By Sally Pobojewski
News and Information Services
Scientists at the Biological Station report that rising levels of atmospheric carbon dioxide (CO2) produce fundamental changes not only in plant growth rates, but also in microorganisms living in the soil and in the levels of carbon and nitrogen deposited in the soil.
Results of their experiment were published in the June issue of Plant and Soil. “This is the most comprehensive study to date detecting changes in soil nitrogen and microbial carbon resulting from rising CO2,” said James A. Teeri, director of the Biological Station and professor of biology. “The study suggests that rising levels of atmospheric carbon dioxide alone, regardless of any global-warming-induced temperature increases, potentially can produce significant changes in ecosystem nutrient cycling.”
As the amount of carbon dioxide in the atmosphere continues to increase, the ex-periment suggests that major changes will occur in the way carbon and nitrogen are transferred from the atmosphere to living organisms, according to Teeri. The long-term effects of changes in these processes, which are basic to life on Earth, are unknown.
The experiment compared three groups of bigtooth aspen seedlings grown outdoors in identical root boxes at the Biological Station for one summer. Two groups of seedlings were grown in open-top chambers; the third (control) group was grown without a chamber.
CO2 levels in one set of chambers were maintained at current atmospheric levels of 350 ppm. Carbon dioxide levels in the second set of chambers were set at 700 ppm, or twice current ambient levels. Many scientists predict atmospheric CO2 levels will reach 700 ppm during the latter half of the next century, if fossil fuel emissions continue to increase.
The aspen seedlings were grown with identical amounts of water and fertilizer. At summer’s end, the trees were harvested and the entire root system excavated by hand. Leaf, stem and root tissues were dried and weighed. The soil in each root box was analyzed to determine the amount of microbes or bacteria present in the soil, along with its total organic carbon and nitrogen content.
While there were few differences between the unchambered trees and those grown in chambers with current levels of CO2, trees grown at twice-ambient levels showed significant differences:
“Our hypothesis is that in the short term more carbon dioxide in the atmosphere sets off a self-fertilizing feedback loop that affects plants, microbes and soil,” Teeri said. “Any change in one component of the loop affects the next component.”
The basic feedback loop as described by Teeri and his colleagues goes like this: More carbon dioxide in the atmosphere speeds up photosynthesis in plants. Higher rates of photosynthesis produce bigger plants with larger roots. Decayed root material and cells sloughed off from growing roots create increased amounts of carbon in the soil. Nourished by an abundant carbon food supply, the number of microorganisms in the soil increases. More microbes produce more mineralized nitrogen in the soil, possibly from the decay of dead organisms.
Teeri cautioned that these preliminary results must be confirmed by long-term experiments in many parts of the world. “We only have results for a single growing season in northern Michigan,” he said. “It’s possible that the self-fertilizing feedback system could reach a plateau over longer periods of time.”
Using a $735,000 Department of Energy grant, Teeri and his colleagues will begin a three-year version of the experiment at the Biological Station near Pellston next spring. He also is providing the complete experimental protocol to researchers in other parts of the country to encourage replication in different environments.
Teeri and his colleagues are currently conducting experiments to determine how increased carbon dioxide changes the chemical composition of plant leaves. “There could be cascading effects down the entire food chain,” Teeri said.
“Rising levels of carbon dioxide in the atmosphere may have the potential to alter the carbon and nitrogen cycles in ecosystems all over the Earth,” Teeri said. “Our job is to discover the extent of these changes and how they will affect plant and animal life.”
Researchers from Michigan State University and Ohio State University also are participating in the Biological Station experiment. The research was funded by grants from the U.S. Department of Agriculture and the University.
