Prescribed versus wildfires in California grasslands: Bacterial and fungal succession

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James W. J. Randolph, M. Fabiola Pulido-Chavez and Sydney I. Glassman, Microbiology and Plant Pathology, University of California-Riverside, Riverside, CA, Sameer S. Saroa and Loralee Larios, Botany and Plant Sciences, University of California-Riverside, Riverside, CA

Background/Question/Methods
Microbial communities play an important role mediating plant establishment and plant
community succession. As management actions are often used to simulate lower intensity
natural disturbances, for example a prescribed burn versus a wildfire, it is important to
understand the differing impacts of these events on microbial communities. Sampling both soil
microbial and plant communities from both a prescribed fire and wildfire in a southern California
annual grassland, we ask 1) how does bacterial and fungal resilience (in terms of biomass,
richness, and composition) differ between these two disturbances? and 2) what soil properties
drive soil microbial recovery? We collected soils from 1m2 plots along three transects separated
a range of 5 to 100 meters. In total, we set up fifteen 1m2 plots at the prescribed fire (twelve
burned; three unburned), and seventeen plots at the wildfire (twelve burned, five unburned).
Within weeks of fire, we estimated soil burn severity with percent char, and we sampled soils by
mixing three 5cm soil cores at each plot during at three timepoints post-fire. We measured soil
nutrients, moisture, and pH of all soils, used qPCR to measure bacterial and fungal biomass,
and we assessed bacterial (16S) and fungal (ITS2) diversity with Illumina MiSeq.
Results/Conclusions
Plant composition was predominately non-native within both fire sites and greater in the
unburned vs burned plots. For both fires, bacterial richness did not differ between the unburned
and burned plots but did vary seasonally with a distinct decline from the fall to the spring. In
contrast, fungal richness decreased in the burned plots compared to the unburned in the wildfire
but not prescribed fire. Bacterial and fungal biomass were impacted by the percent of initial char
after the fire, but these effects differed in the two fires. In the wildfire, bacterial and fungal
biomass decreased with increasing char. In the prescribed burn the relationship between
bacterial and fungal abundance and initial char varied seasonally. Lastly, bacterial and fungal
community composition differed between unburned and burned plots in both fires. These results
suggest that soil microbial community succession does differ between these two disturbances,
with fungal communities more strongly impacted by wildfire than prescribed fire. Understanding
the soil ecology of management actions like prescribed burns that try to simulate natural
disturbances is important for increasing the efficacy of ecosystem management.