Paleoclimate contributes to soil carbon storage in subtropical shrublands

Shrubland soils play a significant role in global carbon sequestration, yet the factors influencing soil organic carbon stocks at different depths in subtropical shrublands remain poorly understood, introducing large biases in understanding and predicting terrestrial carbon dynamics. This study examines the relative impacts of abiotic and biotic drivers on SOC density (SOCD) in topsoils (0–30 cm) and subsoils (30–100 cm) by capitalizing on a consistent broad-scale sampling across 297 sites in subtropical shrublands of China. Contrary to earlier findings suggesting greater SOC storage in subsoils, we found that topsoils and subsoils contribute equally to SOC stocks per unit area on average. Existing SOC maps overestimated SOCD by 30.13 % in topsoils and 22.23 % in subsoils. Past climate conditions had a lasting positive influence on topsoil SOC, while current climate emerged as the dominant indirect driver of SOCD in both soil layers. Edaphic properties emerged as a major driver of SOCD and dominated the indirect effects of paleoclimate legacy rather than that of current climate on the geographic pattern of SOCD, whereas vegetation attributes and recent human disturbances had relatively minor impacts. Our findings reveal that subtropical shrublands store less SOC than previously estimated, underscoring the urgent need to refine SOC assessments and reconsider their long-term carbon sequestration potential. We further illustrate the critical role of past climate over extended timescales in shaping current SOC distribution and highlight the importance of integrating paleoclimate legacies and edaphic interactions into Earth System Models to improve predictions of SOC dynamics under climate change.