Afforestation increases soil carbon in warmer karst and is associated with calcium and bacterial networks rather than microbial carbon-use efficiency

Afforestation is widely promoted to enhance soil carbon sequestration, yet its climatic dependence and the microbial and mineral mechanisms that mediate soil organic carbon (SOC) dynamics remain poorly understood. We compared 20-year-old plantations with long-term croplands across a subtropical karst climatic gradient−characterized by high microbial activity and mineral-rich soils−to evaluate how climate, minerals, and microbial processes jointly influence SOC. Across the study region, afforestation increased SOC by 58%; however, this increase was concentrated in warmer zones, with little change in cooler zones. SOC in croplands declined with increasing mean annual temperature (MAT), whereas SOC in plantations increased with MAT. The mineral-associated organic carbon fraction showed a matching temperature dependence. Multiple analyses indicated that microbial carbon use efficiency contributed little to SOC variation. Instead, microbial necromass, bacterial network complexity, and exchangeable calcium emerged as the strongest correlates of SOC and increased with temperature in plantations. These linked associations are consistent with a pathway in which temperature-driven shifts in microbial networks and calcium coincide with greater SOC accumulation after afforestation. These findings indicate a strong temperature dependence of afforestation-driven SOC gains in subtropical karst soils. Mineral-associated processes and microbial community structure, rather than microbial carbon use efficiency, were most closely associated with SOC accumulation and can inform targeted restoration strategies under future climate change.