Impacts of summer extreme events on soil respiration in grassland in a context of future climate change

Future scenarios foresee more frequent and severe extreme events, such as heat wave and drought. Soil CO2 efflux constitutes the second largest carbon flux between terrestrial ecosystems and atmosphere and derives from two main components, autotrophic respiration (Ra) being associated with root and rhizosphere respiration and heterotrophic respiration (Rh) associated with the turnover of soil organic matter by microorganisms. The aim of this work was to study how increased atmospheric CO2 can mediate the effects of extreme events on grassland soil respiration and to analyze its components Ra and Rh. Grassland monoliths were exposed, from May 2010, to air temperature and precipitation expected for the period 2040-2060. Since March 2011 a CO2 enrichment was applied and during summer a heat wave and drought stress was applied too. Soil respiration and its components were not significantly affected by elevated CO2 until 4 months after the start of the CO2 fumigation, but it increased under elevated CO2 by about 20% during August and September. The extremes treatment decreased drastically soil respiration both at ambient (59%) and at elevated CO2 (67%). Soil respiration recovered from extremes events, even if Ra and Rh showed different recovery times. This was associated to a recover of root growth and to an increase in root decomposition. This behavior agrees with preliminary data on daily ecosystem carbon uptake showing a stronger recovery after rewetting and under elevated CO2 compared to ambient CO2. In view of these results, the interactions between elevated CO2 and extreme events will be discussed.