2002-07-08_GENERAL DOCUMENTS - M2002004 (3)

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June 1993., (Schuman et al. 1998). Root segments examined from the study showed an infection rate of 66 -76 %. They believe that the time between topsoil placement (late summer 1990) and June 1993 was more than adequate for reinoculation of the stockpiled topsoil. Loree and Williams (1984) found that native grasses became infected with AM within a year of establishment on long- term stockpiled topsoil indicating inoculum is spread quite readily under natural conditions. However, this finding should not diminish the importance of topsoil management for AM concerns. Stahl et al. (1998), in a greenhouse study, found that the sagebrush seedling age groups of 30 to 150 -days old that were mycorrhizal were able to tolerate greater drought stress (moisture tension) before dying than non - mycorrhizal seedlings. Non - mycorrhizal, 45- day -old sagebrush seedlings died when the moisture stress level was -2.8 MPa compared to the mycorrhizal seedlings which tolerated soil moisture tensions of -3.2 MPa before dying (Figure 1). Sagebrush seedling age and mycorrhizae treatment interacted, such that as sagebrush seedlings aged the beneficial influence of AM on soil moisture stress tolerance increased (Figure 2). Those seedlings >120 days of age that were non - mycorrhizal were much less tolerant of soil moisture stress than younger non- mycorrhizal seedlings (Figure 1) indicating that sagebrush seedlings become more dependent upon the benefits of mycorrhizae as they age. This phenomena could partially explain the lack of infection differences observed by Schuman et al. (1998) in seedlings grown on direct - placed vs stockpiled topsoil. Those seedlings growing in stockpiled topsoil failing to form AM early in their development may have not tolerated repeated drying cycles experienced in a typical spring - summer period in a semiarid climate. Hence, seedlings sampled a year later may not have been representative of the seedling population that originally germinated and emerged because non- mycorrhizal seedlings may have died early in their development. The presence of mulch also greatly affected sagebrush seedling establishment in 1992 (Schuman and Booth 1998, Schuman et al. 1998). No seedlings were evident in the first year where mulch was not applied (Table 1). Both stubble and surface mulch treatments had similar or greater seedling establishment than the stubble + surface mulch treatment. Soil moisture content of the surface 7.5 cm was greater under all mulch treatments compared to the no -mulch treatment. Schuman et al. (1980) demonstrated that stubble mulch enhanced grass seedling establishment through reduced diurnal temperature fluctuations and increased soil moisture. Grass competition reduced sagebrush seedling densities throughout the duration of the study on direct- placed topsoil treatment where stubble or straw mulch was used (Schuman and Booth 1998, Schuman et al. 1998). They reported grass seedling densities of 0, 196, and 250 grass seedlings m" for the 0, 16, and 32 kg PLS ha' grass seeding rates, respectively. No differences in grass seedling density among topsoil management treatments were observed. They concluded that successful establishment of big sagebrush on mined lands might require seeding in the absence of any grass or perhaps at very low grass seeding rates. These findings have led to further research by Fortier et al. (2000) evaluating effects of grass competition and big sagebrush seeding rates on sagebrush seedling establishment reported at this conference.