PARENT SESSION
Oral Session 15: Soil Ecology I: Mycorrhizal Communities.
Presiding: V Borowicz and X He
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room D 137.

Patterns of rhizosphere carbon flux in sugar maple and yellow birch saplings.

Phillips, Richard ^*,1, Fahey, Timothy ¹, ¹ Cornell University, Ithaca, NY

ABSTRACT- Rhizosphere carbon flux (RCF) has rarely been measured for intact root-soil systems. We measured RCF for eight year-old saplings of sugar maple (Acer saccharum) and yellow birch (Betula allegheniensis) collected from Hubbard Brook Experimental Forest (HBEF), NH and transplanted into pots with native soil horizons intact. We hypothesized birch roots which support ectomycorrhizal fungi would release more C to the rhizosphere than sugar maple roots which support vesicular-arbuscular mycorrhizal fungi. Five saplings of each species were pulse labeled with ¹³CO₂ at ambient CO₂ concentrations for 4-6 hours, and the ¹³C label was chased through rhizosphere and bulk soil pools in organic and mineral horizons for 7 days. We observed immediate appearance of the label in rhizosphere soil of both species, and a striking difference between species in the timing of ¹³C released to soil. In sugar maple, peak concentration of the label appeared one day after labeling and declined over time whereas in birch the label increased in concentration over the chase period. Approximately 3% of the assimilated label was lost from the system as root and rhizomicrobial respiration, though differences between species were not significant. We estimate total RCF from sugar maple was 5.9% and 12.3% of assimilated C in sugar maple and yellow birch which represents 31 and 64 g C/m²/yr in forests at HBEF. These results suggest RCF likely represents a substantial flux of C to soil in northern hardwood forests with important consequences for soil microbial activity, nutrient availability and C storage.

Key words: rhizodeposition, pulse labeling, rhizosphere carbon flux, mycorrhizae