Nitrogen Fixation of Alfalfa in the Seeding Year1

Abstract

Alfalfa (Medicago sativa L.) obtains N from the soil and from the atmosphere by symbiotic nitrogen (N₂) fixation. It is necessary to identify the contribution from each N source to evaluate N₂‐fixation of different alfalfa germplasms. Field experiments were undertaken using ¹⁵N as a tracer of N metabolism to determine the N₂‐fixation of two alfalfa populations in the seeding year. Additional objectives were to investigate the effects of establishment method, stage of plant development, and non‐uniform distribution of isotope in the plant on assessment of N,‐fixation capability.During the seeding year the two populations averaged about 43% of their N needs from symbiosis and fixed an average of 148 kg/ha of N during the growing season. The isotope‐dilution and A‐value methods gave similar assessments of performance over four harvest intervals. Nitrogen fixation measured by isotope dilution was least (8 to 20 kg/ha) in the first and fourth harvest intervals when 25 to 30% of the N needs were met by symbiosis; it was greatest (47 to 87 kg/ha) during the second and third harvest intervals, when the populations derived more than 60% of their N from symbiosis.The proportion of N derived from symbiosis for whole plants of seeded and transplanted materials differed significantly (P2‐fixation per plant were observed between the two populations at several harvests and over the growing season. However, a 14% difference in average plant stand precluded differences between populations in N₂ fixation on a land area basis. The rates of N₂ fixation of field grown plants mirrored growth rates, which varied with the changing field environment and especially with onset of fall dormancy. The use of stable N isotopes can be a powerful technique in assessing progress in breeding alfalfa for increased N₂‐fixation capability.