Dinitrogen Fixation Measurements in Alfalfa‐Ryegrass Swards Using Nitrogen‐15 and Influence of the Reference Crop

Abstract

The accuracy of 15N estimates of N2 fixation may be influenced by the choice of the reference crop, especially in mixed legume‐grass swards, where transfer of fixed N2 from the legume to the reference crop may occur. To investigate this, a 2‐yr field experiment was conducted in a Typic Eutrocrepts soil in Austria to measure N2 fixed in alfalfa (Medicago sativa L.) grown alone or in two seeding ratios with ryegrass (Lolium perenne L.), using pure or mixed sward ryegrass as the reference crop. Alfalfa, on the average, derived over 80% of its N from fixation over the 2‐yr period, equivalent to about 415 kg N/ha. Compared to N2 fixation in pure alfalfa sward, the % N derived from atmospheric N2 (% Ndfa) in alfalfa increased significantly in the mixed sward and even more by increasing the seeding ratio of ryegrass in the mixture. Except for the first harvest, the atom % 15N excess in mixed ryegrass was slightly lower than in the pure grass. These differences in 15SN enrichment were insignificant, suggesting that the possible release of N from alfalfa and subsequent uptake by ryegrass may have been small. The highest proportion of ryegrass N that could have been derived from the legume was estimated to be 16% in the second harvest, equivalent to only 4 kg N/ ha and <10 kg N/ha over the 2 yr. These differences in 15SN enrichments of pure and mixed ryegrass reference plants did not significantly affect the estimates of % Ndfa in alfalfa.