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Plants may absorb less carbon dioxide than initally believed University of Minnesota April 12, 2006 The world's land plants will probably not be able to absorb as great a share of the rising atmospheric carbon dioxide as some models have predicted, according to a new study led by Peter B. Reich, professor in the department of forest resources at the University of Minnesota. The work showed that limitations on the availability of nitrogen, a necessary nutrient, will likely translate to limitations on the ability of plants to absorb extra carbon dioxide (CO2). Given that a large proportion of the world's soils are nitrogen-limited, the study implies that the rate of increase in atmospheric CO2 levels could turn sharply upward as nitrogen-limited plants lose their ability to take advantage of the extra CO2 "food." Since rising atmospheric CO2 levels are the largest cause of global "greenhouse" warming, this raises the possibility of accelerated global climate change. The work will be published in the April 13, 2006, issue of Nature. As fossil fuel burning continues and increases, more CO2 is pumped into the atmosphere. Some reports, including the Intergovernmental Panel on Climate Change's "Climate Change 2001: The Scientific Basis," predict that terrestrial plants will be a considerable "sink" for excess CO2. But the researchers, including University of Minnesota scientists Sarah Hobbie, David Tilman and Jared Trost, conclude that the sink may not be as big as projected because the effects of low nitrogen on plants' ability to increase their growth wasn't taken into account. "After a few years, there was only a very modest increase in growth due to CO2 unless extra nitrogen was added," Reich said. "This lack of CO2 growth stimulation without accompany nitrogen fertilization was also seen in a two-year study in a pine plantation. And in a third study, conducted over 10 years on a heavily managed pasture, there was no response to elevated carbon dioxide even with additions of three to four times as much nitrogen fertilizer as in our study. Given that plant productivity around the globe is commonly limited by insufficient nitrogen supply, these studies collectively suggest that terrestrial ecosystems will not soak up future anthropogenic CO2 emissions as vigorously as had been thought. Hence, atmospheric CO2 levels may rise faster than anticipated."
The researchers also found that the presence of leguminous plants, which are capable of turning atmospheric nitrogen gas into a biologically usable form of nitrogen, did not confer any advantage. Plants in plots with and without legumes were statistically indistinguishable in their ability to increase their growth in response to elevated CO2 levels. Nor did it matter whether the plots contained one, four, nine or 16 species of plants. "Different kinds of species respond differently to elevated carbon dioxide and nitrogen because of their physiological needs and capabilities," Reich noted. "Regardless, the diminished response to elevated CO2 at natural nitrogen supply levels was common to plots with different kinds of species as well as different numbers of species. This seems like strong evidence that the nitrogen limitation of CO2 fertilization is a general response." This is a news release from the University of Minnesota
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