C-labeled hardwood forest

• Characterization of turnover times of fine roots is essential to understanding patterns of carbon allocation in plants and describing forest C cycling. We used the rate of decline in the ratio of 14 C to 12 C in a mature hardwood forest, enriched by an inadvertent 14 C pulse, to investigate fine-root turnover and its relationship with fine-root diameter and soil depth. • Biomass and ∆ 14 C values were determined for fine roots collected during three consecutive winters from four sites, by depth, diameter size classes ( < 0.5 or 0.5– 2 mm), and live-or-dead status. • Live-root pools retained significant 14 C enrichment over 3 yr, demonstrating a mean turnover time on the order of years. However, elevated ∆ 14 C values in dead-root pools within 18 months of the pulse indicated an additional component of live roots with short turnover times (months). Our results challenge assumptions of a single live fine-root pool with a unimodal and normal age distribution. • Live fine roots < 0.5 mm and those near the surface, especially those in the O horizon, had more rapid turnover than 0.5–2 mm roots and deeper roots, respectively.