ithout 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."

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: augmented soil nitrogen, elevated atmospheric CO2, augmentation of both nitrogen and CO2, or augmentation 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 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 nitrog
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Contact: Deane Morrison
morri029@umn.edu
612-624-2346
University of Minnesota
12-Apr-2006

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