Time-dependent responses of soil CO2 efflux components to elevated atmospheric [CO2] and temperature in experimental forest mesocosms

We previously used dual stable isotope techniques to partition soil CO2 efflux into three source components (rhizosphere respiration, litter decomposition, and soil organic matter (SOM) oxidation) using experimental chambers planted with Douglas-fir [ Pseudotsuga menziesii (Mirb.) Franco] seedlings. The components responded differently to elevated CO2 (ambient + 200 mu mol mol(-1)) and elevated temperature (ambient + 4 degreesC) treatments during the first year. Rhizosphere respiration increased most under elevated CO2, and SOM oxidation increased most under elevated temperature. However, many studies show that plants and soil processes can respond to altered climates in a transient way. Herein, we extend our analysis to 2 years to evaluate the stability of the responses of the source components. Total soil CO2 efflux increased significantly under elevated CO2 and elevated temperature in both years (1994 and 1995), but the enhancement was much less in 1995. Rhizosphere respiration increased less under elevated temperature in 1995 compared with 1994. Litter decomposition also tended to increase comparatively less in 1995 under elevated CO2, but was unresponsive to elevated temperature between years. In contrast, SOM oxidation was similar under elevated CO2 in the 2 years. Less SOM oxidation occurred under elevated temperature in 1995 compared with 1994. Our results indicate that temporal variations can occur in CO2 production by the sources. The variations likely involve responses to antecedent physical disruption of the soil and physiological processes.