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Soil Impacts

Soil takes many years to form. Once it is lost or altered, these changes can’t be reversed on human time scales. Many invasive weeds increase the overall rate of erosion because they don’t provide adequate soil protection. Others produce or accumulate compounds that change soil chemistry.

Soil Organic Matter
The decomposition of roots and above-ground plant litter gives rise to soil organic matter. Image depicting the change in soil organic matter with cheatgrass invasionWith weed invasions, inputs to soil organic matter can change. For example, roots of cheatgrass are very fine, and concentrated in the upper 20 cm of soil. Also, cheatgrass litter decomposes easier than most native vegetation. Because of these differences, soils under cheatgrass dominance have shallow, rapidly cycling soil organic matter. There is an overall loss of soil organic matter, which could make restoration of perennial plants more difficult.

Changes in nitrogen concentration
In contrast to soil organic matter, nitrogen concentrations are higher beneath cheatgrass than native vegetation. Because cheatgrass thrives with more available nitrogen, this disproportionately benefits cheatgrass.

Picture of microbiotic crustAltered microbial community
Weed invasions can change the composition of mycorrhizal fungi and other soil microbial species including bacteria, cyanobacteria, fungi, lichens, mosses, and nematodes. These microbes help in organic matter decomposition and nutrient cycling.

Changing the composition of these organisms can modify soil attributes, affecting current plant growth, and may impact revegetation efforts. Microbial soil crusts are a thin layer of bacteria, mosses, and lichen on the soil surface that can prevent soil erosion, reduce soil moisture loss, and make nitrogen available for plants. Weed invasions and subsequent control efforts can damage these living crusts.

Images:
-Microbiotic crust: Nicole Decrappeo.
-Sidebar photo: Southwest Regional Gap Analysis Project. Field site database. (http://earth.gis.usu.edu/swgap/trainingsites.html, 26 September 2006).

Text references:
-Belnap, J., and S.L. Phillips. 2001. Soil biota in an ungrazed grassland: Response to annual grass (Bromus tectorum) invasion. Ecological Applications. Vol. 11 No. 5 pp. 1261-1275.
-Kuske, C.R. et.al. 2002. Comparison of soil bacterial communities in rhizospheres of three plant species and the interspaces in an arid grassland. Applied and Environmental Microbiology. Vol. 68 No. 4 pp. 1854-1863.
-Norton, J.B. et.al. 2004. Soil morphology and organic matter dynamics under cheatgrass and sagebrush-steppe plant communities. Journal of Arid Environments Vol. 57 pp. 445-466.

 

Picture of desert pavement