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Keywords:

  • 14C;
  • carbon cycling;
  • fine-root lifespan;
  • fine-root turnover;
  • live fine-root turnover time;
  • root biomass;
  • root diameter;
  • root mortality

Summary

  • • 
    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 14C to 12C in a mature hardwood forest, enriched by an inadvertent 14C pulse, to investigate fine-root turnover and its relationship with fine-root diameter and soil depth.
  • • 
    Biomass and Δ14C 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 14C enrichment over 3 yr, demonstrating a mean turnover time on the order of years. However, elevated Δ14C 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.