Effects of summer extreme events on grassland soil CO2 efflux in a contest of future climate change

Climate models forecast, for the coming future, an increase in frequency of extremes events, such as heat waves and severe droughts. In grassland ecosystem most of the carbon is stored in the soil and its flux toward the soil is the second larger among all ecosystems. Soil CO2 efflux derives from two main components, autotrophic respiration being associated with root and rhizosphere respiration and heterotrophic respiration associated with the turnover of soil organic matter by microorganisms. The aim of this work was to evaluate the vulnerability of a grassland ecosystem under summer extreme events in a context of future climate and the possible mitigation effect of increased CO2. In particular, we studied the extreme event effects on soil efflux and on its autotrophic and heterotrophic components. Grassland monoliths were exposed, from 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 were applied too. Total efflux was strongly reduced during the extreme event treatments both at ambient and at elevated CO2; this reduction was due mainly to the autotrophic component since the treatments seemed to have a mild effect on heterotrophic respiration. The reduction in total flux was well described by the reduction in soil moisture. Soil efflux strongly recovered from extreme events, indeed, total respiration showed a faster recovery compared to heterotrophic respiration. This was associated to a sustained recovery in root respiration, root growth rate and root dry matter content. Instead, root decomposition in extreme treatments, in the same period, was still lower than control. This behavior agrees with preliminary data on daily ecosystem carbon uptake showing a stronger recovery after rewetting. In view of these results, the interactions between extreme events and elevated CO2 will be discussed.