PARENT SESSION
Contributed Oral Session 75: Biogeochemistry: Soil Chemical Flux
Wednesday, August 10, 8:00 AM - 11:30 AM, Meeting Room 513 E, Level 5, Palais des congrès de Montréal

Seven years of fine root regrowth under elevated atmospheric CO₂ following fire in a scrub oak ecosystem in central Florida, USA.

Day, Frank^*,1, Stover, Daniel¹, Pagel, Alisha¹, Hungate, B³, Drake, Bert², ¹ Old Dominion University, Norfolk, VA³ Northern Arizona University, Flagstaff, AZ² Smithsonian Environmental Research Center, Edgewater, MD

ABSTRACT- We have monitored the effects of elevated atmospheric CO₂ on fine root growth in open top chambers (350 ppm and 700 ppm CO₂) in a scrub oak ecosystem at Kennedy Space Center, Florida USA since 1996. The scrub oak ecosystem is influenced by fire, with a natural burn cycle of 10 to 15 years. The study site was control burned prior to initiation of treatments, resulting in loss of aboveground vegetation and die back of fine roots. We hypothesized that root abundance would increase in both treatments as the system recovered from fire, then equilibrate, and the CO₂ response would dissipate with time. Using minirhizotron tubes, we measured fine root length density (mm/cm²) every three months. Fine root mortality, productivity and turnover were determined for the first two years and the most recent two years of the study. During the first 20 months of the study, fine root growth was rapid as the system recovered from fire, and the elevated chambers exhibited significantly enhanced growth compared to the ambient chambers. CO₂ enrichment resulted in higher fine root turnover, production, and mortality. However, after several years of exposure to elevated CO₂, fine root densities equilibrated in the two treatments (relative CO₂ effect = 181% increase in root density in December 1997; less than 3% the past several years), and there were no treatment differences in turnover, production or mortality. Total root biomass obtained from soil cores taken during the sixth year of the study also indicated that the early CO₂ effect has not been sustained. The results suggest that ambient and elevated chambers may have reached closure of the root systems within a few years following the burn, but the CO₂ fertilized chambers reached closure first. Progressive nitrogen limitation is a possible explanation for the equilibration of root abundance. A remaining unknown is carbon sequestration in coarse roots, which are not adequately sampled by soil cores.

Key words: CO₂, fine roots, fire, scrub oak