Environmental filtration and host selection effect influence bacterial community assembly in broad-leaved Korean pine forests: A regional scale study

In forest ecosystems, the bacterial communities primarily establish symbiotic relationships with host plants, facilitating various ecological niches across different spatial scales. However, our understanding of bacterial community assembly mechanisms at both regional distribution scales and within different niche habitats of hosts remains limited. In this study, we investigated bacterial community dynamics, assembly processes, and co-occurrence network patterns in bulk soil, rhizosphere, root endosphere, and leaf endosphere spanning from north to south in the broad-leaved Korean pine forests of northeastern China. Our findings revealed that bacterial composition varied across distribution regions and compartment niches, with diversity also varying among these compartment niches. Deterministic processes predominantly shaped bacterial community assembly across geographic distribution gradients, whereas stochastic processes were more influential in differentiating bacterial communities within specific compartment niches. The co-occurrence network indicated lower stability of bacterial communities at the southernmost site of DD across different geographic distributions and in the leaf endosphere across different compartment niches. Furthermore, the co-occurrence network identified Jatrophihabitans and Betaproteobacteriales as keystone taxa at HH and DD sites, while Aliihoeflea and Actinoplanes were identified in the root endosphere and leaf endosphere, associated with essential functions like carbon metabolism, nitrogen fixation, and pathogen resistance. Source tracking showed bulk soil was the main source of bacterial communities in the rhizosphere, root endosphere, and leaf endosphere. Environmental factors such as soil pH in shaping bacterial community assembly across different geographical areas, while host selection was influential in determining bacterial community assembly within compartment niches. Furthermore, bacterial community similarity and abundance were influenced by environmental factors such as soil pH, total phosphorus, and mean temperature. Overall, the insights gained contribute to a deeper understanding of the relationship between bacterial communities and host plants, carrying important implications for plant conservation and practices related to forest ecosystem functioning.