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Photosynthesis and soil respiration represent the two largest fluxes of CO2 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 CO2 (eTₑCO2). In a multifactor experiment on managed C3 grassland, we studied the individual and interactive effects of drought and eTₑCO2 (drought, eTₑCO2, drought × eTₑCO2) on ecosystem C dynamics. We performed two in situ 13 CO2 pulse-labeling campaigns to trace the fate of recent C during peak drought and recovery. eTₑCO2 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 eTₑCO2 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 eTₑCO2 (50%, 84%, 70%). Under drought, the diel coupling of photosynthesis and SR persisted only in the eTₑCO2 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 eTₑCO2, but reduced the small fraction remaining in soil under eTₑCO2. After rewetting, C uptake and the proportion of recent C in soil respiration recovered more rapidly under eTₑCO2 compared to ambient conditions. Overall, our findings suggest that in a warmer climate under elevated CO2 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.
Meeran et al. (Sat,) studied this question.