Variability of Root Exudate δ13C and Fluxes in Relation to Environmental Conditions and Plant Characteristics in a Bottomland Temperate Forest

Abstract:

Plant root exudation of organic carbon (OC) is thought to be an important, yet poorly quantified and highly variable component of net primary productivity that influences soil biogeochemistry and ecology. In situ measurements of plant root OC exudation are relatively rare, and δ¹³C measurements of root exudates are generally lacking. Understanding both exudate fluxes and δ¹³C relative to other plant components, root characteristics and environmental parameters (e.g., vapor pressure deficit and soil moisture) will lead to better quantitative understanding of atmosphere—plant—soil linkages. We used a field based collection system to obtain root exudates from fine roots (diameter <5mm) over five sampling periods in a ~20 year old bottomland forest in central Ohio, USA. Exudates were analyzed for dissolved OC concentration and δ¹³C signatures. Exudate flux estimates were made at both the individual root level and also scaled to the entire sampling area. Preliminary data analysis suggests the mean root exudation rate was 26 µmol C g root ^-1 day^-1 and when scaled to the 5600 m² sampling area represents a mean flux of 4,200 µmol C m^-2 day^-1 from tree roots. The flux estimates presented here suggest root exudation may account for as much as 6% of net ecosystem production at the field location. Available data also suggests that exudate δ¹³C is enriched by 1-2 ‰ compared the root material from which exudates were collected. We will also assess the relationship between exudate, root and leaf δ¹³C, environmental parameters, and C fluxes at the site. If root exudation rate or δ¹³C varies as a function of environmental conditions this may suggest that heterotrophic remineralization of root exudates is one potential driver of correlations between soil δ¹³C-CO₂ and environmental parameters.