Nature can help reduce greenhouse gas, but only to a point

FLAGSTAFF, Ariz.--Plants apparently do much less than previously thought to counteract global warming, according to a paper to be published in next week's online edition of Proceedings of the National Academy of Sciences.

The authors, including Bruce Hungate of Northern Arizona University and lead author Kees-Jan van Groenigen of UC Davis, discovered that plants are limited in their impact on global warming because of their dependence on nitrogen and other trace elements. These elements are essential to photosynthesis, whereby plants remove carbon dioxide, a greenhouse gas, from the air and transfer carbon back into the soil.

"What our paper shows is 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," said Hungate of NAU's Merriam-Powell Center for Environmental Research.

The paper notes that various plants can pump nitrogen from the air into soils, and some researchers expected rising carbon dioxide to speed up this natural nitrogen pump, providing the nitrogen needed to store soil carbon. However, the research team found that this process, called nitrogen fixation, cannot keep up with increasing carbon dioxide unless other essential nutrients, such as potassium, phosphorus and molybdenum, are added as fertilizers.

The study, which analyzed all published research to date, challenges recent assessments and model projections from the Intergovernmental Panel on Climate Change that anticipated large increases in soil carbon with rising carbon dioxide.

"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 said.

That's not to say plants are not effective deterrents to global warming. Hungate said about half of the carbon dioxide emitted into the atmosphere is stored, at least temporarily, by the ecosystems on land and oceans.

"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," he said. "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."

In addition to Hungate and van Groenigen, the authors of the study are Johan Six, Marie-Anne de Graaff and Chris van Kessel of the Department of Plant Sciences at the University of California Davis, and Nico van Breemen of the Laboratory of Soil Science and Geology at Wageningen University, the Netherlands.

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