Temperature dependence of soil CO 2 efflux is strongly modulated by seasonal patterns of moisture availability in a Mediterranean ecosystem

Abstract

Extensive research has focused on the temperature sensitivity of soil respiration. However, in Mediterranean ecosystems, soil respiration may have a pulsed response to precipitation events, especially during prolonged dry periods. Here, we investigate temporal variations in soil respiration ( R s), soil temperature ( T) and soil water content (SWC) under three different land uses (a forest area, an abandoned agricultural field and a rainfed olive grove) in a dry Mediterranean area of southeast Spain, and evaluate the relative importance of soil temperature and water content as predictors of R s. We hypothesize that soil moisture content, rather than soil temperature, becomes the major factor controlling CO 2 efflux rates in this Mediterranean ecosystem during the summer dry season. Soil CO 2 efflux was measured monthly between January 2006 and December 2007 using a portable soil respiration instrument fitted with a soil respiration chamber (LI-6400-09). Mean annual soil respiration rates were 2.06 ± 0.07, 1.71 ± 0.09, and 1.12 ± 0.12 μmol m −2 s −1 in the forest, abandoned field and olive grove, respectively. R s was largely controlled by soil temperature above a soil water content threshold value of 10% at 0–15 cm depth for forest and olive grove, and 15% for abandoned field. However, below those thresholds R s was controlled by soil moisture. Exponential and linear models adequately described R s responses to environmental variables during the growing and dry seasons. Models combining abiotic (soil temperature and soil rewetting index) and biotic factors (above-ground biomass index and/or distance from the nearest tree) explained between 39 and 73% of the temporal variability of R s in the forest and olive grove. However, in the abandoned field, a single variable – either soil temperature (growing season) or rewetting index (dry season) – was sufficient to explain between 51 and 63% of the soil CO 2 efflux. The fact that the rewetting index, rather than soil water content, became the major factor controlling soil CO 2 efflux rates during the prolonged summer drought emphasizes the need to quantify the effects of rain pulses in estimates of net annual carbon fluxes from soil in Mediterranean ecosystems.