Warming and elevated CO 2 intensify drought and recovery responses of grassland carbon allocation to soil respiration

Photosynthesis and soil respiration represent the two largest fluxes of CO 2 in terrestrial ecosystems and are tightly linked through belowground carbon (C) allocation. Drought has been suggested to impact the allocation of recently assimilated C to soil respiration; however, it is largely unknown how drought effects are altered by a future warmer climate under elevated atmospheric CO 2 (eT_eCO 2 ). In a multifactor experiment on managed C3 grassland, we studied the individual and interactive effects of drought and eT_eCO 2 (drought, eT_eCO 2 , drought × eT_eCO 2 ) on ecosystem C dynamics. We performed two in situ 13 CO 2 pulse‐labeling campaigns to trace the fate of recent C during peak drought and recovery. eT_eCO 2 increased soil respiration and the fraction of recently assimilated C in soil respiration. During drought, plant C uptake was reduced by c. 50% in both ambient and eT_eCO 2 conditions. Soil respiration and the amount and proportion of 13 C respired from soil were reduced (by 32%, 70% and 30%, respectively), the effect being more pronounced under eT_eCO 2 (50%, 84%, 70%). Under drought, the diel coupling of photosynthesis and SR persisted only in the eT_eCO 2 scenario, likely caused by dynamic shifts in the use of freshly assimilated C between storage and respiration. Drought did not affect the fraction of recent C remaining in plant biomass under ambient and eT_eCO 2 , but reduced the small fraction remaining in soil under eT_eCO 2 . After rewetting, C uptake and the proportion of recent C in soil respiration recovered more rapidly under eT_eCO 2 compared to ambient conditions. Overall, our findings suggest that in a warmer climate under elevated CO 2 drought effects on the fate of recent C will be amplified and the coupling of photosynthesis and soil respiration will be sustained. To predict the future dynamics of terrestrial C cycling, such interactive effects of multiple global change factors should be considered.