Towards a rhizocentric view: The importance of root exudation in forests exposed to elevated CO2

Previous conceptual models of plant-microbial interactions under elevated CO₂ have rarely considered how root exudation may influence ecosystem response to elevated CO₂.  Root exudates are notoriously difficult to collect in situ, but have the potential to alter nutrient availability due to their chelating properties, their provision to mycorrhizal symbionts, and their role in stimulating soil microbial activity.  We designed a novel experimental system for collecting exudates from intact roots of field-grown trees using cuvettes filled with sterile glass beads.  We conducted our study at the Duke Forest FACE site, NC, where loblolly pine (Pinus taeda) trees have been exposed to two levels of atmospheric CO₂ (380 and 580 ppmv) at two levels of soil N availability for ten years.  Based on our previous work with pine seedlings, we hypothesized elevated CO₂ would increase exudation rates due to both increases in mass-specific rates (at the lowest N) and increases in fine root biomass (at the highest N).  Exudation patterns at FACE were spatially and temporally variable such that treatment effects were non-significant in the first months of the experiment.  Mass-specific rates varied by a factor of two between roots from the same tree over a two week period in fall of 2006.  Overall, exudation rates were correlated with fine root length, the percentage of mycorrhizal tips and PAR (p = 0.0001).  These results suggest that root exudation in trees may be sensitive to changes in belowground C allocation, but the degree to which such changes affect nutrient availability warrants further investigation.