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Dinitrogen Fixation Estimates in Alfalfa-Ryegrass Swards Using Different Nitrogen-15 Labeling Methods

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

G. Hardarson and F. Zapata

IAEA Agric. Lab., Seibersdorf, Wagramerstrasse 5, A-1400 Vienna, Austria

^* Corresponding author.

The amount of ¹⁵N-labeled fertilizer and the method by which it is added to soil may influence the estimates of N₂ fixed (Ndfa) in perennial forage stands. A 2-yr field experiment was conducted to compare the estimates of N₂ fixed in an alfalfa (Medicago sativa L.)-ryegrass (Lolium perenne L.) sward, using two N rates, different methods of introducing JSN into soil, and ¹⁵N uptake from previous applications (residual ¹⁵S). During the 2 yr, alfalfa derived about 210 kg N/ha or 88% of its N from atmospheric N₂. Pure as well as mixed sward ryegrass reference crops estimated similar % Ndfa in alfalfa. All of the three approaches by which 5 kg N/ha of lSNlabeled fertilizer was applied to soil resulted in similar % Ndfa at each harvest, in contrast to the application of 45 kg N/ha, which frequently resulted in significantly lower estimates of % Ndfa. Labeling soil with 5 kg N/ha before each regrowth consistently gave higher % Ndfa than when ₁₅N uptake was from residual ₁₅N in soil, but these reached significant levels only at one or both of the secondyear harvests (depending on the reference crop). Even then, these differences were of little agronomic significance because total N₂ fixed was similar for all methods. The consecutive addition of small amounts of 15N at frequent intervals therefore appears to he satisfactory for N₂ fixation measurements in perennial pastures.

Key Words: Forage legume • ¹⁵N Isotope dilution • Lolium perrenne L. • Medicago sativa L. • Mixed stand • Pastures • Residual ¹⁵N • Soil ¹⁵N/¹⁴N pools

Received for publication February 19, 1987.

This article has been cited by other articles:

				V. A. Haby, S. A. Stout, F. M. Hons, and A. T. Leonard Nitrogen Fixation and Transfer in a Mixed Stand of Alfalfa and Bermudagrass Agron. J., June 5, 2006; 98(4): 890 - 898. [Abstract] [Full Text] [PDF]