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 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 (eT_eCO ). In a multifactor experiment on managed C3 grassland, we studied the individual and interactive effects of drought and eT_eCO (drought, eT_eCO , drought × eT_eCO ) on ecosystem C dynamics. We performed two in situ CO pulse-labeling campaigns to trace the fate of recent C during peak drought and recovery. eT_eCO 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 conditions. Soil respiration and the amount and proportion of C respired from soil were reduced (by 32%, 70% and 30%, respectively), the effect being more pronounced under eT_eCO (50%, 84%, 70%). Under drought, the diel coupling of photosynthesis and SR persisted only in the eT_eCO 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 , but reduced the small fraction remaining in soil under eT_eCO . After rewetting, C uptake and the proportion of recent C in soil respiration recovered more rapidly under eT_eCO compared to ambient conditions. Overall, our findings suggest that in a warmer climate under elevated CO 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.