Forest development and seasonal variation drive functional reorganization of soil microbiomes and enzymatic activities

Soil microbiomes are pivotal for the functionality of forest ecosystems. However, how microbial community structure and function vary across forest stand development and seasonal dynamics remains insufficiently understood. Using a space-for-time substitution approach along an afforestation chronosequence of Metasequoia glyptostroboides (6-45 years), we investigated the interactive effects of stand development and seasonal variation on soil microbiome structure and enzymatic functioning. We found that forest development induced a shift toward fungal dominance, accompanied by an increase in microbial biomass and a decrease in α-diversity. Functionally, a greater stand age suppressed carbon-acquiring enzyme activities while enhancing nitrogen- and phosphorus-acquiring enzyme activities. Enzyme vector analysis further indicated a shift from nitrogen to phosphorus limitation during succession. Seasonality significantly modified soil microbiome structures. Fungal and bacterial β-diversity responded strongly to seasonal variations. Variation partitioning revealed that stand age was the dominant driver of changes in microbial biomass and enzyme activity, while season exerted a stronger influence on fungal community structure and microbial carbon use efficiency. Structural equation modeling revealed that soil pH, electrical conductivity, total nitrogen, nitrate, and fine root biomass were key ecological drivers of soil microbiome structure and enzymatic functions across forest succession and seasonal variation. Our findings underscore the necessity of integrating multiple processes into assessments of soil microbial dynamics. They highlight the importance of concurrent measurement of microbial community structure and function, providing new insights into how community assembly links to ecosystem processes. Collectively, this work provides a basis for understanding and predicting microbial dynamics during forest development, with direct implications for sustainable plantation management.