Fine root biomass estimates from minirhizotron imagery in a shrub ecosystem exposed to elevated CO2

Fine root biomass and C content are critical components in ecosystem C models, but they cannot be directly determined by minirhizotron techniques, and indirect methods involve estimating 3-dimensional values (biomass/ soil volume) from 2-dimensional measurements. To estimate biomass from minirhizotron data, a conversion factor for length to biomass must be developed, and assumptions regarding depth of view must be made. In a scrub-oak ecosystem in central Florida, USA, root length density (RLD) was monitored for 10 years in a CO2 manipulation experiment using minirhizotron tubes. In the seventh year of the study, soil cores were removed from both ambient and elevated CO2 chambers. Roots from those cores were used to determine specific root length values (m/g) that were applied to the long-term RLD data for an estimation of root biomass over 10 years of CO2 manipulation. Root length and biomass estimated from minirhizotron data were comparable to determinations from soil cores, suggesting that the minirhizotron biomass model is valid. Biomass estimates from minirhizotrons indicate the < 0.25 mm diameter roots accounted for nearly 95% of the total root length in 2002. The long-term trends for this smallest size class (< 0.25 mm diameter) mirrored the RLD trends closely, particularly in relation to suspected root closure in this system. Elevated CO2 did not significantly affect specific root length as determined by the soil cores. A significant treatment effect indicated smallest diameter fine roots (< 0.25 mm) were greater under elevated CO2 during the early years of the study and the largest (2-10 mm) had greater biomass under elevated CO2 during the later years of the study. Overall, this method permits long-term analysis of the effects of elevated CO2 on fine root biomass accumulation and provides essential information for carbon models.