Published online 20 August 2008 | Nature | doi:10.1038/news.2008.1053

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Tibetan meadows emit methane

Field survey confirms that plants can boost levels of the greenhouse gas.

A three-year field study on the Tibetan plateau has shown that plant species differ in their ability to emit or consume methane, a potent greenhouse gas. This could shed fresh light on the role of plants in global methane budgets.

mountainsThe meadows of the Tibetan plateau appear to be emitting the potent greenhouse gas methane.Punchstock

The saga began in early 2006 when Frank Keppler, of the Max Planck Institute for Chemistry in Mainz, Germany, and his colleagues, revealed that plants — thought to be helping combat climate change by taking in carbon-dioxide — may collectively emit millions of tonnes of methane1.

Based on his laboratory experiments, Keppler estimated that globally, plants could emit up to 236 million tones per year, some 30% or so of total methane emissions.

Since then, researchers from other labs have got mixed results, putting Keppler's claim under scrutiny and questioning the relevance of those lab findings to what happens in the real world.

Now a team led by Zhao Xinquan, director of the Northwest Plateau Institute of Biology, Chinese Academy of Sciences, in Xining, Qinghai province, report in Biology Letters2 evidence from field experiments that plants can emit methane in the natural environment.

Sealed meadow

The researchers studied two types of grass meadow and a shrub community in a grass meadow on the Tibetan plateau — all of which contain a mixture of plant species. To trap the gas emission from the plants, they covered a fixed area of the land with a transparent, plastic chamber. Methane emission from the soil was also similarly measured in plots where the above ground part and live roots of the plants were removed.

To preventing overheating, the researchers installed small electric fans to circulate the air and covered the chambers with foam and white waterproof cloth.

After subtracting the soil contribution from the total methane emission, the researchers found that the two grass meadows emitted methane, whereas the shrub community consumed the greenhouse gas. And there are clear seasonal variations: methane emission by grasses peaks in summers, whereas shrubs are more active at consuming methane in winters.

They calculate that meadows on the Tibetan Plateau emit about 130,000 tonnes of methane every year, a small but important part of the regional methane budget.

"Our findings suggest that the question of the role of methane in plant metabolism is an extremely complex one," says Zhao. "There seems to be great variations in plants' ability to emit methane, which are affected by a multitude of variations under natural conditions." This may explain the mixed results from different laboratories, he adds.

Mechanistic mystery

Keppler speaks highly of the study: "It's a good indication that plants can emit methane in the natural environment." He maintains, however, that the study does not rule out the possibility that part of the methane emission captured by the chambers came from alternative sources.

Proof should come from uncovering the biological mechanisms by which plants produce methane. Earlier this year, Keppler's team announced that pectin, a type of sugar molecule derived from the cell wall of higher terrestrial plants, could be a precursor of atmospheric methane3.

More recently, studies by both an independent group and by Keppler have demonstrated that ultraviolet radiation boosts methane emission from pectin4,5, which is consistent with the seasonal variations of methane emission on the Tibetan plateau.

Zhao stresses that further research is needed to understand the basic biology of methane emission and consumption by plants. "Until then, it might be a bit premature to speculate on their role in global climate change," he says. 

  • References

    1. Keppler, F., Hamilton, J. T. G., Bras zlig, M. & Röckman, T. Nature 439, 187–191 (2006).
    2. Cao, G. et al. Biol. Lett. doi:10.1098/rsbl.2008.0373 (2008).
    3. Keppler, F. et al. New Phytol. 178, 808–814 (2008).
    4. McLeod, A. R. et al. New Phytol. doi: 10.111/j.1469-8137.2008.02571.x (2008).
    5. Vigano, I., van Weelden, H., Holzinger, R., Keppler, F. & Röckmann, T. Biogeosciences 5, 937–947 (2008).
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