Sagebrush steppe focus of Andrew Austreng’s research
By Cindy Salo
One of the most promising ways to reduce atmospheric carbon dioxide levels is right under our feet. Trapping carbon and storing it as organic matter in the soil is simple and inexpensive because plants do the work.
Graduate student Andrew Austreng’s thesis, “The Carbon Budget Impact of Sagebrush Degradation,” details results of research that shows that restoring cheatgrass-degraded sagebrush grasslands, or sagebrush steppe, by reestablishing native shrubs and grasses could remove as much carbon dioxide from the atmosphere as that produced by 100 million Americans in one year.
Now an environmental compliance specialist in Redmond, Wash., Austreng has completed requirements for a master’s of science degree in Hydrologic Sciences from Boise State University and will graduate in May.
As part of the project, geosciences professor Shawn Benner and Austreng measured the amount of plant material produced and the amount of carbon stored in soil under three different types of vegetation. They found that native sagebrush steppe plants can store much more carbon that either nonnative cheatgrass or nonnative crested wheatgrass.
These results indicate that restoring degraded sagebrush steppe can sequester carbon dioxide released from the burning of fossil fuels and help reduce net carbon emissions in the U.S., according to Austreng.
“Boise State gave me the opportunity to use instrumentation and resources that students at other universities can only observe or read about,” Austreng said about his research project, which received funding from the U.S. Bureau of Land Management. “The skills I gained at Boise State have opened many doors for me.”
The Boise State researchers worked with Mike Pellant of the BLM, which manages about one-fifth of Idaho’s land and is working to restore cheatgrass-degraded sagebrush steppe rangelands in the Intermountain West.
“The U.S. has pledged to reduce its net production of carbon dioxide by 17 percent by 2020,” said Benner. “Carbon sequestration in soil can help us meet that goal by offsetting human-caused emissions.”
Plants capture carbon dioxide during photosynthesis and create simple organic molecules, from which they build all of the compounds they need. When plants drop old leaves, replace roots, or die, the plant materials are broken down by soil microbes into carbon-rich organic matter. Although researchers have studied organic matter and carbon in farm land quite extensively, little is known about carbon storage in the uncultivated rangelands that cover much of the U.S. West.
The Impact of Cheatgrass on Soil Carbon
Austreng found that after cheatgrass invades the sagebrush steppe, much of the carbon that was in the soil under the native vegetation is lost. The carbon that remains is concentrated near the surface of the soil, where the shallow roots of the small cheatgrass plants grow.
Cheatgrass can move into sagebrush steppe after wildfires kill sagebrush and weaken native grasses. Once it gains a foothold, this nonnative invasive grass competes with native plants and carpets the ground with fine stems and leaves. This material creates an excellent fuel bed, where fire spreads easily. This increases the likelihood of subsequent fires during the hot, dry summers.
Crested wheatgrass was brought to the U.S. from the grasslands of Eurasia in the early 20th century. Land managers wanted grasses that could thrive in dry, harsh conditions and provide food for livestock. Crested wheatgrass has often been planted after wildfires because it sprouts quickly and competes strongly with cheatgrass.
Austreng found that the soil under crested wheatgrass contains 40 percent more carbon than soil in areas invaded by cheatgrass. In addition, the more extensive root systems of the larger crested wheatgrass plants add carbon to deeper soil layers than do those of cheatgrass. Crested wheatgrass helps keep an area from reburning, as it stays green longer into the summer than cheatgrass and does not produce the carpet of fine fuel that cheatgrass does.
Crested wheatgrass plants also live for many years, unlike cheatgrass, which dies each spring after it produces seeds to sprout the following fall. This year-round cover of living plants and network of roots protect the soil from wind and water erosion, even when the plants are brown and dormant during the summer. Protecting the soil from erosion also protects the organic matter and carbon stored in it.
Native Vegetation and Carbon Storage
While Austreng’s research showed that crested wheatgrass stands did sequester significantly more carbon than cheatgrass, these plants were not able to capture and store nearly as much carbon as the diverse mix of native shrubs and grasses found in non-degraded sagebrush steppe. Soil under the native vegetation contained almost twice as much carbon as soils in cheatgrass-degraded areas.
The large, deep roots of sagebrush and native perennial grasses store more carbon, and in deeper soil layers, than either of the nonnative types of vegetation. In addition, the variety of plants in the diverse native communities protects the soil, and its organic matter, more completely throughout the year than the other vegetation types studied.
Cheatgrass has invaded and degraded an estimated 39,000 square miles of sagebrush steppe in the West. This is equal to about one-half of the area covered by the state of Idaho. Austreng’s work indicates that restoring these areas to native sagebrush and grasses would remove a significant amount of carbon dioxide from the atmosphere. He estimates that by the time the plants reached their full size they would have sequestered as much carbon dioxide as that produced by 100 million Americans in one year.
“Much of the low elevation public land in the West is sagebrush steppe rangelands,” explained Benner. “Our work indicates that our public lands can help us cope with the increasing levels of atmospheric CO2. And the added benefits to wildlife habitat and increased recreation value are just icing on the cake.”
Summer Field Work at Kuna Butte
Austreng carried out his research at Kuna Butte, southwest of Boise. “This was an ideal place to work because there had been a fire there in 1983,” he explained. “Part of the area was seeded with crested wheatgrass and part of it wasn’t; that part had been invaded by cheatgrass. And right across the road from the fire we had sagebrush that hadn’t burned. So we had all three types of vegetation on the same type of soil.”
Before he started his work, Austreng first developed efficient methods to collect samples of the plants and soil. He used a soil corer to collect samples at several different depths down to 60 cm (two feet) deep. In order to accurately compare soils in the three vegetation types, he had to collect samples at the exact same depths in each without mixing any of the layers.
Once he had polished his technique, Austreng and an undergraduate student assistant headed for Kuna Butte. Getting to the field site on the rough, unmaintained roads was challenging enough to require a four-wheel drive vehicle. But wrestling soil samples out of rocky soil was far more challenging.
“There’s nothing more frustrating than coring down for half a meter (20 inches) and hitting a rock, forcing you to start all over,” Austreng remembered.
The two field workers had only an occasional startled jackrabbit or circling hawk for company while they worked. They saw evidence of badgers, which are common in the area. “Badger holes are everywhere,” Austreng said.
“They threaten to snap an ankle as you’re carrying boxes of samples back and forth, if you’re not careful.”
Collecting plant and soil samples was only the first step in Austreng’s research. Each soil sample had to be dried and then mixed and remixed until a tiny subsample could be taken to accurately represent the entire sample. Only 60 mg of soil, less than 3 percent of the weight of a dime, was analyzed from each field sample in a carbon analyzer. This machine burns soil completely in pure oxygen at high temperature and then measures the amount of CO2 gas produced.
“A sample that takes a few minutes to collect in the field takes over an hour total processing time in the lab, where we processed about 1,000 samples in total,” Austreng said.
Austreng came to Boise State after completing his B.S. at the University of North Dakota in Grand Forks. He currently is preparing to submit his research results to a scientific journal.
Cindy Salo earned a Ph.D. in Renewable Natural Resources from the University of Arizona and a graduate certificate in Technical Communications from Boise State University. She has
more than 20 years of experience as a plant ecologist and writes about science and natural resource issues for general and technical audiences