Soil moisture as the dominant driver of CO ₂ efflux in Mediterranean urban green spaces: evidence for a Gaussian temperature response and mechanistic modelling of moisture and temperature interaction

Soil respiration is a key component of the global carbon cycle, yet its dynamics in urban Mediterranean environments remain poorly understood. This study investigated the effects of soil temperature, soil moisture, and plant community on soil CO ₂ efflux in urban green spaces of Madrid (Spain), characterized by a Mediterranean pluvioseasonal-oceanic bioclimate with pronounced summer drought. Four dominant ruderal plant communities ( Diplotaxis virgata, Hordeum leporinum, Malva spp., and Dactylis glomerata ) were monitored across three urban green spaces. Soil CO ₂ efflux, temperature, and moisture were measured biweekly over one year using an infrared gas analyzer. Contrary to the Q ₁₀ exponential assumption, soil respiration showed a Gaussian relationship with temperature, with a positive correlation below a breakpoint of 18.4 °C and a negative effect above this threshold, consistent across plant communities. Soil respiration exhibited a positive exponential relationship with soil moisture and a logarithmic relationship with a rewetting index for values below 20. A mechanistic model described soil respiration as the joint outcome of temperature-driven moisture loss and moisture-stimulated CO ₂ emissions. Plant community identity had a limited effect, with the exception of Malva spp., which consistently produced higher emissions. These findings challenge the universal applicability of temperature-based respiration models and highlight soil moisture as the dominant driver of CO ₂ efflux in water-limited urban ecosystems. As climate change is expected to intensify both the urban heat island effect and summer drought in Mediterranean cities, soil moisture emerges as a critical variable for projecting urban soil CO ₂ fluxes and for designing evidence-based management strategies for Mediterranean urban green spaces, including improved soil infiltration capacity, moisture-sensitive irrigation planning, and the incorporation of moisture-temperature coupling into urban carbon monitoring protocols.