Dynamics of soil CO 2 efflux under varying atmospheric CO 2 concentrations reveal dominance of slow processes

We evaluated the effect on soil CO 2 efflux ( F CO 2 ) of sudden changes in photosynthetic rates by altering CO 2 concentration in plots subjected to +200 ppmv for 15 years. Five‐day intervals of exposure to elevated CO 2 ( eCO 2 ) ranging 1.0–1.8 times ambient did not affect F CO 2 . F CO 2 did not decrease until 4 months after termination of the long‐term eCO 2 treatment, longer than the 10 days observed for decrease of F CO 2 after experimental blocking of C flow to belowground, but shorter than the ~13 months it took for increase of F CO 2 following the initiation of eCO 2 . The reduction of F CO 2 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 CO 2 ‐temperature sensitivity. These asymmetric responses pose a tractable challenge to process‐based models attempting to isolate the effect of individual processes on F CO2 .