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

Abstract

Previous research has indicated that a potentially large portion of root-respired CO 2 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 CO 2 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 [CO 2 ] in the xylem (i.e., [CO 2 *]) and calculated F S using the [CO 2 ] gradient method. During the study, stem and soil CO 2 concentrations, temperature, and sap flow were measured continuously. We determined that F T accounted for 33% of daily total belowground CO 2 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 CO 2 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. K E Y W O R D Sautotrophic respiration, belowground carbon allocation, dissolved inorganic carbon, forest carbon cycle, soil CO 2 efflux, tree carbon budget

Differences between tree stem CO2 efflux and O2 influx rates cannot be explained by internal CO2 transport or storage in large beech trees

“…Colours denote different sapwood depths. [Color figure can be viewed at wileyonlinelibrary.com] nonneglectable contribution of CO 2 transport to R S , up to 1/3 in the lower stem segment, highlighting the potentially significant role of CO 2 xylem transport in diverting root-respired CO 2 from soil measurements(Aubrey & Teskey, 2021). Nevertheless, we found two lines of evidence refuting our hypothesis, as CO 2 internal fluxes could not bridge the gap between stem O 2 influx and CO 2 efflux.First, the relation between O 2 influx and R S diverged from the 1:1 line (Figure5b), as indicated by a slope different from the unit and a significant intercept.…”

“…If true, this would be evident in our mature beech trees, whose large sapwood conducting area provides room for potentially high transport and storage of respired CO 2 (Fan et al, 2017). We found a nonneglectable contribution of CO 2 transport to R S , up to 1/3 in the lower stem segment, highlighting the potentially significant role of CO 2 xylem transport in diverting root‐respired CO 2 from soil measurements (Aubrey & Teskey, 2021). Nevertheless, we found two lines of evidence refuting our hypothesis, as CO 2 internal fluxes could not bridge the gap between stem O 2 influx and CO 2 efflux.…”

The Effect of Rapid Development on Soil CO2 Efflux in a Cellulosic Biofuel Stand

“…We also did not measure the potential of respired CO 2 being transported through the xylem, which was outside the scope of this study. It is well known that as LAI increases with stand development so does transpiration [50], which could have led to R a bypassing soil respiration [51,52].…”

Twenty Years of Progress, Challenges, and Opportunities in Measuring and Understanding Soil Respiration

“…However, it may include CO 2 from both (3) aerobic methanotrophy (Serrano‐Silva et al., 2014) and (4) non‐biological CO 2 production (Rey, 2015; Sánchez‐Cañete et al., 2016), processes particularly important in saline and alkaline soils (J. Ma et al., 2013) and geothermally active areas (e.g., F. Liu et al., 2023). In addition, CO 2 produced in soils is not always directly released to the atmosphere (Maier et al., 2011; Sánchez‐Cañete et al., 2018), and thus the surface‐measured flux may deviate from total belowground CO 2 production due to (5) the transformation to bicarbonate ions under high‐pH conditions (Angert et al., 2015); (6) dissolution of respired CO 2 in soil water and subsequent transport (M. S. Johnson et al., 2008; Tamir et al., 2011), potentially via plants' transpiration (Aubrey & Teskey, 2009, 2021); and (7) pressure pumping (Bain et al., 2005; Moya et al., 2019). These and other belowground processes remain important active research areas crucial for reliable estimates of Rs in many ecosystems (Rey, 2015).…”