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Title: CHANGES IN MICROBIAL ACTIVITY AND COMPOSITION IN A PASTURE ECOSYSTEM EXPOSED TO ELEVATED ATMOSPHERIC CARBON DIOXIDE

Author
item MONTEALEGRE, CONSTANZA - UNIV OF MINNESOTA
item VAN KESSEL, CHRIS - UNIV OF CALIF-DAVIS
item Russelle, Michael
item SADOWSKY, MICHAEL - UNIV OF MINNESOTA

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/11/2002
Publication Date: 6/1/2002
Citation: Montealegre, C.M., Van Kessel, C., Russelle, M.P., Sadowsky, M.J. 2002. Changes in microbial activity and composition in a pasture ecosystem exposed to elevated atmospheric carbon dioxide. Plant and Soil. 243:197-207.

Interpretive Summary: It is apparent that human activities are increasing carbon dioxide concentrations in the air. This gas reflects heat that radiates from the Earth's surface and participates in the 'greenhouse effect.' Carbon dioxide is the source of nearly all food on Earth, because green plants convert this gas to sugars and other carbohydrates and to amino acids and proteins. With some plants, higher carbon dioxide concentrations result in more plant growth, but with others, there appears to be little effect. We discovered that soil bacterial populations also increased around clover roots when these plants were grown at high carbon dioxide concentrations. Because clover can obtain its nitrogen from the air, its growth can be stimulated by improved carbon dioxide availability. This is not the case with a grass, and we found no increase in soil bacteria around grass roots. These changes are probably due to the effect plant roots have on food availability in the soil. Roots, themselves, are food for some soil organisms, and roots also excrete organic compounds that are used by bacteria, fungi, and other organisms. In addition, we discovered that higher carbon dioxide levels caused changes in the kinds of bacteria present in the soil. This may mean that some functions of the soil may change as carbon dioxide levels rise. Soil organisms help process nutrients, like nitrogen and phosphorus, making them available for plant uptake or loss. This research is important because it shows that as carbon dioxide in the air increases it will also affect the ecology underground. This information will be useful to ecologists, soil microbiologists, and soil scientists as they consider strategies to deal with excess carbon dioxide.

Technical Abstract: Elevated atmospheric CO2 increases above-ground plant growth and productivity. Carbon dioxide-induced alterations in plant growth also are likely to affect below-ground processes, including the composition of soil biota. We investigated the influence of increased atmospheric CO2 on bacterial numbers and activity and on the soil microbial community composition in a pasture ecosystem under Free-Air Carbon Dioxide Enrichment (FACE). Composition of the soil microbial communities under two atmospheric CO2 levels was evaluated by phospholipids fatty acid analysis (PLFA), and total and respiring bacterial counts were determined by epifluorescence microscopy. Although populations increased with increased atmospheric CO2 under white clover (Trifolium repens L.), elevated atmospheric CO2 concentration did not affect total or metabolically active bacteria in bulk soil of perennial ryegrass (Lolium perenne L.). Moreover, there was an 85% increase in total rhizosphere bacteria, and a 170% increase in respiring rhizosphere bacteria, with elevated atmospheric CO2, when assessed on a per unit land area basis. However, no change in rhizosphere bacterial numbers was observed when assessed on a per g soil basis. Differences in microbial community composition between rhizosphere and bulk soil were evident in samples from white clover, as well as the response of such communities to CO2 enrichment. Results of this study indicate that changes in soil microbial activity, numbers, and community composition are likely to occur under elevated atmospheric CO2, but the extent of those changes depends on plant species and the distance that microbes are from the immediate vicinity of the plant root surface.