Responses of grassland soil CO2 production and fluxes to drought are shifted in a warmer climate under elevated CO2

As the climate warms, drought events are expected to increase in intensity and frequency, with consequences for the carbon cycle. Soil respiration (R s ) accounts for the largest flux of CO 2 from terrestrial ecosystems to the atmosphere. While the drought responses of R s have been well studied, it is uncertain how they will be modified in a future world, when higher temperatures will occur in combination with elevated atmospheric CO 2 concentrations. In a global change experiment in a managed temperate grassland, we studied drought and post-drought responses of R s dynamics under current versus likely future conditions (+3 ° , +300 ppm CO 2 ). Furthermore, to understand the soil CO 2 production (P s ) and transport dynamics underlying R s fluxes we continuously monitored in-situ soil CO 2 concentrations across the soil profile. Our results show that R s was higher and that drought-induced reductions in R s were delayed under future compared to current conditions. Peak drought reductions and post-drought pulses of R s were more pronounced in the future scenario. Annual R s was reduced by drought only under current but not under future conditions. An in-depth analysis of soil CO 2 gradients and fluxes across the soil profile showed that elevated CO 2 stimulated P s primarily in the main rooting horizon and that warming affected P s also in deeper soil layers. We found that both in the current and the future scenario drought led to the strongest reductions of P s in the most productive soil layers, which also exhibited the largest depletion of soil moisture. We conclude that a future warmer climate under elevated CO 2 amplifies soil CO 2 production and efflux and their peak drought and post-drought responses, but delays the onset of the drought responses and thereby eliminates the overall drought effect on annual soil CO 2 emissions. • Warming and elevated CO 2 alter soil CO 2 production and its temperature sensitivity. • Warming and elevated CO 2 amplify drought and rewetting responses of soil respiration. • Warming and elevated CO 2 delay the onset of drought responses. • Warming and elevated CO 2 eliminate drought effects on annual soil CO 2 emissions.