Stabilities of soil organic carbon and carbon cycling genes are higher in natural secondary forests than in artificial plantations in southern China

Abstract Our understanding of how reforestation affects microbial carbon cycling genes remains limited, restricting our ability to evaluate reforestation methods. We hypothesized that natural reforestation produces soil organic carbon (SOC) with significantly more complex chemical composition and more diverse carbon cycling genes than artificial restoration. The SOC fractions and carbon cycling (primarily fixation and degradation) genes present in natural restoration/regrowth (natural secondary forests), and artificial restoration (Masson and slash pine plantations) were compared. The SOC in natural secondary forests was significantly more abundant and structurally more complex than that of Masson and slash pine plantations. The natural secondary forest soils contained significantly more abundant and diverse carbon cycling genes than plantations. Plant‐, soil‐, and nutrient‐associated factors explained 42.1% of the variation in microbial carbon cycling gene compositions. These factors included tree diversity, fine root surface area, litter stock, clay and silt, labile and recalcitrant organic carbon, available nitrogen and nitrate‐nitrogen contents. These factors affect the microbial carbon cycling gene diversity but not abundance in the natural secondary forest soils. These genes' abundance and diversity in the Masson and slash pine plantation soils were enriched by increasing litter stock and decreasing clay and silt contents; they were also enhanced by higher tree diversity, labile and recalcitrant organic carbon, available nitrogen, and nitrate‐nitrogen contents. These findings indicate that current reforestation methods affect carbon cycling genes, carbon cycling potential, and thus, biogeochemical carbon cycling processes. Natural restoration is better than artificial restoration in terms of organic carbon stability and cycling.