Dinitrogen Fixation Estimates in Alfalfa-Ryegrass Swards Using Different Nitrogen-15 Labeling Methods
- S. K. A. Danso ,
- G. Hardarson and
- F. Zapata
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.
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