The ability of plants to counteract global warming by removing carbon dioxide from the atmosphere and storing it as carbon in the soil is limited, report researchers at the University of California, Davis, Northern Arizona University and the Netherlands.

Their findings will appear this week in the online version of the Proceedings of the National Academy of Sciences.

"The discovery implies that future carbon storage by land ecosystems may be smaller than previously thought, and therefore less of a solution to global warming," said Johan Six, a study co-author who is an agroecologist in UC Davis' Department of Plant Sciences.

The research team summarized the statistical findings from numerous published studies on the effect of atmospheric carbon dioxide on plant growth and soil carbon. They found that soils are limited in their impact on global warming because of their dependence on nitrogen and other nutrients.

"Our paper shows that in order for soils to lock away more carbon dioxide as carbon, there has to be quite a bit of extra nitrogen available -- far more than what is normally available in most ecosystems," said Bruce Hungate, a researcher from Northern Arizona University'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. However, the studies analyzed in this paper revealed 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 researchers acknowledge that plants do play a role in mitigating global warming, with about half of the carbon dioxide emitted into the atmosphere stored, at least temporarily, by land or marine ecosystems.

"But soils of non-managed ecosystems appear to have a limited and diminished capacity to clean up excess carbon dioxide in the atmosphere," said Six. He stressed that reducing reliance on fossil fuels is likely to be far more effective than expecting natural ecosystems to absorb increasing levels of carbon dioxide in the atmosphere.

The findings challenge recent assessments and model projections from the Intergovernmental Panel on Climate Change, which have anticipated increases in soil carbon with rising carbon dioxide.

Lead author on the study is Kees-Jan van Groenigen, a doctoral student at UC Davis. Other collaborators include Marie-Anne de Graaf and Chris van Kessel of UC Davis' Department of Plant Sciences Department, and Nico van Breeman of the Laboratory of Soil Science and Geology at Wageningen University, the Netherlands.

Funding for the study was provided by the National Science Foundation and the Dutch Ministry of Agriculture, Nature Management and Fisheries.