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.