• fine roots;
  • root biomass;
  • root diameter;
  • root mortality;
  • root N content;
  • root turnover;
  • soil C storage


  • • 
    Greater fine-root production under elevated [CO2] may increase the input of carbon (C) and nitrogen (N) to the soil profile because fine root populations turn over quickly in forested ecosystems.
  • • 
    Here, the effect of elevated [CO2] was assessed on root biomass and N inputs at several soil depths by combining a long-term minirhizotron dataset with continuous, root-specific measurements of root mass and [N]. The experiment was conducted in a CO2-enriched sweetgum (Liquidambar styraciflua) plantation.
  • • 
    CO2 enrichment had no effect on root tissue density or [N] within a given diameter class. Root biomass production and standing crop were doubled under elevated [CO2]. Though fine-root turnover declined under elevated [CO2], fine-root mortality was also nearly doubled under CO2 enrichment. Over 9 yr, root mortality resulted in 681 g m−2 of extra C and 9 g m−2 of extra N input to the soil system under elevated [CO2]. At least half of these inputs were below 30 cm soil depth.
  • • 
    Increased C and N input to the soil under CO2 enrichment, especially below 30 cm depth, might alter soil C storage and N mineralization. Future research should focus on quantifying root decomposition dynamics and C and N mineralization deeper in the soil.