A scrub-oak ecosystem at Kennedy Space Center, Florida was exposed to 11 years of atmospheric CO₂ enrichment (ambient + 350 ppm) using open-top chambers. The objective of our study was to determine the effect of elevated CO₂ on fine roots < 2 mm diameter using minirhizotrons and on coarse roots > 5 mm diameter using ground-penetrating radar (GPR). Fine root measurements of root length density (RLD) were applied to specific root length values to calculate fine root biomass to a depth of 100 cm. GPR data were applied to previously-established relationships between GPR signal reflectance and root biomass to estimate coarse root biomass to a depth of 60 cm. 3-dimensional GPR data imaging was also used to assess root architecture and depth distributions.

Results/Conclusions

Total root biomass after 11 years of CO₂ enrichment was not greater in elevated plots (7670 ± 467 g/m²) than in ambient plots (6655 ± 399 g/m²; P = 0.1208). Coarse roots comprised 76% and 77% of root biomass for elevated and ambient plots, respectively. Root-to-shoot ratios were 3.9 for elevated and 5.5 for ambient CO₂ plots, demonstrating that root biomass contributes the most to total plant biomass in this system. Belowground plant structures in this fire-dominated system sequester substantial carbon from the atmosphere and provide an important sink for atmospheric CO₂. Under elevated CO₂, depth distribution of root biomass was affected, with an increase in biomass at 10-50 cm depth compared to a decrease of biomass with increasing depth in ambient CO₂ plots.