Xylem transport of root-derived CO2 caused a substantial underestimation of belowground respiration during a growing season

Previous research has indicated that a potentially large portion of root-respired CO2 can move internally through tree xylem, but these reports are relatively scarce and have generally been limited to short observations. Our main objective was to provide a continuous estimate of the quantity and variability of root-respired CO2 that moves either internally through the xylem (F-T) or externally through the soil to the atmosphere (F-S) over most of a growing season. Nine trees were measured in a Populus deltoides stand for 129 days from early June to mid-October. We calculated F-T as the product of sap flow and dissolved [CO2] in the xylem (i.e., [CO2*]) and calculated F-S using the [CO2] gradient method. During the study, stem and soil CO2 concentrations, temperature, and sap flow were measured continuously. We determined that F-T accounted for 33% of daily total belowground CO2 flux (i.e., F-S + F-T; F-B) during our observation period that spanned most of a growing season. Cumulative daily F-T was lower than F-S 74% of the time, equivalent to F-S 26% of the time, and never exceeded F-S. One-third of the total CO2 released by belowground respiration over most of the growing season in this forest stand followed the F-T pathway rather than diffusing into the soil. The magnitude of F-T indicates that measurements of F-S alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in F-T observed during the growing season demonstrated the importance of making long-term, high-frequency measurements of different flux pathways to better understand physiological and ecological processes and their implications to global change.