Soil Moisture Thresholds for the Temperature Sensitivity of Ecosystem Respiration.

Ecosystem respiration (ER) is the largest source of biogenic CO2 to the atmosphere, and its temperature sensitivity (Q10) before reaching maximum values is crucial for understanding land-climate feedback. However, despite decades of studies showing that Q10 varies considerably across space, time, and biomes, the mechanisms underlying this variability remain unresolved. Here we demonstrate that global variation in Q10 can be reconciled within a unified hydrothermal framework. Using data from 142 eddy covariance sites around the world, we show that Q10 exhibits unimodal responses to soil moisture. At each site, Q10 first increases with soil moisture, peaks at a threshold (SMth), and then declines. This SMth is ecosystem-specific, which consistent with mechanisms involving plant-soil-microbial interactions, shaped by long-term hydroclimatic regimes and soil physical constraints. Global mapping of SMth shows that about 25% of the planet's vegetated land currently operates above SMth, including many carbon-rich peatlands and tropical forests, where moderate drying may amplify temperature sensitivity and accelerate carbon loss. By identifying soil moisture thresholds as a first-order control on Q10, our study provides a unifying mechanism that links hydrological state to the thermal sensitivity of carbon fluxes. This framework offers a predictive basis for anticipating respiration responses to climate change by explicitly resolving whether shifts in soil moisture move ecosystems toward or away from these critical thresholds.