CO(2) enrichment increases carbon and nitrogen input from fine roots in a deciduous forest

Greater fine-root production under elevated [CO(2)] 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 [CO(2)] 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 CO(2)-enriched sweetgum (Liquidambar styraciflua) plantation. CO(2) 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 [CO(2)]. Though fine-root turnover declined under elevated [CO(2)], fine-root mortality was also nearly doubled under CO(2) 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 [CO(2)]. At least half of these inputs were below 30 cm soil depth. Increased C and N input to the soil under CO(2) 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.