Impact of drought and increasing temperatures on soil CO2 emissions in a Mediterranean shrubland (gariga)

In arid and semiarid shrubland ecosystems of the Mediterranean basin, soil moisture is a key factor controlling biogeochemical cycles and the release of CO2 via soil respiration. This is influenced by increasing temperatures. We manipulated the microclimate in a Mediterranean shrubland to increase the soil and air night-time temperatures and to reduce water input from precipitation. The objective was to analyze the extent to which higher temperatures and a drier climate influence soil CO2 emissions in the short term and on an annual basis. The microclimate was manipulated in field plots (about 25 m2) by covering the vegetation during the night (Warming treatment) and during rain events (Drought treatment). Soil CO2 effluxes were monitored in the treatments and compared to a control over a 3-year period. Along with soil respiration measurements, we recorded soil temperature at 5 cm depth by a soil temperature probe. The seasonal pattern of soil CO2 efflux was characterized by higher rates during the wet vegetative season and lower rates during the dry nonvegetative season (summer). The Warming treatment did not change SR fluxes at any sampling date. The Drought treatment decreased soil CO2 emissions on only three of 10 occasions during 2004. The variation of soil respiration with temperature and soil water content did not differ significantly among the treatments, but was affected by the season. The annual CO2 emissions were not significantly affected by the treatments. In the semi-arid Mediterranean shrubland, an increase of soil CO2 efflux in response to a moderate increase of daily minimum temperature is unlikely, whereas less precipitation can strongly affect the soil processes mainly limited by water availability.