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

Soil respiration (S R ) 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 S R 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 S R was quantified across the different phases defined by the SWC threshold. Soil moisture exerted a stronger control on S R than soil temperature. Including a SWC threshold to delineate distinct soil moisture phases improved model performance (R 2 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 S R below the SWC threshold was further weakened. To further investigate the threshold effect on the temperature sensitivity of S R , we calculated Q 10 values, which confirmed that under the SWC threshold, the response of S R 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.