The six-year study is the first long-term examination of how soil nitrogen affects the abilities of long-lived plants in realistic "natural" open-air ecosystems to increase their size by absorbing extra CO2. Only two other long-term experiments in the world are asking similar questions.
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."
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 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."
Peter Reich, forest resources professor, (612) 624-4270
Jeff Falk, University News Service, (612) 626-1720, email@example.com
Deane Morrison, University Relations, (612) 624-2346