1. Fire-Induced Mineralogical Changes in Midwest Tallgrass Prairie Soils
Lacey LeGrand
University of Nebraska - Omaha
Hi my name is Lacey LeGrand. I’m a graduate student in the department of geography and geology at UNO.
My thesis is on Fire Induced Mineralogical Changes in Midwest Tallgrass Prairie Soils
Here I am helping with a prescribed burn at my study site

2. Introduction- Prairie history
historical extent
fertile soils formed in glacial loess
settlement, agriculture, and
fire suppression → loss of prairie
Historically much of the Midwest was a prairie ecosystem
Prairie type varied with moisture availability, with shortgrass prairies occurring in the drier west and tallgrass prairies in the east
Prairie soils are fertile soils, classified as Mollisols that are rich in organic matter and formed in glacial loess deposits from the last Ice Age.
But, most of the prairie has been lost due to settlement, ag… and today only isolated remnants remain

3. Prairie management- precribed burns
importance of fire in grassland ecosystems
removes litter
reduces woody/invasive vegetation
stimulates growth of native prairie plants
returns nutrients to the soil
result is healthier, more diverse plant communities
what are the effects on soil?
For the areas of prairie that remain or have been restored, prescribed burning is an important tool to maintain this ecosystem
Fire is important in grassland ecosystems and many prairie plants are adapted to frequent fire.
Fire removes litter…
The result is healthier, more diverse prairie plant communities, but what are the effects fire is having on the soil?

4. Effects of fire on soil mineralogy
Minerals can be transformed or destroyed as a result of fire
goethite FeO(OH) → maghemite or hematite Fe₂O₃
gibbsite AlOH₃ and kaolinite Al₂Si₂O₅(OH)₄ are destroyed
calcite CaCO₃ forms as plants burn
Depends on fire intensity, duration, type of fuel
changes are better understood at higher temperatures
prescribed fires burn at lower temperatures, shorter duration
We know that fire can affect soil minerals, causing them to be transformed or destroyed
For example, the mineral goethite is transformed to maghemite at high temperatures, or to hematite at lower temps…
But these changes depend on fire intensity, duration, and the type of fuel being burned, and are better understood for higher temps
However, prescribed fires burn at low temperatures and for a shorter duration
Changes in soil mineralogy can influence soil structure, hydrology, and nutrient availability

5. Effects of fire on soil properties
Fire affects many soil properties over a range of temperatures
Temperature is shown here on the X axis and we see organic matter content, soil water repellency, and abundance of soil fungi being impacted at lower temperatures.
Mineralogical changes (seen here at the top of the graph) are fairly well understood for higher temperatures, but at lower temperatures there is a gap in the data, partly because people haven’t seen dramatic changes at these temperatures, but also that not many studies have looked specifically for effects of fire at low temperatures
Prescribed fires typically burn below 200 degrees
Mataix-Solera et al., 2011

6. Research questions
Does low-intensity prescribed burning induce mineralogical changes in prairie soils compared to unburned prairie soils?
identity or abundance of minerals  XRD
degree of crystallinity  Fe, Al elemental extractions
Which leads me to my research questions
In this study I want to know if low-intensity burning induces mineralogical changes…
Specifically, I am focusing on changes in the identity or abundance of minerals, which will be tested using X Ray Diffraction Analysis, or XRD
It could be that these fires are not intense enough to cause changes in mineral identity, but maybe mineral structure is being impacted, and this will be tested with extraction analysis of the elements Fe and Al which are common in soil minerals

7. Study site- Glacier Creek Preserve
long term study on effects of different management strategies
treatment
timing
My study site is a restored prairie in NW Omaha near 144 and State Street, owned by the University of Nebraska
It is 320 acres and additional land is still being acquired
Most of the preserve is burned in sections every few years, but for over 40 years there has been an ongoing study of prairie management strategies in experimental plots to test the effects of either burning or mowing at different intervals
For my study I was interested in the annually burned plots

8. Research design collect soil samples from burned and unburned plots
Surface (top 1-2 cm)
Depth (~10 cm)
12 plots sampled, for a total of 24 samples
9 burned
3 unburned
analysis by XRD and elemental extractions
So here you can see this large area has just been burned, and here are the experimental plots, and you can see a couple of them that were recently burned
For my project I collected soil samples from burned and unburned plots from two depths, the surface or top 1-2 cm, and about 10 cm deep, which is deep enough to be unaffected by fire
12 plots were sampled, 3 unburned or control plots, and 3 each that were burned in the spring, summer, and fall. Time of burning was not expected to affect my research questions

9. So here I am collecting samples from an unburned plot on the left and a burned plot on the right
We used a bulk density sampler to take a core of soil. The surface and depth portions were separated and bagged.
5 cores from each plot were amalgamated to get an representative sample

10. Results- XRD Some plots showed difference between surface and depth
So now some results from XRD analysis
These are data from a single burned plot
On the Y axis we have intensity and on the X axis the angle of the sample in degrees 2 theta. In the photo you see a sample mounted on a slide, which is rotated through different angles and X-rays are diffracted off the minerals in the sample.
The top pattern is the surface sample, from 1-2 cm and the bottom pattern is the depth sample from about 10 cm
Peak intensities were normalized to the quartz peak… to account for any systematic differences in intensity of the XRD patterns, because some runs had overall stronger patterns than others
Some plots had a difference in the peaks following the quartz peak between surface and depth, so we see this extra peak here in the surface pattern that is not present in the depth sample
Some plots showed difference between surface and depth
Peak intensities normalized to quartz peak

11. Results- XRD Unburned plots similar to burned plots
However, when we compare the surface burned data to the unburned plots, we see similar patterns and similar peak intensities.
So the new peak that looked interesting also occurred in the unburned plots, indicating that fire is not responsible for the difference
Unburned plots similar to burned plots

12. XRD data do not support conclusion that burning changes mineralogy
No clear trend
XRD data do not support conclusion that burning changes mineralogy
Here I have plotted representative patterns from an unburned plot on the top and 3 burned plots below.
When we compare the patterns there is no clear trend after burning.
So the data from XRD are inconclusive because we see a range of patterns in both the burned samples and the unburned samples
When we look at the normalized peak intensities we see no systematic shifts in patterns that would indicate mineralogical changes as a result of burning.

13. Methods- Extractions Surface sample elemental extractions
dithionite-citrate: dissolves free Fe and Al
sodium pyrophosphate: dissolves Fe/Al organic complexes
ammonium oxalate: dissolves amorphous/non-crystalline oxides
Analysis by Inductively Coupled Plasma mass spectrometer
Changes may indicate fire is impacting mineral structure
So on to my next method of analysis, elemental extractions
Surface samples will be analyzed for extractable Fe and Al content by 3 different chemical extractions
Dithionite-citrate will dissolve…
The resulting solutions will be sent to an analytical lab to be run on an Inductively Coupled Plasma mass spectrometer
Results will be in percent concentration of the element and will allow for inferences regarding mineral structure. XRD works best for crystalline mineral structures, so the extraction data will complement what we learned from XRD.
More amorphous structures should have more extractable Fe and Al compared to more crystalline structures.
Any changes between burned and unburned samples may indicate that fire is impacting mineral structure
So far I have 2 of the 3 extractions ready to go, and you can see part of the process in the photos here.

14. Extraction data example
dissolves poorly crystalline
dissolves organic complexes
dissolves free Fe and Al
Here is an example of extraction data for Al. There are a series of samples that were unheated and heated to 500 degrees.
You can see the amount of Al that was extracted from each of the 3 chemical extractions, indicated by the different colors.
There are differences between the heated and unheated samples, though in this case it seems that heating reduced the extractable Al content in the samples
Scullett-Dean and Santini, 2016

15. Effects of fire on soil properties
Controlled heating experiment to induce changes
One thing I would like to add to my project, especially since the XRD data were inconclusive, is an experiment to heat my samples systematically until I see changes in mineralogy.
Once I know at what temperature these changes start to occur, I will be able to say more definitively that prescribed burning is not impacting soil mineralogy because the temperatures are too low.
Mataix-Solera et al., 2011

16. Implications and applications
baseline data that could lead to other research questions, such as nutrient availability after fire
better understanding of prairie ecosystems
prairie management decisions and controlled burns
Results of my project will provide baseline data for Glacier Creek Preserve and other prairie sites
Understanding how fire affects soil mineralogy is one piece of the puzzle that will help us make more informed prairie management decisions, including controlled burning practices

17. Funding and acknowledgements
Project funding from:
University Committee on Research and Creative Activity (UCRCA)
Nebraska Geological Society (NGS) with matching funds from the AAPG
Thanks to my committee:
Dr. Ashlee Dere
Dr. Christina Dando
Dr. Tom Bragg
I would like to acknowledge my funding sources- UCRCA at UNO, The Nebraska Geological Society, and the AAPG.
Thank you!
I would also like to thank my committee including my wonderful advisor Dr. Ashlee Dere 

18. Questions