Fire Effects on Soil Properties
Authors: Schuler Sabrina Marie 
Published: April 2022

Within the sagebrush steppe, fire has been shown to affect biogeochemical properties and the microbial community composition in
soils. However, there is uncertainty about the magnitude and direction of these effects, since they vary by sites that differ in
abiotic and biotic conditions. Moreover, differences in post-fire management strategies are likely to mediate the effect of fire
on soil properties, thus further compounding this uncertainty. Any changes in soil biogeochemical properties following fire can
prevent successful restoration of Artemisia tridentata sp. wyomingensis (sagebrush), leading to
variable outcomes of restoration success in the sagebrush steppe. Previous research has shown that addition of native soil microbes
and biochar can improve ecosystem restoration efforts, but the effects of these soil amendments on post-fire soil properties and
sagebrush performance across sites are uncertain. With this study, I investigated how fire impacts soil properties (i.e., soil
organic matter (SOM), soil structure, carbon (C) and nitrogen (N) concentrations, soil pH, net mineral N, microbial richness and
composition) at a variety of sites that differ in time since exposure to fire, post-fire plant communities, and post-fire site
management. I then implemented a greenhouse study to evaluate how amending soils with native microbial community inocula and
biochar impact soil properties of the post-burn sites and sagebrush germination and growth. Taken together, these findings
capture the influences of multiple fires and separate management strategies on soil properties, and how certain soil amendments may redirect soil recovery to aid in sagebrush restoration.

In my first chapter, I asked two questions: (1) how does fire affect soil biochemical properties across sites that differ in
fire history, post-fire plant communities, and post-fire site management, and (2) how does fire affect soil microbial richness
and community composition across sites that differ in post-fire plant communities, and post-fire site management. To assess these
questions, soils were collected from three south of Boise, Idaho within the Orchard Combat Training Center (OCTC) that contrasted
in fire history, plant community, and post-fire management. The northern part of the Union Fire (180 acres; hereafter: UFN2011)
burned in 2011, and was treated with a mix of imazapic and glyphosate during the spring of 2019, after which sagebrush was
handplanted 8 months later that yea. The southern part of the Union Fire (160 acres; hereafter: UFS2011) burned in 2011 and
seeded with native grass species and planted with sagebrush. The Christmas Fire (hereafter: CF2018) burned in 2018, and was
subjected to the same seeding and handplanting treatments as UFS2011. At each site, I selected five locations within the
perimeter of the burn, and five locations outside the burn, representing the unburned control plots. In these unburned control
plots, the five locations were stratified by sagebrush canopy and interspace microsites separately. At each one of the five
locations, I collected four soil cores (10cm depth, 2.5cm diameter). I evaluated differences in soil pH, soil organic matter
(SOM), soil carbon (C) and nitrogen (N) contents, soil structure, N cycling, and soil microbial communities between burned and
unburned sites. Fire reduced SOM and soil C contents, and these losses were greater in burned areas that received an herbicide
treatment. This suggests that suppression of plant growth using herbicides may limit the recovery of soil properties that are
foundational to sagebrush steppe ecosystem functioning. Furthermore, I found a loss of arbuscular mycorrhizal fungi (AMF)
richness with fire and significant changes in soil microbial community structure when herbicide had been used. Finally,
increased soil mineral N concentrations across all burned sites indicate that fire may significantly reduce ecosystem stability
and increase the risk of invasion. These changes in soil properties are likely to lead to a persistent ecosystem state-changes
in the sagebrush steppe, and future studies should evaluate which management approaches could be used to restore both soils and plant communities.

In my second chapter, I investigated two management approaches that may be used to restore the soils and plant communities impacted
by fire. I asked (1) How does a live native soil microbial inoculum impact sagebrush performance and soil properties, (2) how do biochar
additions impact sagebrush performance and soil properties, and (3) how does prior management (e.g., herbicide) mediate the impact of
soil microbial inoculation and biochar amendment on sagebrush performance? A full factorial greenhouse experiment was conducted for
three months with soils collected from the three post-burn sites described in Chapter 1. In the greenhouse experiment I incorporated
the following treatments: (1) sterile native inoculum [-Inoculum] and no biochar [-Biochar], (2) live native inoculum [+Inoculum] and
no biochar [-Biochar], (3) sterile native inoculum [-Inoculum] and biochar [+Biochar], and (4) live native inoculum [+Inoculum] and
biochar [+Biochar]. Inocula was derived from sagebrush canopies at unburned sites and either added as live native inocula or autoclaved
to sterilize the microbial community. Biochar was crushed into planted, watered daily until cotyledons showed, and continually monitored
throughout the growing period. Germination, soil moisture content and pH, above- and below-ground measurements, total mineral N, fungal
root colonization proportional abundances, and microbial richness and composition were assessed. I found that inoculations did not
significantly benefit sagebrush performance, most likely due to the ratio of inocula administered. In contrast, biochar consistently
enhanced soil moisture, pH, sagebrush germination and other performance variables while its effects on total mineral N and fungal root
colonization varied by site location. Lastly, presence of herbicide in post-burn soils significantly altered soil bacterial and fungal
community composition, and its effects persisted enough to inhibit sagebrush performance. Together, my data show that addition of biochar
has a greater positive impact on sagebrush germination and performance than addition of soil microbial inocula, and that herbicide addition
has persistent negative impacts on sagebrush performance.

My study captured the varying levels at which fire impacts ecosystem structure and function, and how different soil amendments affected
sagebrush performance at these post-fire soils. My findings support the notion that soil properties will remain degraded without appropriate
management strategies supporting restoration, and herbicide may actually suppress successful restoration, residing longer in the soil than
previously documented. When growing sagebrush in post-burn soils within the greenhouse, biochar enabled soil recovery, and this benefited
sagebrush performance. However, herbicide impacts persisted and decreased sagebrush biomass even when soil amendments were incorporated.
Fire can have profound, yet vastly different, influences on soil properties, and soil amendments may be able to augment soil recovery.
Future studies should investigate various soil amendments and their impacts on sagebrush performance in the midst of changing fire regimes,
post-fire vegetation shifts, and current post-fire management.