Fine-root respiration in a loblolly pine and sweetgum forest growing in elevated CO₂

•  The loss of carbon below-ground through respiration of fine roots may be modified by global change. Here we tested the hypothesis that a reduction in N concentration of tree fine-roots grown in an elevated atmospheric CO₂ concentration would reduce maintenance respiration and that more energy would be used for root growth and N uptake. We partitioned total fine-root respiration (R_T ) between maintenance (R_M ), growth (R_G ), and N uptake respiration (R_N ) for loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) forests exposed to elevated CO₂ . •  A substantial increase in fine-root production contributed to a 151% increase in R_G for loblolly pine in elevated CO₂ . Root specific R_M for pine was 24% lower under elevated CO₂ but when extrapolated to the entire forest, no treatment effect could be detected. •  R _G (< 10%) and R_N (< 3%) were small components of R_M in both forests. Maintenance respiration was the vast majority of R_T , and contributed 92% and 86% of these totals at the pine and sweetgum forests, respectively. •  The hypothesis was rejected because the majority of fine-root respiration was used for maintenance and was not reduced by changes in root N concentration in elevated CO₂ . Because of its large contribution to R_T and total soil CO₂ efflux, changes in R_M caused by warming may greatly alter carbon losses from forests to the atmosphere.