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.