Altered edaphic parameters couple to shifts in terrestrial bacterial community structure associated with insect-induced tree mortality

Unprecedented insect-induced tree mortality has been observed globally and while hydrologic and biogeochemical changes have been recorded, alterations to terrestrial microbial communities, which influence as well as respond to these shifts, are not well understood. The objective of this work was to better understand how bacterial communities are coupled to perturbations in biogeochemically-relevant soil physicochemical parameters resulting from beetle-induced tree death. To this end, soils beneath trees across a beetle-kill spectrum were contrasted in the central Rocky Mountains at both heavily impacted (Chimney, 85% mortality) and moderately impacted (Niwot, 18% mortality) field sites. Soil organic matter (OM), dissolved organic carbon (DOC) and dissolved organic nitrogen were all significantly altered when contrasting soils under healthy versus beetle-attacked trees at Chimney. Bacterial alpha diversity measurements were found to increase with tree death and beta diversity measures showed significant clustering with relation to tree phase. The site was characterized by a significantly higher relative abundance of bacterial clades under healthy trees that were correlated to OM and DOC concentrations. In contrast, compositional changes in soil bacterial communities and edaphic parameters associated with tree phase were not observed at the less impacted Niwot site. Our findings reveal a coupled response between shifts in organic carbon cycling and the bacterial assemblage as a result of large-scale, beetle-induced tree mortality with implications for heterotrophic respiration in near-surface soils and suggests a possible dependence on the level of forest mortality before manifestation.