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 CO 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 (, , 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.IMPORTANCEAlong with increased fire frequency, the wildfire-urban interface has been expanding, requiring the need for fire mitigation strategies, such as pre-fire vegetation thinning near urban structures. Pre-fire vegetation thinning contributes to vegetation-type conversion and decreases burn severity, but its effect on the soil microenvironment is largely unknown. Here, we compared soil sites that experienced burns of different severity due to pre-fire vegetation thinning and vegetation-type conversion at one site but not the other. We identified changes in soil chemistry and longitudinal shifts in soil bacterial abundance and metabolic capacity that are associated with decreased burn severity due to pre-fire vegetation-type conversion. Our work contributes to improved understanding of the effects of pre-fire vegetation thinning to manage wildfire impact on urban structures on the soil microenvironment. These findings demonstrate ecological implications for fire management strategies and recovery of the chaparral ecosystems following wildfire.