Research Feature: Forest Wildfire Effects on Soil Microbial Communities

Thesis Citation

Hebel, Cassie, 2007, "Effects of Burn Severity on Soil Microbial Communities and Invasive Plant Species after Wildfire in the Cascade Range of Oregon," M.S. Thesis, Department of Forest Science, Oregon State University.

In April 2007, Forest Science student Cassie Hebel defended her M.S. on soils burned by wildfire.  Her project is part of a larger effort led by Jane Smith (USFS PNW Research Station) and Elizabeth Sulzman (OSU Crop and Soil Science) to assess the physical, chemical, and biological rates of soil recovery post-fire.  The group's research is funded by the USFS PNW Research Station and the Joint Fire Science Program.

In this short Web interview, Cassie describes her research project.

What was your Master’s project about?

My project investigated the effects of wildfire on soil microbial communities and invasive plant species.  I worked in the Booth and Bear Butte (B&B) Fire Complex – an area that burned in 2003.  You pass through part of the complex as you drive over Highway 20 toward Sisters, Oregon. 

I compared differences between “red soils” and “black soils” within the B&B complex.  The red soils are areas of soil where a fallen tree or stump had burned in place.  The intense heat generated by the burning debris causes the soil to oxidize and change to a red color.  Through the burning process, these areas of red soil sustained extensive damage to the soil structure and microbial communities and therefore have historically been labeled “detrimentally burned soils.”  These soils have also been hypothesized  to be more susceptible to invasion by non-native plants, due to the severe alteration by the smoldering fire.  This research was the first to investigate the effects of red soils across a post-fire landscape. I was hoping to discover facts about red soils by comparing them to adjacent black soils -- areas where the fire had passed over, but with a lower burn intensity.  My project had several parts comparing red and black soil:  I measured soil nutrients, carried out growth experiments with native and non-native plants, conducted a field assessment of plant growth two years after the fire, and investigated the soil microbial community.

Photo showing "red soils".

The "red soils" show up as light colored patches in this photo from the B&B Fire Complex. Their shape often outlines the location where downed logs or other debris burned.

What did the soil nutrient and plant growth experiments show?

What stands out most was that the soil chemistry of the red and black soils were vastly different – soil nutrients were less abundant in the severely burned soils – for example, there was less organic carbon and less nitrogen and phosphorous.

For the growth experiments, I grew three native and three non-native plant species in field collected samples of the red and black soils in a controlled growth chamber.  This was a way to test if native or non-native plants had reduced growth in the red soil.  The results were surprising – the non-native plants had reduced growth in the burned soils while the native plants did not show growth differences between the soils with different burn severities. This result was suprising because we initially hypothesized that the non-natives would be negatively influenced by the low abundance and diversity of soil microbes (especially arbuscular mycorrhizal fungi) in the red soil; we did not take into account how dependent non-native, invasive plant species are to soil nutrient availabity.  What these results suggest is that many native plants species may be better adapted to regenerate in the severely burned redsoils because they are less dependent than non-native, invasive plants on the flush of nutrients after a fire.

What did your field survey of plant regrowth on the two soil types show?

Two years after the burn, I revisted each red and adjacent black soil plot. Of the sixteen species of plants growing in our plots none were non-native. Much like our controlled growth chamber experiment, we found that, on average, the black soils contained about twice the plant cover of the red soils.

How did you study the soil microbial communities and what did you learn about them?

To study the soil microbial community, I conducted a Phospholipid Fatty Acid (PLFA) Analysis. This procedure is commonly used to, in a sense, take a snapshot of the microbial community structure in a given sample of soil. To explain the procedure you must understand that all living organisms have fatty acids in their cells which are specific to that organism or group of organisms. The fatty acids make up a relatively constant proportion of the biomass, so from a small soil sample, you can extract lipids and get a good idea of what microbial groups are in the soils and in what quantities. 

My work with PLFA showed that the microbial communities in the black and red soils were distinct and that the difference is associated with the difference in soil chemistry. Also, the PLFA showed that there was a greater abundance of microbial biomass in the black soils but that bacteria, especially gram negative bacteria, were more abundant relative to other microorganisms in the red soils.

Photo of Cassie collecting soil samples.

Cassie Hebel collecting soil samples.

What were the most important lessons from your project?

Hmmm…maybe, don’t judge a soil by its color!  This project is very preliminary but what it suggests is that calling red soils “detrimentally burned” may be a misnomer since they may provide a benefit.  They create diversity across a post-fire landscape and therefore create the potential for increased plant and microbial biodiversity. In addition, they may not provide the foothold for non-native species that people originally feared. 

This research project also made me realize the importance of below ground systems. I feel below ground research is a neglected area of study because of the difficulty in studying organisms you cannot see. Therefore, you have to be creative about the techniques you use. For me as a fledgling researcher, another one of the great things about this project was that it helped foster my inquisitive nature. Now I feel like I can’t stop with just one question answered! I plan to continue on my academic track to obtaining a PhD degree in hopes of answering some more of life’s mysteries.

How did your project fit in with the work of others in your research group?

My research is part of a larger study investigating burned soils and post-fire salvage logging in central Oregon.  My collegues have been collecting data on soil respiration, soil chemistry and hydrologic properties as well as identifying soil fungi and bacteria using molecular techniques.  We hope to continue monitoring our red soil plots to further assess post-fire soil recovery in these severely-burned red soils.