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

Int. Atomic Energy Agency (IAEA) Agric. Lab. in Seibersdorf, Wagramerstrasse 5, A-1400 Vienna, Austria

S. K. A. Danso^*

Joint Food Agric. Organization of the United Nations (FAO)/IAEA Division, Soil Fertility, Irrigation, and Crop Production Section, A-1400 Vienna, Austria

^* Corresponding author.

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 N~ 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 % ¹⁵SN 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.

Key Words: Competative ability • Fertilizer utilization effeciency • Isotope dilution • Lolium perenne L. • Medicago sativa L. • Nitrogen transfer

Received for publication February 18, 1987.

This article has been cited by other articles:

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