11 April 2006
Plants’ Capacity To Soak Up Carbon Limited
by Kate Melville

Plants do much less than previously thought to soak up carbon dioxide, say Bruce Hungate of Northern Arizona University and Kees-Jan van Groenigen of the University of California Davis. Their paper, appearing in the Proceedings of the National Academy of Sciences, suggests that plants are limited in their capacity to clean up excess carbon dioxide in the atmosphere. And unfortunately, even their current abilities may be diminishing.

According to the paper, the limitation on carbon take-up stems from a dependence on nitrogen and other trace elements that are essential for photosynthesis; the process that removes carbon dioxide from the air and transfers it back into the ground. "Our paper shows that in order for soils to lock away more carbon as carbon dioxide rises, there has to be quite a bit of extra nitrogen available - far more than what is normally available in most ecosystems," explained Hungate.

It was previously thought that rising carbon dioxide levels would also speed up the process of nitrogen fixation, where plants "pump" nitrogen back into the soil. But this process can only increase if higher levels of other essential nutrients, such as potassium, phosphorus and molybdenum are available. "The discovery implies that future carbon storage by land ecosystems may be smaller than previously thought, and therefore not a very large part of a solution to global warming," Hungate lamented.

While plants may not save the planet, they still play an important role in reducing carbon dioxide levels. "We do know that CO₂ in the atmosphere would be increasing faster were it not for current carbon storage in the oceans and on land," said Hungate. "But land ecosystems appear to have a limited and diminishing capacity to clean up excess carbon dioxide in the atmosphere. Reducing our reliance on fossil fuels is likely to be far more effective than expecting natural ecosystems to mop-up the extra CO₂ in the atmosphere."

Source: Northern Arizona University

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