Rhizobacteria provide resistance to Gaultheria mucronata (Ericaceae) against seasonal variability in volcanic scoria soils

Plants growing in volcanic scoria soils face severe edaphic challenges, including nutrient scarcity, high porosity, and marked seasonal variability. This study explores the structural and functional stability, microbial interactions, and plant growth-promoting capacities of rhizosphere bacterial communities associated with Gaultheria mucronata (L. f.) Hook. & Arn. (Ericaceae), a native shrub adapted to extreme environments in southern Chile. Rhizosphere and bulk soil samples were collected in contrasting seasons (summer and winter) across two volcanic scoria sites. By employing 16S rRNA metabarcoding, functional inference, microbial isolation, and plant-beneficial trait assays, we demonstrated that rhizosphere bacterial communities maintain structural stability and distinct functional profiles, significantly differentiated from bulk soils despite seasonal fluctuations in soil nutrient availability. Network co-occurrence analyses revealed season-dependent patterns, with higher complexity and balanced interactions in winter. Functional predictions highlighted enrichment in rhizosphere processes related to nutrient mobilization, organic matter degradation, and antioxidant mechanisms. Arthrobacter sp. emerged as a keystone taxon with consistent high abundance, central network positioning, and multiple plant growth promoting traits, including phosphate solubilization, siderophore production, and ammonia synthesis. Our findings demonstrate that rhizosphere bacterial communities associated with Gaultheria mucronata maintain structural stability and exhibit functional plasticity across contrasting seasons in nutrient-poor volcanic scoria soils. These traits reflect the resistance of rhizobacteria to seasonal variability and suggest a role in supporting plant adaptation to extreme edaphic environments.