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A jungle

Without enough nitrogen, jungles and forests like this one cannot absorb as much CO2 from the atmosphere.

Plants to reduce less of our carbon dioxide mess

A University study shows that limited access to a key nutrient will curb the ability of plants to absorb carbon dioxide

By Deane Morrison

April 17, 2006

As fossil fuel burning continues to pump carbon dioxide (CO2) into the air, the world's land plants will be unable to absorb as much of it as had been predicted, according to a new study led by Peter B. Reich, professor in the University's department of forest resources. 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 the extra CO2. Given that a large proportion of the world's soils are nitrogen-limited and that supplying extra nitrogen would be impractical, 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 warming, this raises the possibility of accelerated global climate change. The work appears in the April 13, 2006, issue of the journal Nature. The six-year study, carried out at the University's Cedar Creek Natural History Area north of the Twin Cities, is the first long-term examination of how soil nitrogen affects the abilities of long-lived plants to increase their size by absorbing extra CO2. Only two other long-term experiments in the world are asking similar questions.

"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 [human-generated] CO2 emissions as vigorously as had been thought, "says Reich. "Hence, atmospheric CO2 levels may rise faster than anticipated."

A report from the Intergovernmental Panel on Climate Change's "Climate Change 2001: The Scientific Basis," predicts that terrestrial plants will be a considerable "sink"--or sponge--for excess CO2. But the U researchers, including scientists Sarah Hobbie, David Tilman and Jared Trost, concluded 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 accompanying 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 [human-generated] CO2 emissions as vigorously as had been thought. Hence, atmospheric CO2 levels may rise faster than anticipated." In their study, the researchers grew 296 field plots containing different numbers and combinations of perennial grassland species. They subjected each plot to one of the following: extra soil nitrogen, extra atmospheric CO2, more of both, or more of neither. They tallied responses twice each year by measuring the amount of plant material-which is 45 percent carbon-produced under each regimen, both above ground and down to 20 centimeters below ground. After four to six years, plots receiving augmented nitrogen acquired at least three times as much extra carbon in response to elevated CO2 as did plots without additional nitrogen supply. 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 how many species of plants--one, four, nine or 16--were growing on the plots. "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."