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 (SM_th), and then declines. This SM_th 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 SM_th shows that about 25% of the planet's vegetated land currently operates above SM_th, 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.