Root uptake and transport of soil dissolved CO2 in tree seedlings

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.