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

• 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 CO 2 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 CO 2 . • A substantial increase in fine-root production contributed to a 151% increase in R G for loblolly pine in elevated CO 2 . Root specific R M for pine was 24% lower under elevated CO 2 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 CO 2 . Because of its large contribution to R T and total soil CO 2 efflux, changes in R M caused by warming may greatly alter carbon losses from forests to the atmosphere.