Differential assembly and functional roles of bacterial communities in coniferous and mixed conifer-broadleaf forest soils.

Forest soils harbor a diverse array of bacteria that play a crucial role in nutrient cycling. However, the differential effects of coniferous versus mixed conifer-broadleaf forests on the distribution of both abundant and rare bacterial taxa remain poorly understood. In this study, we integrated 16S rRNA gene amplicon sequencing with metagenomic shotgun sequencing to conduct a comparative analysis of soil bacterial communities in a conifer plantation and an adjacent mixed conifer-broadleaf forest, specifically examining their community structure, assembly mechanisms, co-occurrence networks, and functional potential. Both abundant and rare taxa showed significant differences in community composition between the two forest types. Soil pH and organic matter content significantly influenced the total and abundant bacterial communities, while available phosphorus and potassium were key determinants of rare community composition. Co-occurrence network analysis revealed that abundant communities formed highly clustered, simplified networks, contrasting with more fragmented and keystone-rich networks in rare communities. Null model analyses indicated that community assembly was largely driven by stochastic processes, with ecological drift accounting for about 80% of the variation in total and rare communities, and dispersal limitation explaining nearly 72% of the variation in abundant communities. Functional predictions indicated that bacterial communities in mixed forests were enriched in pathways linked to glycosylation, carbohydrate degradation, and nitrogen fixation, while coniferous forests favored pathways related to autophagy, signaling, and stress responses. This study highlights the complementary roles of abundant and rare bacterial taxa in forest soil ecosystems and underscores the importance of preserving mixed forests to sustain microbial functional diversity.IMPORTANCEForest soils host a complex web of common and rare bacteria that quietly regulate nutrient cycles. By comparing pure conifer stands with mixed conifer-broadleaf forests, we found that abundant species underpin essential functions while rarer microbes fill specialized niches. Acidity and nutrients strongly influence which bacteria thrive; mixed stands favored microbes that break down carbohydrates and fix nitrogen, whereas conifer soils supported organisms adapted to stress and nutrient-poor conditions. These findings emphasize the importance of preserving diverse forest ecosystems for soil health, carbon storage, and effective forest management strategies in climate change adaptation.