Effects of elevated atmospheric carbon dioxide and temperature on soil respiration in a boreal forest using δ13C as a labeling tool

This study examines the effect of elevated atmospheric carbon dioxide [CO2] (+340 ppm, C-13-depleted) and/or elevated air temperature (2.8-3.5 degrees C) on the rate and delta C-13 of soil respiration. The study was conducted in a boreal Norway spruce forest using temperature-controlled whole-tree chambers and C-13 as a marker for root respiration. The delta C-13 of needle carbohydrates was followed after the onset of the CO2 treatment in August 2001 and during a 2.5-week period in the summer of 2002. Averaged over the growing seasons of 2002 and 2003, we observed a 48% and 62% increase, respectively, in soil respiration in response to elevated [CO2], but no response to elevated air temperature. The percentage increase in response to elevated [CO2] varied seasonally (between 10% and 190% relative to the control), but the absolute increase varied less (39 +/- 11 mg C m(-2) h(-1); mean +/- SD). Data on delta C-13 of soil respiration indicate that this increase in soil respiration rate resulted from increased root/rhizosphere respiration of recently fixed carbon. Our results support the hypothesis that root/rhizosphere respiration is sensitive to variation in substrate availability.