he said. Many others remained cautious, believing instead that the soil contained reactive chemicals. The results are still enigmatic.
Last year, methane was at the centre of another surprise. Living plants, it emerged, liberate vast amounts of methane into Earth鈥檚 atmosphere by some mysterious process (New 杏吧原创, 12 January 2006, page 13). By extrapolating from experiments in which plants were grown in incubation chambers, the researchers estimated that living plants produce as much as 30 per cent of the planet鈥檚 methane output.
This result was greeted with varying degrees of shock and confusion. How could we have overlooked such a huge source of what is the second most important greenhouse gas? We must have badly overestimated the output of some other source, such as rice farming or flatulent ruminants. Climate models would need readjusting. Worse, the practical implications looked bad: methane production would increase as the world warmed.
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But wait. Findings by a second team, published this week, found no more methane in containers in which plants were growing than in others devoid of vegetation. The researchers went to extreme lengths to pin down the source of any methane, raising their plants from seed on a diet of carbon-13, rather than the common carbon-12, and growing the plants hydroponically to exclude methane from soil microbes. Plants, they conclude, produce little if any methane (see 鈥淧lants 鈥榥ot to blame鈥� for potent greenhouse gas鈥�).
They suggest that what the first group found was methane left in air spaces between cells and in soil that had not been properly degassed in advance. The first group counters that methane production varies widely, depending on the plant and growing conditions, which the second group did not address. In any case, plants containing carbon-13 are of limited value when trying to calculate emissions from plants grown under natural conditions.
So we have a stand-off. It is a reminder that a single study does not bring scientific understanding; it takes confirmation by others to do that. Inevitably, it will take further research to decide if living plants are major methane producers. Given the importance of the consequences, let鈥檚 hope we solve this one in less time than the mystery of life on Mars.
