CO₂ enrichment increases carbon and nitrogen input from fine roots in a deciduous forest

• 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.