No abstract

The effect of grazing on the colonization of range plants by veskular-arbuscular mycorrhizal (VAM) fungi was investigated within an cxclosure and on degraded Wyoming big sagebrush (Artemisia tridentatu ssp. wyominge&s) rangelands at Medell Flat, near Reno, Nev. Implications of the interaction between mycorrhiue and grazing, relevant to the ecology and management of rangelands, are discussed. Density of forage grasses and their colonization by VAM fungi was significantly reduced as a result of grazing, in some cases by more than 50%. No differences in colonization were found in forage or nonforage broadleaf plants. A significant shift in the floristic composition and density of range plants occurred as a result of the presence or absence of grazing pressure. The decrease in VAM-funpl colonization of grasses under grazing is ascribed to a decrease in leaf areas and an increase in root to shoot ratios-conditions which result ln decreased source capacity and increased sink demand. The interactionsbetween vesicular-arbuscular mycorrhizal (VAM) fungi, their host plants, and the soil are complex (Masse 1973) and little known in semiarid rangelands (Trappe 1981). Plants benefit from their VAM-fungal endophyte when available P in the soil is limiting (Mosse 1973) and within a certain concentra-The authors are presently research plant physiologist and soil scientist, U.S.

Perennial plants of 19 families were surveyed for colonization by vesicular–arbuscular mycorrhizal (VAM) fungi at four sites in the Anza–Borrego Desert State Park, California, an area characterized as arid to extremely arid. Soils at all sites were very low in phosphorus and nitrogen and had a coarse sandy texture. The sites were distinct in the floristic composition of their vegetation. All plants (38 species) were colonized by VAM fungi (six species). The distribution of the VAM mycoflora was not random. Site preference by VAM-fungal species was ascribed to an interaction of factors pertaining to the host plants and to edaphic and climatic conditions.

Plant roots have been shown to increase soil shear resistance through direct mechanical reinforcement and thereby enhance soil stability on slopes. Because of their potential large diameter and length, the roots of trees may be especially significant in such soil reinforcement. To provide large rooting volumes simulating natural conditions and shear cross sections many times larger than potential tree root diameters, we constructed 12 1.22‐ by 1.22‐m cylindrical soil containers in which two artificial soil profiles were prepared. Replicates of each profile type were planted to alfalfa, yellow pine, or were kept clear of plants. A large pneumatic direct shear device sheared both root‐free and root‐permeated materials along a horizontal plane at the 0.6‐m depth at either constantly maintained shear stress levels (creep shear) or at constant shear displacement rates.Creep shear at the 0.6‐m depth showed that roots of 14‐month‐old alfalfa increased the shear resistance of homogeneous clay loam and a clay loam/gravel interface simulating a soil‐weathered rock boundary by 32 and 50%, respectively. Constant shear displacement rate experiments at the same depth on similar samples permeated by roots of yellow pine planted 54 months earlier showed shear resistance increasing steadily with displacement over the entire test displacement range. At 75 mm displacement, the shear resistance of the pine‐rooted soil was about two times that of the nonrooted in both profile types. The superiority of pine roots to alfalfa roots in increasing soil shear strength in these experiments is in accord with field observations that woody plants are more effective than herbaceous plants in stabilizing soil against slips and slides. Pine root‐size distributions determined 11 months after the last shear test were combined with root tensile strengths and Young's moduli previously measured and used in model simulations of root reinforcement with good results.

Plants enhance soil stability against downslope mass movement through the removal of soil water by transpiration and by the mechanical reinforcement of their roots. To assess the magnitude of this reinforcement, direct shear measurements were made on 0.25‐m diam cylindrical soil columns packed both homogeneously and in layers which simulated water and/or root‐impeding horizons. In all cases the matric potential was adjusted to zero before shearing. Twelve plant species were used including seven grasses: Phalaris tuberosa, Lolium rigidium, Dactylis glomerata, Bromus mollis, Sorgum bicolor sudanense, Triticum oestivum, Hordeum vulgare. Two legumes were used: Vicia dascarpa, Medicago sativa. and two trees were used: Pinus ponderosa and Quercus agrifolia.The ratio of the shear resistance at 25‐mm displacement of the rooted and unrooted specimens was used as a measure of root reinforcement. Roots of several grasses planted in early fall and sheared the following spring gave about a threefold increase in shear resistance at the 0.3‐m depth in homogeneous saturated clay loam. In the same material, roots of oak produced a similar increase only after 3 years' growth. One‐year‐old alfalfa produced a fourfold increase. At the 0.45‐m depth at the interface between soil and a dense gravel‐sand mixture simulating weathered rock, yellow pine gave a 1.5‐fold increase after 16 months and a 2.5‐fold increase after 52 months. Hardinggrass was almost equally effective after only 7 months.In almost all cases where roots increased soil shear resistance, the resistance continued to increase beyond 25‐mm displacement so that the selection of 25‐mm displacement was conservative, i.e., it may underestimate the root reinforcing effect.Factor of safety calculations for shallow planar slides using measured shear strengths show that plant roots can make large increases in slope stability.

Soybean plants were grown in pots with or without vesicular-arbuscular mycorrhizal (VAM) fungi in three soils of low plant-available P content, different texture and different water-holding capacities. Mineral nutrients, except P, were provided in a complete nutrient solution. The biomass of non-VAM plants was positively and fungal colonization negatively correlated with increasingly coarse soil texture. There was no correlation of soil P with host or endophyte growth. Plant growth enhancement was positively correlated with soil water content at --1.5 MPa. These observations suggest soil water status and the mycorrhizal condition interact in influencing plant 'growth.

Onion plants (Allium cepa L.) inoculated in the root zone with a vesicular‐arbuscular mycorrhizal (VAM) fungus, or left uninoculated, were grown in potted soil for 230 d to determine the influence of the VAM fungus (Glomus macrocarpum Tul. and Tul.) on soil structure. The silty clay loam soil was maintained at a moisture content between 25 and 30%. Paired inoculated (+M) and uninoculated (−M) plants were harvested (20 pairs over 150 d) beginning 80 d after planting. Relationships between plant, fungal, and soil parameters and changes with time were evaluated by regression analysis. Root colonization by the VAM fungus in +M plants ranged from 49% to 60% over the sampling period, while no mycorrhizae were detected in the ‐M plants. Total dry mass of VAM plants was five to six times that of the non‐VAM plants. Soil from the +M treatment was significantly better aggregated, more porous, and had greater water permeability than ‐M soil. Root dry mass and VAM hyphal density in the +M soil were both significantly correlated with the relative abundance of water‐stable soil macroaggregates. Correlation of root mass with aggregate abundance was stronger, however, suggesting that soil changes were mainly mediated by direct root effects of a host plant whose growth was stimulated dramatically by its VAM fungal endophyte.

A research project was undertaken to assess the relative effectiveness and durability of a wide variety of liner materials when exposed to hazardous wastes under conditions simulating various aspects of service in waste storage and disposal facilities. The materials studied included compacted soil, admixes, sprayed-on asphalt and 32 polymeric membranes. Four partially crystalline polymeric sheetings, though not compounded for use as liners, were included in the study because of their known chemical resistance and their use in applications requiring good chemical and ageing resistance.The lining materials were exposed in test cells to 10 hazardous wastes (two acidic, two alkaline, three oil, a blend of lead, a pesticide and a briny industrial waste) and three media of known composition—deionized water, 5% aqueous solution of salt and a saturated solution of low-concentration (0.1%) tributyl phosphate. The polymeric materials were also exposed to wastes or environmental conditions under a variety of conditions that included primary one-sided exposure, immersion testing, two types of outdoor exposure and a pouch test. Some of the exposures were as long as 2700 days. New methods for testing polymeric materials are presented. Results indicated that some of the liner materials performed satisfactorily in contact with certain wastes but, because waste combinations can be highly specific, compatibility testing is needed to select a liner for a given waste.