Effects of quicklime application rate on carbon mineralization in strongly acidic soil of Cunninghamia lanceolata plantations

Abstract Aims Strongly acidic Chinese fir ( Cunninghamia lanceolata ) plantation soils face significant constraints in nutrient and carbon dynamics. This study aimed to investigate the biogeochemical mechanisms governing carbon cycling in these soils under varying quicklime amendment rates. Methods Soils collected from Chinese fir plantations treated with six quicklime amendment rates (0–3,000 kg hm⁻²), and incubated for 49 days in carbon mineralization experiment. The analyses integrated high-throughput sequencing, PICRUSt2 functional prediction, Mantel tests, and partial least squares structural equation modeling (PLS-SEM) to evaluate the relationships among soil physicochemical properties, microbial communities, and carbon mineralization. Results Quicklime amendment exerted significant nonlinear, dose-dependent effects on soil carbon cycling, with 2,250 kg hm⁻² (T4) identified as the optimal threshold, whereas excessive application (3,000 kg hm⁻²) produced significant inhibitory effects ( P  < 0.05). Key mechanistic insights reveal that optimal quicklime application: (i) Alleviates acidity stress by increasing soil pH from 4.71 to 5.25, thereby increasing microbial biomass carbon and phosphorus by 113.96% and 192.75%, respectively; (ii) Elevates β-glucosidase and cellobiohydrolase activities ( P  < 0.05) to drive an 88.5% increase in cumulative carbon mineralization, while exhibiting a decoupling between high enzymatic capacity and lower functional gene abundance indicative of a high-efficiency metabolic strategy; (iii) Drives carbon mineralization through a sequential mediation pathway (PLS-SEM GoF = 0.519) where improved physicochemical properties promote microbial biomass accumulation (path coefficient = 0.860), which subsequently enhances extracellular enzyme activity (path coefficient = 0.899). Notably, microbial biomass magnitude (0.899) rather than community composition (0.074) was the primary regulator of enzymatic capacity, with nitrate nitrogen acting as a key factor linking environmental conditions to microbial functional profiles. Conclusion Optimal quicklime application ameliorates physicochemical constraints and stimulates microbial biomass growth, thereby enhancing extracellular enzyme-mediated organic carbon decomposition in acidic Chinese fir plantation soils. These findings highlight that microbial biomass magnitude, rather than community structure, is the primary driver of enzymatic capacity, providing a scientific basis for optimizing carbon cycling management in acidic forest soils.