Hydro-thermal controls on soil respiration revealed by continuous measurements at a Mediterranean forest flux site

Soil respiration (SR) is a major component of carbon fluxes in forest ecosystems, encompassing contributions from both heterotrophic and autotrophic respiration. In this study, we analysed two years of continuous CO₂ flux measurements obtained using an automated chamber monitoring system to investigate the seasonal drivers of SR in an evergreen holm oak forest under a typical Mediterranean climate. Soil water content (SWC) thresholds, marking the transition from moisture- to temperature-limited conditions, were determined using a stepwise regression-based approach. The relative importance of each predictor controlling SR was quantified across the different phases defined by the SWC threshold. Soil moisture exerted a stronger control on SR than soil temperature. Including a SWC threshold to delineate distinct soil moisture phases improved model performance (R2 increased from 0.79 to 0.85). A complementary litter manipulation experiment was conducted to isolate the role of litter surface in modulating soil respiration and to quantify its contribution to soil CO₂ fluxes. Results showed that, under bare-soil condition, the control exerted by soil temperature on SR below the SWC threshold was further weakened. To further investigate the threshold effect on the temperature sensitivity of SR, we calculated Q10 values, which confirmed that under the SWC threshold, the response of SR to temperature is weak. These findings provide important insights for modelling soil respiration in Mediterranean ecosystems exposed to abrupt variations in soil moisture associated with seasonality and climate-change extreme events.