Consistent soil organic carbon loss rate across depth under warming

Soil organic carbon (SOC) stock is the largest terrestrial carbon reservoir, with a substantial portion stored in the subsoil below 20 cm. The near-synchronous warming of the subsurface poses a threat to SOC storage across the whole soil profile. However, whether topsoil or subsoil is more vulnerable to warming still remains highly debated. Here, we utilize 213 matched observations from 60 field experiments to compare the responses of SOC stock to warming across depth. We find that warming causes SOC losses both in topsoil and subsoil. Moreover, multiple lines of evidence indicate no significant difference in SOC responses to warming at different soil depths, suggesting a consistent SOC loss rate throughout the whole soil profile. Despite the consistent loss rate, subsoil below 20 cm is projected to contribute over 60% absolute SOC losses across the 0-100 cm soil profile under the shared socioeconomic pathways 5-8.5 scenario due to its large SOC stock. We show that SOC in subsoil is as susceptible to warming-induced loss as in topsoil. Neglecting subsoil carbon loss will significantly underestimate the positive climate-carbon cycle feedback.