Climate Effects on Subsoil Carbon Loss Mediated by Soil Chemistry.

Subsoils store at least 50% of soil organic carbon (SOC) globally, but climate change may accelerate subsoil SOC (SOC) decomposition and amplify SOC-climate feedbacks. The climate sensitivity of SOC decomposition varies across systems, but we lack the mechanistic links needed to predict system-specific SOC vulnerability as a function of measurable properties at larger scales. Here, we show that soil chemical properties exert significant control over SOC decomposition under elevated temperature and moisture in subsoils collected across terrestrial National Ecological Observatory Network sites. Compared to a suite of soil and site-level variables, a divalent base cation-to-reactive metal gradient, linked to dominant mechanisms of SOC mineral protection, was the best predictor of the climate sensitivity of SOC decomposition. The response was "U"-shaped, showing higher sensitivity to temperature and moisture when either extractable base cations or reactive metals were highest. However, SOC in base cation-dominated subsoils was more sensitive to moisture than temperature, with the opposite relationship demonstrated in reactive metal-dominated subsoils. These observations highlight the importance of system-specific mechanisms of mineral stabilization in the prediction of SOC vulnerability to climate drivers. Our observations also form the basis for a spatially explicit, scalable, and mechanistically grounded tool for improved prediction of SOC response to climate change.