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, 13C-depleted) and/or elevated air temperature (2.8–3.5°C) on the rate and δ13C of soil respiration. The study was conducted in a boreal Norway spruce forest using temperature-controlled whole-tree chambers and 13C as a marker for root respiration. The δ13C 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 δ13C 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.