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

Abstract

As the climate warms, drought events are expected to increase in intensity and frequency, with consequences for the carbon cycle. Soil respiration (Rs) accounts for the largest flux of CO2 from terrestrial ecosystems to the atmosphere. While the drought responses of Rs 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 Rs dynamics under current versus likely future conditions (+3°, +300 ppm CO2). Furthermore, to understand the soil CO2 production (Ps) and transport dynamics underlying Rs fluxes we continuously monitored in-situ soil CO2 concentrations across the soil profile. Our results show that Rs was higher and that drought-induced reductions in Rs were delayed under future compared to current conditions. Peak drought reductions and post-drought pulses of Rs were more pronounced in the future scenario. Annual Rs 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 Ps primarily in the main rooting horizon and that warming affected Ps also in deeper soil layers. We found that both in the current and the future scenario drought led to the strongest reductions of Ps 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.