Soil organic carbon fraction mediated microbial community assembly along stand age and seasonal gradients in temperate poplar plantations

Boreal forests play a pivotal role in the global carbon cycle, contributing 45% of total forest carbon sinks, where soil microorganisms are critical for maintaining carbon stability. However, microbial community assembly processes and their linkages to carbon fractions remain poorly understood in these ecosystems. We sampled soils across four stand ages (3, 6, 10, and 13 years) and three seasons (spring, summer, autumn) in temperate populus plantations. Integrated approaches including high-throughput sequencing, co-occurrence network analysis, null and neutral modeling, and soil physicochemical characterization revealed that stand age, season, and their interaction significantly affected soil organic carbon (SOC), easily oxidizable organic carbon (EOC), particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and recalcitrant organic carbon (ROC) (P < 0.001), but not dissolved organic carbon (DOC). In summer, all carbon fractions (excluding EOC) accumulated with increasing stand age. Afforestation enhanced bacterial diversity and richness, with age driven shifts in bacterial composition (P < 0.001) governed by stochastic assembly (peaking in spring) mediated by pH, SOC, ROC, cellulase (CL) activity, and C/P ratio. Fungal composition responded strongly to seasonality (P < 0.001) via deterministic assembly (weakest in spring), exhibiting higher dispersal limitation than bacteria. Specific microbial modules (B-Mod3, F-Mod2) significantly predicted carbon fractions (P < 0.05), stand age promoted stable organic carbon accumulation, while seasonal variations intensified labile organic consumption. Critically, the demonstrated divergence in carbon microbial assembly linkages across stand age and seasonal gradients underscores microbial community assembly as a fundamental driver of carbon sequestration and stabilization in forest soils, providing a mechanistic basis for enhancing forest carbon sink functionality.