Effects of low wildfire burn severity due to pre-fire shrub thinning on the chaparral soil bacteriome in the Santa Monica Mountains of Southern California

Our objective was to study the longitudinal effect of decreased burn severity due to vegetation-type conversion (VTC) induced by chaparral shrub thinning prior to the Woolsey wildfire (November 2018) on soil chemistry and bacteriome composition and function. We compared soils from two study sites on the Malibu campus of Pepperdine University in the Santa Monica Mountains: one site had dense, unaltered chaparral shrubland and experienced a 4.5-fold increase in vegetation burn severity (high severity burn) compared to an adjacent altered site where the vegetative fuel load was 80% less prior to the fire (low severity burn). We analyzed soil nutrient concentrations and pH in 2019 and 2021 and soil respiration, measured by CO₂ efflux, in 2019, 2020, and 2021. DNA was isolated from soil samples collected in 2019, 2020, 2021, and 2023 for Illumina Miseq paired-end 16S V3-V4 sequencing. We predicted the functional bacteriome from the 16S data using PICRUSt2. Relative to high severity soils, low severity soils showed decreased nutrient concentrations, pH, and % organic matter in 2019. The low severity burned site showed greater compositional stability over time, with increased pyrophilous taxa in 2021 and 2023 (Massilia, Conexibacter, etc.). High severity burned soils showed decreased metabolic capacity over time. We identified correlations between bacterial taxa and diversity and functional pathways, which remained only in the high severity soil samples after stratification. Our findings contribute to an improved understanding of bacterial succession in soil from sites that experienced VTC prior to wildfire, highlighting microbial ecological implications for fire management strategies.