The composition and functional roles of soil autotrophic microorganisms in vegetation restoration of degraded karst forest

Autotrophic microorganisms play a significant role in atmospheric CO₂ fixation and soil organic carbon (SOC) sequestration in diverse ecosystems, but little is known about their role in karst forests. To investigate the composition and changes of autotrophic microbial communities during degraded karst forest restoration, the related functional genes and microorganisms from three restoration stages (shrubbery, TG; secondary forest, SG; old-growth forest, OG) were examined through metagenomic sequencing. Their underlying drivers and contributions to SOC were investigated using structural equation modeling (SEM) and regression analysis. Karst forest restoration resulted in the synchronous recovery of above-ground plants and soil conditions. When TG was restored to OG, soil autotrophic CO₂ fixation microbes changed significantly, indicated by an increase in microbial functional strength and diversity. Among the six examined functional pathways, the rTCA cycle contributed the most (0.074–0.082%), while the WL pathway contributed the least (0.008–0.010%) to CO₂ fixation functions. Except the Calvin cycle, genes involved in the other five pathways showed an increase with karst forest restoration. SEMs further revealed that soil pH and available nitrogen directly drive the increase in microbial autotrophic CO₂ fixation functions. In karst forests, autotrophic CO₂-fixing microorganisms play a crucial role in enhancing SOC, particularly through the DC/4-HB cycle, 3-HP/4-HB cycle, and WL pathway. Soil microbial communities involved in autotrophic CO₂ fixation were predominantly attributed to Proteobacteria (43.02–32.42%) and Actinobacteria (18.83–30.89%), although their contributions varied across different stages. These results highlight the significant contribution of autotrophic microorganisms to the SOC of karst forests and enhance our understanding of the microbial mechanisms behind soil C sequestration.