Decadal Effects of Elevated CO2 and O3 on Forest Soil Respiration and Belowground Carbon Cycling at Aspen FACE

Three northern temperate forest communities in the north-central United States were exposed to factorial combinations of elevated carbon dioxide (CO₂) and/or tropospheric ozone (O₃) for 11 years, advancing from open-grown seedlings <0.25 m in height at the beginning to the experiment to closed-canopy stands that were >8 m tall. Here, we report results from measurements of soil respiration that occurred during the experiment from 1999 to 2008. In order to better understand this flux, we compare changes in soil respiration to the effects of CO₂ and O₃ on net primary productivity (NPP), fine root biomass, and leaf litter production.

Elevated CO₂ enhanced soil respiration by an average of 28%. This stimulation of soil respiration varied from +19% to +44%, but did not change consistently during the 10 year measurement period (r² = 0.04). The effect of elevated O₃ on soil respiration was dynamic. In year two of the experiment (1999), elevated O₃ decreased soil respiration by 7%. However, soil respiration consistently increased through time under elevated O₃ (r² = 0.71) and was 9% greater than under ambient O₃ in the final year of the experiment (2008). Overall, elevated O₃ had no meaningful effect on soil respiration (+0.3%). The annual effects of elevated CO₂ on soil respiration were not correlated with NPP or fine root biomass, but was positively correlated with leaf litter production (r = 0.57). Annual leaf litter production was also related to the annual effects of elevated O₃ on soil respiration (r = 0.78), but relationship was tighter between annual O₃ effects on NPP and soil respiration (r = 0.83).