Competition between native perennial grasses and Bromus tectorum has been studied for many years. Recently, soil nutrients have been immobilized in an effort to preferentially inhibit growth of B. tectorum relative to native species. Most of this work has focused on soil N, but interactions with soil P are less studied and may be important as well. Additionally, although competitive effects and nutrient responses of several Centaurea spp. are well documented, data are lacking on the competitive effects that Centaurea virgata exerts on the establishment of native and exotic communities. A field experiment and a greenhouse experiment were conducted to evaluate influence of N and P on the native perennial bunchgrass Pseudoroegneria spicata in competition with B. tectorum and C. virgata. Chapter 2 describes the effects of nutrient immobilization and subsequent 2 x 2 factorial additions of N and P. Treatments were applied to transplanted individuals of the study species in monocultures and mixtures. Analysis of variance (ANOVA) was performed on shoot dry mass, soil water content, and physiological responses including photosynthetic rate, stomatal conductance, and transpiration. Results suggest that B. tectorum may be P-limited in certain circumstances, and that physiological activity of all species is subject to interactions between available N and P. In Chapter 3, I assess response of the study species to 2 x 2 factorial additions of N and P on a very low-nutrient soil in a greenhouse. Relative effects of different species mixtures on P. spicata are reported. I report ANOVA results of tiller number, leaf number, specific leaf area, shoot dry mass, root dry mass, and water addition. Results indicate that N and P co-limit P. spicata and B. tectorum, and to a lesser extent, C. virgata. B. tectorum showed the highest percentage response to fertilization. Land managers may be more successful establishing native grasses if they can minimize soil fertility.