Date of Award

8-2008

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Richard J. Norby, Aimée T. Classen

Committee Members

James A. Fordyce, Louis J. Gross, Jennifer A. Schweitzer

Abstract

Increased forest growth in response to rising atmospheric concentrations of CO2 may mitigate a portion of fossil fuel emissions, especially if carbon is sequestered in longlived biomass or soil pools. Greater carbon uptake under elevated atmospheric [CO2] in forested ecosystems may facilitate the production of small diameter (i.e. “fine”) roots used for nutrient acquisition. Increased fine-root production in forested ecosystems may affect soil carbon storage and nitrogen cycling because fine roots live and die in the span of a year. My dissertation research took advantage of a long-term, on-going Free-Air CO2-Enrichment experiment in a sweetgum (Liquidambar styraciflua L.) forest stand at Oak Ridge National Laboratory to investigate the causes and consequences of increased fine-root production under elevated [CO2]. To examine the premise that N limitation was the cause of increased fine-root production in the CO2-enriched sweetgum stand, I fertilized plots in an adjacent sweetgum plantation with 200 kg ha-1 of N as urea. The relative C flux to wood production that I observed in the fertilized treatment is consistent with the premise that increased root production in the adjacent FACE experiment is in response to N limitation. To examine the consequencesof increased fine-root production under elevated [CO2], I: (1) quantified fine-root biomass and N inputs at several soil depths using a long-term minirhizotron data set combined with continuous, root-specific measurements of root mass per unit length and [N], and (2) allowed fine roots grown under current and elevated [CO2] to decompose in a common garden experiment by modifying existing litterbag methodology. I found that C and N inputs via root mortality were doubled under elevated [CO2], and half of the inputs were below 30 cm soil depth. However, CO2-enrichment had no effect on fine-root chemistry or decomposition rate, and therefore more root detritus may be incorporated into long-lived soil organic matter under elevated [CO2]. Quantification of the effects of elevated CO2 on the fate of a greater quantity of fine-root detritus, especially at depth in the soil, will provide critical information needed for predicting processes such as long-term soil C storage and N cycling in response to environmental change.

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