ALLOCATION AND CYCLING OF NITROGEN IN AN ALFALFA-BROMEGRASS SWARD

The N requirements of grasses grown in legume-grass mixtures can be me t in part via transfer of symbiotically fixed N from the legume to the nonlegume. The objectives of this study were to determine above- and belowground biomass production and N accumulation of alfalfa (Medicago sativa L.) and meadow bromegrass (Bromus riparius Rhem.) in a mixed s tand and to investigate N cycling between these species. Available soi l inorganic N was monitored and biomass production and N accumulation were determined over 3 yr Symbiotic Nt fixation and N transfer were es timated using N-15-enriched isotope dilution, natural N-15 abundance, and N-difference methods. Both species depleted available soil N to co mparable levels (less than 5 mg kg(-1) soil). Following the establishm ent year, the majority of alfalfa N was allocated to aboveground plant production, whereas N-stressed meadow bromegrass favored belowground allocation. Estimates of the proportion of N derived from fixation in alfalfa using N-15-enriched isotope dilution and N-difference methods ranged from 74 to 89%, and 76 to 93%, respectively, after the establis hment year. Use of the natural N-15 abundance method resulted in consi stently lower estimates of N-2 fixation as compared with the N-15-enri ched isotope dilution and N-difference methods. The average potential annual net N input was 86 kg N ha(-1) in the mixed sward and 168 kg N ha(-1) in monocropped alfalfa. Maximum net transfer of N from alfalfa to meadow bromegrass, estimated using N-15-enriched isotope dilution, was 55 kg ha(-1) Evidence suggests that N transfer largely occurred in directly via net N mineralization of belowground plant components.