The roles of mycorrhizae, soil microbes, and microbially-mediated soil processes in forest tree responses to rising atmoshperic CO₂

Author

Elizabeth Gerry O'Neill

Date of Award

5-1996

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Susan E. Richert

Committee Members

Richard J. Norby, W. M. Post, Stephen C. Nodvin, Dewey L. Bunting II

Abstract

Carbon dioxide (CO₂) has been increasing exponentially in the earth's atmosphere since the mid-18th century. There is a need to understand the responses of the terrestrial component of the biosphere, including feedbacks from terrestrial systems to the atmosphere and vice versa. Understanding effects of elevated CO₂ on terrestrial systems includes an understanding of what constrains, or limits, responses of vegetation. Evaluating responses of soil processes and the organisms that mediate these processes may be second only to quantifying changes in photosynthetic potential in determining the status of future terrestrial ecosystems. Increased productivity in forested ecosystems may be dependent upon microbially-mediated soil processes that increase nutrient availability in what are commonly nutrient-poor systems. Chemical transformations mediated by rhizosphere (root-associated) bacterial populations represent one mechanism by which soil nutrient availability might increase in a higher CO₂ world. However, in my studies, numbers of bacteria in the rhizospheres of yellow-poplar seedlings were not increased when seedlings were grown at twice-ambient CO₂. Selected microbial functional groups (phosphorus-dissolving bacteria, ammonium-oxidizing bacteria, and nitrite-oxidizing bacteria) in the rhizosphere also showed no response to a doubling of atmospheric CO₂. A second mechanism proposed to increase nutrient uptake is stimulation of the mycorrhizal symbiosis, a ubiquitous relationship between plants and certain soil fungi. Mycorrhizal fungi receive the bulk of their metabolic and structural C from the plant host and increase nutrient uptake by the host in return. Differential responses were seen between two major types of symbioses. Elevated CO₂ increased the number of ectomycorrhizal (ECM) root tips per unit fine root length in two forest tree species. However, no change in vesicular-arbuscular mycorrhizal (VAM) colonization was seen in yellow-poplar seedlings when grown in enriched CO₂. Because most deciduous temperate forests contain host plants of both ECM and VAM fungi, there is potential for CO₂ enrichment to change the competitive balance between plant species. Nutrient availability in soils is ultimately dependent upon decomposition, or breakdown, of dead plant material (litter). Furthermore, rates of decomposition are sensitive to the chemistry of the materials being decomposed. There has been concern expressed that growth in elevated CO₂ might alter litter chemistry and retard decomposition, thus reducing nutrient availability. No changes in litter chemistry or decomposition rate were seen in either yellow-poplar or white oak litter when saplings were grown in the ground in open-topped chambers at two levels of CO₂ enrichment. This is in contrast to other studies, conducted in pots under artificial lighting, where litter chemistry was altered in such a way as to suggest dramatically decreased decomposition rates. We investigated the effect of pot culture on the chemistry of sugar maple litter produced under a combination of CO₂ enrichment and increased (+4°C) temperature treatments. Concentrations of nitrogen (N) were increased in litter produced in elevated temperatures, probably due to abnormal senescence (the process of leaf aging which culminates in leaf fall). However, there were no effects of CO₂ enrichment on litter chemistry, and little effect of pot growth. CO₂ enrichment of forest tree species did not substantially alter soil microbial populations nor change plant litter chemistry. Mycorrhizal relationships were affected, however, in a manner that might lead to changes in tree species composition of forests. Forest ecosystems that are dominated by either ECM or VA-mycorrhizae (e.g., coniferous forests or tropical hardwood forests, respectively) might show little compositional change. Forest ecosystems comprised of mixed ECM and VAM host trees, however, could experience substantial changes in composition. This would have a greater impact on ecosystem level decomposition and nutrient cycling than will changes in leaf litter tissue chemistry.

Recommended Citation

O'Neill, Elizabeth Gerry, "The roles of mycorrhizae, soil microbes, and microbially-mediated soil processes in forest tree responses to rising atmoshperic CO₂. " PhD diss., University of Tennessee, 1996.
https://trace.tennessee.edu/utk_graddiss/9817