Forest recovery suppresses soil microbial carbon use efficiency via geochemical and biotic constraints in a subtropical karst region

Forest recovery is a critical nature-based solution for mitigating climate change, yet the microbial mechanisms governing soil microbial carbon use efficiency (CUE) remain poorly understood in geochemically active landscapes. Here, we conducted a large-scale paired-site study across a ∼1200 km transect in the subtropical karst region of southwest China, comparing 23 pairs of maize croplands and naturally regenerated secondary forests. Forest recovery significantly increased SOC stocks but suppressed microbial CUE by 18.8%. This reduction in CUE was coupled with a substantial decline in microbial specific growth rate, while specific respiration showed no significant change. Structural equation modeling revealed that forest recovery suppressed microbial metabolic activity through three convergent pathways: (i) enhanced physical protection by exchangeable calcium/magnesium and carbonates, which restricted substrate accessibility; (ii) a fundamental shift in microbial community structure from copiotrophic to oligotrophic dominance, characterized by lower rrn operon copy numbers; and (iii) reduced phosphorus availability. These findings suggest that in lithologically constrained ecosystems, the enhanced mineral preservation of organic matter, acting in concert with increased plant carbon inputs, may outweigh the impact of microbial residue production efficiency in regulating SOC accumulation. This study highlights that the relationship between microbial CUE and SOC accumulation is highly context-dependent following forest recovery, and emphasizes that Earth system models should integrate these geochemical-biotic feedbacks to accurately predict the carbon sink potential of karst restoration projects.