Elevated CO₂ and warming cause interactive effects on soil carbon and shifts in carbon use by bacteria

Accurate predictions of soil C feedbacks to climate change depend on an improved understanding of responses of soil C pools and C use by soil microbial groups. We assessed soil and microbial C in a 7-year manipulation of CO₂ and warming in a semi-arid grassland. Continuous field isotopic labelling under elevated CO₂ further allowed us to study the dynamics of the existing C (Old C) in soil and microbes as affected by warming. Warming reduced soil C under elevated CO₂ but had no impact under ambient CO₂ . Loss of soil C under warming and elevated CO₂ was attributed to increased proportional loss of Old C. Warming also reduced the proportion of Old C in microbes, specifically the bacteria, but not the fungi. These findings highlight that warming impacts are C pool and microbial taxa dependent and demonstrate interactive effects of warming and atmospheric CO₂ on soil C.