In Area 11-C, granivores (D. merriamt, D. miecrops, and P. ltongtmembris) did not differ significantly (P>.05) from insectivore-omnivores (A. leucurus, 0. torridus, and lizards) in mean Pu tissue burdens from either Activity Stratum. Mean Pu tissue burdens of both trophic categories, however, were significantly greater (P<.01) from the High Activity Stratum where Pu carcass burdens were approximately 2 x 107! nCi/g ash. Mean Pu/Am ratios of both pelt/skin and carcasses were more variable in insectivore-omnivores than in granivores from both strata. As shown in Table 3, however, ratios in GI tracts of both trophic categories were consistent with soil and vegetation ratios and exhibited relatively low variability. In contrast, mean ratios in carcasses of insectivore-omnivores were not only more variable, they were significantly higher (P<.01) than either means of the GI tract samples or soil and vegetation ratios. This phenomenon is difficult to explain in light of the consistent ratios found in GI tracts of rodents. In addition, Romney et az. (1975) present data which indicate relatively consistent values for Pu/Am in soil and vegetation samples and suggest consistent Pu/Am ratios in resuspendable contamination in Area 11-C. The high ratios present in carcass samples of insectivore-omnivores may indicate that these rodents are exposed to different environmental conditions than plants which have been sampled and occupy microhabitats which have not been sampled by current soil sampling design. In addition, insectivore-omnivores have larger home ranges than granivores (McNab, 1963; our studies) and, hence, may be exposed to a wider variety of microhabitats. A supplemental hypothesis may be that some food sources of insectivore-ominvores, such as arthropods and possibly small vertebrates, contain Pu and Am which is more or less soluble, hence, more or less available for uptake, than that of resuspended contaminants. It is well known that plants and animals play a significant role in development and dynamics of soil ecosystems. ing of rock and soil. Plants aid in chemical and mechanical weather- Plant roots help break up large soil aggregates and rocks and provide avenues of access of air and water to subsurface soil. Fungi and bacteria excrete metabolic acids which can dissolve rock aggregates, and possibly can solubilize essentially insoluble soil components, such as plutonium. When plants die, various soil invertebrates, fungi, and bacteria convert plant tissue to available food for microinvertebrates, and the resulting Whereas desert soils are relatively processes add nutrients to the soil. sterile, the available nutrients are not uniformly distributed. We believe that nutrient enrichment associated with burrow systems may be a major link in nutrient cycling of desert ecosystems. Animal activity, particularly burrowing activity, has been shown to be a principal force affecting soil chemistry and structure. Taylor (1935) has reported that ground squirrels move 30 to 40 tons of soil per acre yearly in arid midwest areas. Thorp (1949) estimated that burrow mounds consisted of 7-9 Kg of subsoil for each square meter of surface in semiarid portions of the United States. Burrowing activity has also been shown to be extensive as far as depth is concerned. For example, some ants at Rock Valley, NTS, have been found at depths of 10 meters. Kangaroo rats (Dtpodomys), the most abundant rodents of NAEG Study Areas, have been shown to significantly alter soil chemisty in their burrow systems. Green and Reynard (1932) reported that kangaroo rats defecated extensively throughout their burrow systems. They 203