PS 47-29
Root uptake and transport of soil dissolved CO2 in tree seedlings

Thursday, August 14, 2014
Exhibit Hall, Sacramento Convention Center
Mary Anne McGuire, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA
Thomas Anné, Laboratory of Plant Ecology, Ghent University, Gent, Belgium
Doug P. Aubrey, Department of Biology, Georgia Southern University, Statesboro, GA
Robert Teskey, Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA
Kathy Steppe, Laboratory of Plant Ecology, Ghent University, Ghent, Belgium
Background/Question/Methods

Research has confirmed that root- and stem-respired CO2 can dissolve in xylem sap of trees and be transported upward throughout the stem and crown, where it can flux to the atmosphere and/or be re-assimilated by photosynthetic tissues. It has also been shown that dissolved CO2 can enter plant roots via water uptake, but the magnitude and overall contribution of this CO2 source relative to root- and stem-respired CO2 remains uncertain. To determine if the concentration of CO2 in soil could influence stem CO2 efflux, we supplied water at four CO2 concentrations ([CO2]) to soil of potted oak (Quercus rubra) and pine (Pinus taeda) seedlings and subsequently measured soil [CO2] and stem CO2 efflux. To verify the transport of CO2 from soil to xylem, we supplied water enriched with 13CO2 to seedling roots and subsequently harvested samples of root and stem tissues, which were sealed into individual test tubes, immediately frozen in liquid nitrogen to stop all metabolic activity, and incubated at 3°C to allow the 13CO2 concentration of the xylem water to equilibrate with the gas in the tubes. Gas was then extracted from the tubes and analyzed for 13C by IRMS.

Results/Conclusions

Treatment solution [CO2] (%) (1.5, 5.0, 10.0, 20.0) resulted in soil [CO2] (%) of 0.4, 1.3, 5.1, and 8.6 in pine, respectively; and 0.3, 1.3, 6.4, and 8.5 in oak, respectively. After correcting for pre-treatment values, increases in stem efflux (µmol m-2 s-1) were linearly related to increases in soil [CO2] in pine (from 1.6 at soil [CO2] of 0.4 to 4.9 at soil [CO2] of 8.6, R2=0.99) and in oak (from 0.7 at soil [CO2] of 0.3 to 1.1 at soil [CO2] of 8.5, R2=0.56). Dissolved 13CO2 applied to the soil resulted in higher 13C concentration (atom %) of xylem water of treated roots and stems compared to untreated controls (+0.60 in roots and +0.18 in stems of pine; +0.71 in roots and +0.29 in stems of oak). These 13C results confirmed that dissolved CO2 can be taken up by roots and transported into the stem. Efflux results suggested that [CO2] in stems increased due to transported CO2 in proportion to the [CO2] of the soil. However, since soil [CO2] in natural conditions rarely exceeds 1%, the amount of CO2 transferred from soil into plants is likely small compared to the amount of root- and stem-respired CO2 transported within plants.