Dinitrogen Fixation and Transfer in Legume-Crested Wheatgrass Mixtures

Crested wheatgrasses [Agropyron cristatum L. Gaertn. and A. desertorum Fisch. ex (Link) Schult.] have been extensively seeded on semiarid western rangelands, but without supplemental N many of these seedings decline in vigor, ground cover, and productivity as the stand ages. Biological N2 fixation by legumes may represent a viable alternative to fertilizer N for increasing stand productivity. Two growth-room studies were designed to investigate N2 fixation and N transfer in mixtures of alfalfa (Medicago sativa L.) or annual sweetclover (Melilotus alba Medik. var. annua Coe) and crested wheatgrass. Growth media were enriched with 15N-labeled KNO3 at a rate of 24.2 kg N ha-1 and used to grow mixtures and monocultures of alfalfa or sweetclover and crested wheatgrass. Fixed and transferred N were determined at 3 harvest dates from differences in isotopic composition between the legume species in mixture with crested wheatgrass and crested wheatgrass in monoculture. The percentage of legume N derived from fixation was >80% for the final 2 harvests and increased as the proportion of crested wheatgrass in the mixtures increased. Nitrogen transfer from alfalfa to crested wheatgrass accounted for < 5 % of the grass total N. Conversely, nitrogen transfer from sweetclover to crested wheatgrass accounted for about 20% of the grass total N. Nitrogen transfer from alfalfa or sweetclover to crested wheatgrass may be important in maintaining stand productivity on N-limited western rangelands.