Fine Root Dynamics Alter Biogeochemical Fluxes at Soil Depths up to 60 cm in a CO2-Enriched Sweetgum Plantation.

The production of fine roots (< 2 mm diameter) is expected to increase under elevated atmospheric [CO₂], especially in N-limited forests where increased belowground C allocation may facilitate N acquisition.  Greater root production may lead to increased C storage in long-term soil pools because fine roots turnover quickly in forested ecosystems.  We assessed the effect of elevated [CO₂] on fine-root C and N inputs at soil depths ranging from 0 to 60 cm 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, where the trees had been exposed to current or elevated atmospheric [CO₂] for 9 years.  Fine-root production and mortality nearly doubled under CO₂-enrichment.  Over 9 years, 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.  Unexpectedly, at least half of root biomass inputs were below 30 cm soil depth.  Declining temperature, oxygen content and microbial activity with depth may slow the rate at which C and N are mineralized from root detritus deeper in the soil.  This is demonstrated by a linear decline in the decomposition rate of a standard dowel substrate from 0 to 60 cm soil depth.  Currently, soil organic matter dynamics in many ecosystem models are only simulated in the top 20 cm of the soil profile.  Our results demonstrate the need to measure and model soil organic matter dynamics below 20 cm soil depth; this will provide critical information needed for predicting processes such as long-term soil C storage and N cycling in response to environmental change.