Mycorrhizal Colonization of Crested Wheatgrass as Influenced by Grazing¹

Abstract

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.

Mycorrhizal Colonization Patterns Under Contrasting Grazing and Topographic Conditions in the Flooding Pampa (Argentina)

“…The higher colonization intensity of Dallis grass in the exclosure in winter supports the widespread concept that the less positive carbon balance induced by defoliation reduces colonization intensity (Bethlenfalvay and Dakessian 1984;Bethlenfalvay et al 1985;Hetrick et al 1990). At the community level, in general, when differences existed between grazing situations, colonization intensity was higher in the grazed areas than in the exclosures.…”

“…Grassland ecosystem researchers agree on the need for more knowledge of mycorrhizal colonization patterns under field conditions (Bethlenfalvay et al 1985;Fitter 1985;Wallace 1987;Read 1991;Eom et al 2001). This is a consequence of numerous controversies between many glasshouse experiments and a few field -based experiments and observations (Bethlenfalvay 1992).…”

“…At the individual plant level, a variety of responses have been documented: Under intense defoliation regimes, colonization intensity decreased (Bethlenfalvay and Dakessian 1984;Bethlenfalvay et al 1985;Hetrick et al 1990), whereas, under milder regimes, it was either unaffected or increased (Wallace 1981(Wallace , 1987Wallace and Svejcar 1987;Eom et al 2001). At the community level, there are no studies directly assessing grazing effect on colonization.…”

Mycorrhizas in Grasslands: Interactions of Ungulates, Fungi and Drought

“…Grazing did not influence levels of colonization. Bethlenfalvay et al (1985) also noted no effect on colonization frequency until grazing was severe. However, plants outside exclosures growing in non-fumigated soil had the lowest colonization levels.…”

Arbuscular mycorrhizal fungi in degraded typical steppe of inner Mongolia

“…Some recent studies have reported similar results (Busso et al, 2001;Grigera and Oesterheld, 2004). Effects of grazing on mycorrhizal colonization are regarded as species dependent (Wallace, 1981;Bethlenfalvay et al, 1985). Trends of increasing abundance of AMF vesicles within the roots of the indicator plants in the severely degraded steppes were also found (Figure 1).…”