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Mycorrhizal Colonization of Crested Wheatgrass as Influenced by Grazing¹

Vesicular-arbuscular mycorrhizal (VAM) fungi play a role in crop productivity and soil stability in agricultural ecosystems. This role is accentuated under adverse cultural or climatic conditions, such as grazing or drought. The purpose of this study was therefore to assess the effects of stress (grazing) on VAM-fungal colonization of a major forage species. Changes in root colonization of crested wheatgrass [Agropyron desertorum (Frisch.) Schult.] plants by VAM fungi were measured under three levels of grazing pressure, ranging from no grazing to heavy, continuous grazing. Percent colonization of root length was determined by microscopy, and VAM-fungal biomass spectroscopically. These findings were related to the changes in root/shoot ratios caused by grazing. The study site is characterized as a Wyoming big sage brush (Artemisia tridentata spp. Wyomingensis) - grassland community on an Abgese loam (fine-loamy, mixed, mesic Xerollic Haplargid) soil in central Nevada. Plants studied were introduced previously as part of a range improvement program. Colonization of root length declined significantly (P < 0.05) and seasonally (40.2%, June and 50.6%, October) in plants under heavy grazing as compared to ungrazed plants. Spores (60.8% June and 89.5% October) and biomass (35.0% June and 61.5, October) of the VAM fungi also declined with grazing. Colonization of wheatgrass was favored when the amount of photosynthetic tissue relative to root mass was high. Phosphorus concentration in plant tissues was not significantly affected (P < 0.05) by grazing. It is concluded that severe grazing adversely affects the colonization of crested wheatgrass by VAM fungi. This reduction in the fungal symbiont may have an effect on plant nutrition and soil structure and stability which needs further investigation.

Key Words: Agropyron desertorum • Glomus mosseae • Glomus constrictum • Root/shoot ratio • Phosphorus nutrition

² Plant physiologist (G.J.B.), range scientist (R.A.E.), and professor of animal science (A.L.L.).

Received for publication April 4, 1984.