Early response of soil bacterial communities to soil burn severity in pine and shrub vegetation after wildfire in temperate-humid climate

Pine forests and shrublands worldwide are increasingly threatened by wildfires driven by global change, and those of temperate-humid ecosystems are no exception. Although the impact of soil bacterial of soil burn severity (SBS) on bacterial communities, has been examined in a number of studies, knowledge about their response across gradients of high SBS levels remains limited. In this study, we assessed the early effect (six weeks post-wildfire) of an SBS gradient, from moderate to extreme, previously defined, on bacterial communities in topsoil (0-2 cm) under adjacent pine and shrubland vegetation in northwestern Spain. Alpha diversity declined under moderate SBS, but no further decrease was observed at higher severity. Beta diversity increased, but community composition stabilized from moderate severity onwards, suggesting a threshold above which further compositional shifts are limited. Burned soils were enriched in Proteobacteria, Bacteroidetes, and Firmicutes, while Acidobacteria and Actinobacteria declined. Known pyrophilic genera (e.g. Massilia, Pedobacter), and others not previously reported with that character, were more abundant in burned soils. However, their increase did not extend beyond moderate SBS, reinforcing the notion of a compositional threshold. This stabilization may reflect the fire selection of fire-adapted communities' composition. Vegetation type, although a weaker driver than SBS, modulated bacterial responses: unburned pine and shrubland soils hosted similar communities that diverged after fire, with shrubland soils showing greater sensitivity. Soil pH, C, and C/N were strongly associated with bacterial composition. Our study provides evidence that moderate SBS may be sufficient to drive major shifts in soil bacterial communities, and that higher SBS levels do not necessarily lead to further changes, although confirmation through new studies is needed. The study findings may be useful for developing methods of accelerating the recovery of burned soils.