Taxonomic and Metabolic Novelty in Wildfire-Impacted Soil Microbiomes

Abstract:

Rising temperatures coupled with earlier snowmelt has increased the frequency, duration, and intensity of wildfires in the western United States. Although wildfire severity is being increasingly recognized as a key variable that impacts the ability of forest soils and ecosystems to recover, we lack an understanding of how soil microbial function is altered by the severity of this disturbance. Here, we focused on characterizing changes in soil microbial metabolic function across a wildfire severity gradient associated with two recent wildfires in lodgepole pine forests in Wyoming, USA. In July 2019, we collected both organic and mineral soil samples from low, moderate, and high severity burned areas and adjacent, unburned forests. 16S rRNA gene sequencing revealed that microbial communities in all burned soils had decreased diversity and evenness with distinct community composition relative to unburned soils (ANOSIM = 0.569, p < 0.05). Communities in burned soils were particularly enriched in gram-positive Actinobacteria that likely have higher tolerance to moisture and temperature stresses common after wildfire. Metagenomic sequencing of 12 soils impacted by low and high burn severity yielded 637 medium and high-quality metagenome-assembled genomes (MAGs), representing the majority of dominant taxa. These analyses confirmed the prevalence of Actinobacteria in post-wildfire soils, revealing that MAGs affiliated with this phylum were frequently discriminant for high severity burn conditions. Metabolic profiling of the MAGs indicated that the potential for pyrogenic carbon (C) and amino acid degradation was greater in high severity burned soils, and metatranscriptomic sequencing identified active pyrogenic C degradation genes within Actinobacteria MAGs. Additionally, several fungal MAGs were highly active across both low and high severity burned soils, highlighting the importance of fungal species in post-fire nutrient cycling. These genome-resolved analyses have begun to identify linkages between wildfire severity and the responding functionality of the microbiome and reveal the complex biotic interactions in fire-impacted ecosystems.