The production of fine roots (less than 2 mm in diameter) is expected to increase under elevated atmospheric [CO₂], especially in N-limited forests where increased belowground C allocation may facilitate nitrogen N acquisition.  Greater fine-root production under elevated [CO₂] may drive changes in soil C storage and N cycling because fine roots turnover quickly in forested ecosystems.  However, the rate at which C and N are re-mineralized from fine-root detritus will depend on root population turnover and chemistry, and the soil depth at which the roots are produced.  We assessed the effect of elevated [CO₂] on fine-root biomass and N inputs at several soil depths using a long-term minirhizotron data set combined with continuous, root-specific measurements of root mass per unit length and [N].  We conducted our research at the Oak Ridge National Laboratory (ORNL), Free-Air CO₂-Enrichment (FACE) experiment in a sweetgum (Liquidambar styraciflua L.) plantation in eastern Tennessee, USA where the sweetgum trees had been exposed to current or elevated atmospheric [CO₂] for 9 years. 

Results/Conclusions

CO₂-enrichment had no effect on fine-root tissue density or [N] within a given diameter class.  Root diameter explained 96% of the variation in fine-root mass per unit root length, and 65% of the variation in fine-root [N] across CO₂ treatments.  Fine-root biomass production and peak standing crop doubled under elevated [CO₂].  Though fine-root population turnover was somewhat slower under elevated [CO₂], fine-root mortality was also nearly doubled under CO₂-enrichment.  Over 9 years, fine-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] relative to the current [CO₂] treatment.  At least half of these inputs were below 30 cm soil depth where microbial mineralization of C and N from fine-root detritus may be limited by soil temperature, oxygen availability, or moisture.  Quantification of the effects of elevated CO₂ on fine-root detritus and its subsequent decomposition, especially at depth in the soil, will provide critical information needed for predicting processes such as long-term soil C storage and N cycling in response to environmental change.