Symbiotic interactions among various organisms are often necessary for one or both individual's survival. These symbiotic relationships must be considered in restoration projects to allow for the successful establishment of the species. Rhizobia are nitrogen-fixing bacteria found in symbiotic relations with legumes. By utilizing this relationship, restoration practitioners can establish native legume species more successfully while repopulating soil microorganisms into degraded soils. Despite the potential benefits a rhizobium inoculant can have on restoration efforts, minimal research has been done to understand the impacts this treatment has on specific species and the systems they are employed within. Our research goal was to assess the efficacy of applying a commercial rhizobium product (EXCEED ®) and indigenous rhizobium strains on two lupine species (Lupinus argenteus Pursh and Lupinus sericeus Pursh), commonly used for rangeland seedings in the Great Basin region of the western United States. We conducted laboratory and field trials to meet this research goal, with the results of the laboratory experiments shared in chapter 1 and findings from the field reported in chapter 2. In chapter 1, we evaluated in the laboratory whether indigenous rhizobia strains could be isolated, cultured, and applied as a liquid inoculant or a seed coating to induce root nodulation and increase plant growth. The performance of these inoculums was compared against the commercial rhizobium product. Additionally, we tested in a trial if compost could be applied within the seed coating to improve the efficacy of the rhizobium treatment. Our research demonstrated that the commercial inoculum induced root nodulation, and in one of three trials, this treatment improved plant growth. We also found indigenous strains effectively formed nodules on the plant roots when applied through a liquid culture or a seed coating. However, the number of root nodules and the presence of a pink color (indicating nitrogen fixation) were typically higher in the commercial product than in the indigenous strains when applied through a seed coating. These short-term laboratory studies generally provided minimal evidence that rhizobia impacted plant growth. However, data indicated that having compost in the coating alone improved shoot biomass by 33% (P = 0.025). In chapter 2, research assessed the performance of the same rhizobia inoculums tested in the laboratory trials on a mine in northern Utah at two waste-rock sites, one comprised of crushed waste rock and the other made of waste rock amended with topsoil. One year after seeding, we had high plant recruitment at both study sites, and there were more plants, which were more vigorous, in the amended site (P<0.001). These results demonstrate that reclamation efforts on mineland overburden can be improved when topsoil is incorporated into the growing medium. At this stage in the study, there was no difference in plant establishment and vigor between any seed treatments, but future research is planned to assess these metrics in the next growing season. The lack of improvement in plant growth from a rhizobia treatment in some of our laboratory and field trials may be due to the short period of these studies. Nodules that form on mature root systems provide more nitrogen-fixing benefits than those formed on immature roots. Hence, future research should consider conducting trials for more extended periods to understand how the treatments influence the growth of mature plants. Because we found in the laboratory that the rhizobia inoculums were successful in nodulating the test species, we anticipate that future studies will find that these treatments can improve plant performance and subsequently restoration success.

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