Fixation and transfer of nitrogen (N) from alfalfa (Medicago sativa L.) to different grass species including timothy (Phleum pratense L.) and bromegrass (Bromus inermis Leyss) were studied under field conditions, using the¹⁵N dilution technique.

The percentage of alfalfa N derived from fixation (%NF) increased throughout the growing seasons and ranged from 62 to 83%. Nitrogen transfer (NT) from alfalfa to associated grasses was evident and contributed 26,46 and 38% of the total annual N yield of associated grasses or represented absolute amounts of 5, 20 and 19 kg N ha⁻¹ during the first, second and third year, respectively. The gradual and consistent percentage of NT that occurred before first harvest indicated that this transfer is a result of a direct excretion of N compounds from alfalfa root systems. Decomposition of root and nodule debris seems to contribute to the NT from alfalfa to associated grasses in the later cuts. All grass species benefitted similarly from alfalfa, although earlier maturing species with greater competitive ability were slightly more responsive.