Convergent dynamics and shared mechanisms of three-pool soil carbon mineralization under different grassland managements

The mineralization dynamics of soil organic carbon (SOC) in grasslands are crucial to terrestrial biogeochemical cycles. However, the regulatory mechanisms underlying extracellular enzyme metabolism and microbial community structure during SOC mineralization across different carbon pools remain poorly understood. In this study, a 553-day incubation experiment was conducted to examine temporal changes in CO₂ emissions, extracellular enzyme activities, microbial biomass, and microbial community composition in soils from both enclosed and grazed grasslands. Using a three-pool model, SOC dynamics were quantified within active, slow, and passive carbon pools, revealing a shift in the dominance of mineralization from the active carbon pool to the passive carbon pool during the long-term carbon turnover, with differences observed across grassland management strategies. Compared to grazed grasslands, enclosed grasslands exhibited an approximately 110% larger active carbon pool and higher initial SOC mineralization rates (significantly higher during the first 113 days), yet long-term microbial and enzymatic regulatory mechanisms-particularly shifts in microbial strategies, enzyme activity patterns, and their interactions with carbon pools-were similar across both management regimes. The observed shifts in carbon pool dynamics were driven by enhanced microbial capacity to decompose passive carbon, associated with substantially increased oxidative enzyme production (e.g., mass-specific oxidase activity increased by 190.6% in enclosed soil and by 256.1% in grazed soil) and elevated nitrogen and phosphorus demands. Notably, microbial communities shifted from fast-growing copiotrophic taxa (e.g., Proteobacteria, Bacteroidetes, Ascomycota) to slower-growing oligotrophic taxa (e.g., Acidobacteria, Actinobacteria, Planctomycetes, Basidiomycota), with the oligotroph-to-copiotroph ratio increasing by 55.5–62.6% for bacteria and 96.9–247.5% for fungi. These changes were closely linked to shifts in enzyme activity profiles and stoichiometric ratios. Overall, this study provides mechanistic insights into how microbial ecological strategies and enzyme activities interact to regulate SOC mineralization across different pools under contrasting grassland management regimes. These findings advance our understanding of SOC turnover and improve predictive capabilities for carbon cycling, with broader implications for global climate change feedbacks.