Cheatgrass Impacts Soil Health
Driving across the green Wyoming rangelands this June, the reddish patches that indicate cheatgrass is having a good year remind me of the summer 14 years ago I spent up to my eyeballs in the weedy annual grass. Cheatgrass grows well in the Great Basin of Utah and Nevada but not that tall. I spent much of that summer standing in five-foot-deep holes with my head protruding at ground level in a sea of cheatgrass.
As a researcher at Utah State University, I was collecting data on how conversion from sage-steppe grassland to persistent monocultures of the exotic winter annual grass impacts soil health and organic matter processes. I started by working with long-time Great Basin ecologists like Neil West and Steve Monsen to locate seven long-term infestations with matched areas dominated by native range vegetation.
With help from a crew of muscle-bound Utah college students, I dug holes at each of the 14 locations and carefully described the soil to make sure the paired sites were well matched and to look for differences attributable to the weed infestation, like porosity and root characteristics. Then I took samples back to the lab where I measured many properties that underlie soil health and productivity, such as density, texture, alkalinity, organic carbon and nitrogen, microbial activity and easily decomposable soil organic matter.
It turned out that looking at soil under paired cheatgrass and native range in the Great Basin was almost like looking at soil under paired wheat fields and native range in eastern Wyoming. Cheatgrass grows a thick mat of shallow, very fine roots that die each summer, aerating the soil and adding carbon-rich organic material in almost the same manner as tilling to incorporate wheat straw. Soil microbes thrive on the added air and the carbon energy source, and their rate of decomposing organic material multiplies.
The initial effect of increased microbial activity is increased mineralization of soil organic matter, making more nutrients available for the crop or the weed. But after a few years of enhanced conditions caused by disturbance, microbes start eating themselves out of house and home, depleting soil organic matter that accumulated under millennia of perennial grassland vegetation. Loss of organic matter degrades all the properties that contribute to soil health, including building soil structure, enhancing water infiltration and storage, providing a time-release nutrient supply and supporting sustained productivity.
In cropland, organic matter loss means that more and more tillage and more and more fertilizer might be necessary to keep growing crops. In cheatgrass-infested rangelands, it means that the older and more monocultural the infestation, the more difficult it might be to restore desirable vegetation. Cheatgrass tends to convert slow, conservative nutrient cycles of native grass and shrublands to flashy, leaky nutrient cycles where mineral nutrients accumulate to create conditions for rapid cheatgrass growth – just add water, or to be lost with erosion, leaching or volatilization. In native perennial plant communities a diversity of plants and microbes turn mineral nutrients into more stable organic forms almost all year.
In annual cropping, a great practice is to try to keep plants growing and covering the soil all year or especially to convert fields to perennial cover, like hay or pasture, for a few years. This minimizes disturbance and aeration and shifts the system back toward accumulating, rather than losing, soil organic matter.
The extensive nature of cheatgrass infestation, often on hot, south slopes and rough ground, makes restoration difficult. Range managers in the Great Basin have had some success “re-perennializing” cheatgrass stands by seeding introduced cool-season bunchgrasses like pubescent wheatgrass or crested wheatgrass. After the bunchgrasses become established and reduce competition from cheatgrass, managers increase the diversity by weakening the introduced grasses with fire, heavy grazing, herbicides, mechanical treatment or a combination and then planting native grasses, forbs and shrubs.
Rigorous fire prevention is crucial to restoring cheatgrass infestations to diverse, desirable plant communities. Once a seed bank is established, cheatgrass may always be there in the understory, filling the bare spaces and creating a continuous fine fuel load so that when a fire occurs, it can be much larger and more destructive, and the plant community quickly converts back to cheatgrass.
A comprehensive guide to cheatgrass management in the Rocky Mountain region can be found at the University of Wyoming Extension publications website at wyoextension.org/publications. Search “cheatgrass” or bulletin number B-1246. For more detailed information on the study discussed here, see the papers posted at uwyo.edu/esm/faculty-and-staff/norton/papers/wildland-shrub-proc-2004.pdf and uwyo.edu/esm/faculty-and-staff/norton/papers/jae57-4-6-04.pdf.