Dinitrogen fixation and transfer of fixed nitrogen (N) in mixtures of 'Spredor 2' alfalfa (Medicago sativa L.) and 'Hycrest' crested wheatgrass (Agropyron desertorum Fisch. ex (Link) Schult. x A. cristatum (L.) Gaertn.) and in mixtures of 'Hubam' annual sweetclover (Melilotus alba var. annua Coe) and 'Hycrest' crested wheatgrass, with varying planting ratios, were evaluated using $\sp{15}$N isotope dilution methodology under both growth room and field conditions. The proportion of legume N derived from fixation was more than 53% and increased as the proportion of crested wheatgrass in the mixture increased. Maximum legume N derived from fixation occurred in grass-legume ratios of 75:25. Minimum legume N derived from fixation occurred in legume monocultures. Crested wheatgrass N derived from transfer from the legume ranged from 0 to 31%, depending on legume species, experiment location (growth room or field site), and grass-legume ratio. Maximum grass N derived from transfer occurred in grass-legume ratios of 25:75 and decreased as the proportion of legume in the mixture decreased. Crested wheatgrass N concentrations were higher when grown in mixture with legumes than when grown in monoculture. The proportions of legume N derived from fixation and grass N derived from transfer did not differ significantly between two field sites with different environmental conditions.

Sweetclover, however, experienced severe mortality in mixture with crested wheatgrass during the second year of the field study, and N derived from fixation could not be determined. Despite sweetclover failure in year two, evidence for transfer of fixed N to crested wheatgrass was observed, indicating that decomposition and release of N from sweetclover root and nodule tissue from the previous year contributed to this transfer. Isotope dilution calculations indicated that the transfer of N from both legume species occurred over a distance of at least 25 cm under field conditions. These results indicate that alfalfa and sweetclover are capable of fixing and subsequently transferring significant amounts of N to crested wheatgrass when growing in mixture on drought- and temperature-stressed Great Basin rangelands.