Soil organic carbon is a key determinant of CH sink in global forest soils.

Soil organic carbon (SOC) is a primary regulator of the forest-climate feedback. However, its indicative capability for the soil CH sink is poorly understood due to the incomplete knowledge of the underlying mechanisms. Therefore, SOC is not explicitly included in the current model estimation of the global forest CH sink. Here, using in-situ observations, global meta-analysis, and process-based modeling, we provide evidence that SOC constitutes an important variable that governs the forest CH sink. We find that a CH sink is enhanced with increasing SOC content on regional and global scales. The revised model with SOC function better reproduces the field observation and estimates a 39% larger global forest CH sink (24.27 Tg CH yr) than the model without considering SOC effects (17.46 Tg CH yr). This study highlights the role of SOC in the forest CH sink, which shall be factored into future global CH budget quantification.