Researcher Reports That Invasive Grass Destroys Soil Nitrogen, Depletes Native Desert Plants

Evans will present his findings today (Aug. 10) at a symposium, "Invasive Species in the Soil: Effects on Organisms and Ecosystem Processes," at the 84th annual meeting of the Ecological Society of America in Spokane, Wash. The innocuous looking grass was brought from Eurasia to Washington State in the 1890s, and it quickly spread through the arid areas.

"In 30 years it basically took over the West," Evans said. The grass can now be found from Washington through Nevada, Utah, Idaho and parts of many other states.

In Canyonlands National Park, where Evans and Jayne Belnap of the National Park Service have conducted soil nitrogen studies, undisturbed areas sport small clumps of native grasses with areas of black biological soil crust in between. But when cheatgrass invades an area, it grows in thick blankets, shutting out sunlight that microbiotic crusts need to fix nitrogen in the soil, Evans said. Cheatgrass dies back every year, but it adds little nitrogen back to the soil.

"It uses up all the nitrogen that other plans would normally take up," Evans said.

Large tracts of cheatgrass also use up nitrogen another way. The fields are susceptible to range fires that can spread for miles. The nitrogen tied up in plant litter literally goes up in smoke, Evans said.

The native plants have no tolerance to fire, because they have had little exposure to it in the past, Evans said.

The combination of fires, and low nitrogen content soil may drive out the native plants. But the cheatgrass seems to thrive under these conditions.

"That's what people are finding with invasions. They can't be reversed," Evans said.

The next step in Evans research will be to look at the plant's responses to the nitrogen loss, he said.