THE EFFECTS OF FIRE HISTORY AND BURN SEVERITY ON SOIL BACTERIAL COMMUNITIES IN ANDISOLS OF A TEMPERATE CONIFEROUS FOREST

Abstract

Fire is a crucial disturbance that shapes forested ecosystems both above- and belowground. Fires change the soil environment both directly through heating and combustion of organic matter and indirectly by changes to physical and chemical properties, including pH, soil nutrients such as carbon and nitrogen, and moisture. Fires may have cumulative effects on soil properties, affecting the microhabitats in which microorganisms live, thus impacting microbial community structure and composition. While the impacts following a single fire are relatively well understood, the interaction of effects from repeated fires is less well studied. Studying the effects of repeated fire on soil bacterial communities is essential to understanding how changing fire regimes may affect the numerous critical roles that bacteria play in ecosystems, including plant symbioses, nutrient cycling, and organic matter decomposition. In this study, we compared soil bacterial communities at sites that share the same most recent fire but differ in at least one previous fire to understand how fire history and recent burn severity impact soil pH and shape soil bacterial communities. We sampled soils from Lassen Volcanic National Park, in northeastern California one year after the 2021 Dixie fire, measured soil pH, and characterized their soil bacterial community compositions using 16S rRNA amplicon sequencing. We found that soil pH increased with burn severity, but was lower in areas with more fire occurrences, potentially demonstrating a recovery of soil pH during the fire free interval and suggesting an interaction between fire occurrence and fuel loading. Although soil bacterial community composition was best predicted by soil pH, we found an inconsistent, yet strong, relationship between burn severity and soil bacterial community composition. Fire history was a significant predictor of soil bacterial community composition in one unit, although this was driven by recent burn severity, indicating a bacterial resilience to previous fires. We also identified bacterial taxa differentially abundant with fire history and burn severity, some of which had previously been identified as fire responders. We found that across all three units, fire history was a significant predictor of soil bacterial community composition. Our study examined the effects of recurrent fires on soil pH and bacterial community composition, expanding our understanding of fire ecology in a microbial context.