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 CO2 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 CO2 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 degrees, +300 ppm CO2). Furthermore, to understand the soil CO2 production (P-s) and transport dynamics underlying R-s fluxes we continuously monitored in-situ soil CO2 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 CO2 gradients and fluxes across the soil profile showed that elevated CO2 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 CO2 amplifies soil CO2 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 CO2 emissions.