Microbial regulation of soil carbon stabilization shapes soil carbon projections

Enhancing SOC accumulation is considered as a pathway for mitigating climate change. However, projection of soil organic carbon (SOC) exhibits large uncertainties, and microbial activities, the key regulator of SOC dynamics, are omitted in most Earth System Models (ESMs). Here, we compare the traditional SOC scheme in ESMs (CENURY) with two microbial-explicit models, through constraining SOC stock and its stable component. Unlike net gains of SOC projected by CENTURY, the two microbial models consistently projected turning points of SOC change, and net loss of SOC for 21.9 ~ 61.4PgC were projected under SSP5-8.5 by 2100. SOC loss originates primarily from unprotected SOC in northern high-latitudes. SOC stabilization pathways, instead of the temperature sensitivity of SOC mineralization, drive the divergence among model projections. Our results indicate strong risk of SOC loss in northern high-latitudes, and ESMs should incorporate microbial-regulated SOC stabilization mechanisms as the priority to improve SOC projections.