Dynamics of soil CO₂ efflux under varying atmospheric CO₂ concentrations reveal dominance of slow processes

We evaluated the effect on soil CO₂ efflux (F_CO2 ) of sudden changes in photosynthetic rates by altering CO₂ concentration in plots subjected to +200 ppmv for 15 years. Five-day intervals of exposure to elevated CO₂ (eCO₂ ) ranging 1.0-1.8 times ambient did not affect F_CO2 . F_CO2 did not decrease until 4 months after termination of the long-term eCO₂ treatment, longer than the 10 days observed for decrease of F_CO2 after experimental blocking of C flow to belowground, but shorter than the ~13 months it took for increase of F_CO2 following the initiation of eCO₂ . The reduction of F_CO2 upon termination of enrichment (~35%) cannot be explained by the reduction of leaf area (~15%) and associated carbohydrate production and allocation, suggesting a disproportionate contraction of the belowground ecosystem components; this was consistent with the reductions in base respiration and F_CO2 -temperature sensitivity. These asymmetric responses pose a tractable challenge to process-based models attempting to isolate the effect of individual processes on F_CO2 .