Exudation Rates and δ¹³C Signatures of Bottomland Tree Root Soluble Organic Carbon: Relationships to Plant and Environmental Characteristics

Tree root exudation of soluble organic carbon (SOC) is often considered an important but under-assessed component of terrestrial net primary productivity that also strongly influences rhizosphere and soil biogeochemical processes. Although riparian and bottomland systems are often considered “hot spots” of biogeochemical activity that are potentially supported by root exudate SOC, in situ tree root exudation rates of SOC have not been previously reported for these systems. Additionally, there is an outstanding need to understand the δ¹³C signatures of root exudates in relation to not only different plant components such as leaves and roots but also different ecosystem pools of C, such as CO₂ emitted from soil.

In the present study we used an in situ method to collect root exudate SOC in order to assess root exudation rates in a bottomland forest for Acer saccharinum, Populus deltoides and Platanus occidentalis trees over five sampling dates ranging from mid-summer to late-autumn. Leaves from Acer negundo, Acer saccharinum, Lonicera maackii, Populus deltoides and Platanus occidentalis were also collected. δ¹³C values were determined for all of the root exudates, roots and leaves collected in this study. Exudation rates and δ¹³C values were evaluated in relation to leaf and root morphology, leaf and root C and N contents and a number of environmental parameters (e.g. vapor pressure deficit) and net ecosystem exchange (NEE).

Findings indicate that exudation rates and δ¹³C values of leaves and roots were significantly correlated to time-lagged measurements of NEE, suggesting a strong link between exudation rates and δ¹³C values of leaves and roots and photosynthetic rates. Various time lagged environmental parameters (e.g., vapor pressure deficit) were correlated to the δ¹³C of exudates, leaves and roots—suggesting a rapid transfer of recent photosynthate from the canopy to roots and root exudates and relatively rapid turnover of C in leaves. When pooled together, the leaf δ¹³C values for individual species were found to be significantly related to leaf nitrogen per unit leaf area—suggesting a strong leaf level control of N on leaf δ¹³C values. We also observed that average exudate δ¹³C values became progressively enriched across the first three sampling dates (-32.0 ± 1.0, -29.4 ± 0.7 and -27.9 ± 0.3, respectively), which then leveled off, potentially reflecting a shift in the relative contribution of two or more soluble plant pools that differed in δ¹³C over the course of the study.

When the average net SOC exudation rate (14 ± 3 µmol C g root^-1 d^-1) is scaled to the entire sampling area, root exudation may account for as much as 3.2% of net C uptake. While this may not be major loss of C and energy from plants, this is the net rate, and therefore heterotrophic losses due to bacteria and fungi are not included; therefore, this represents a minimal loss rate. In contrast, this amount of root exudation may represent a potentially important flux of SOC to temperate bottomland soils and their heterotrophic communities.

We suggest that future studies examining δ¹³C within plants or as a natural tracer of measurements of the δ¹³C values of CO₂ would benefit by accounting for both the fluxes and remineralization of root exudates. This is because the findings from the present study suggest that root SOC exudation and δ¹³C values are highly variable and influenced by a number of environmental and plant-level processes that have yet to be fully elucidated.