Dinitrogen Fixation in Alfalfa‐Orchardgrass Pastures1

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Abstract
Information on amounts and proportions of alfalfa N derived from symbiotic N2 fixation is needed to more effectively exploit this source of N in alfalfa‐orchardgrass (Medicago sativa L.‐Dactylis glomerata L.) pastures. Our objectives were to describe the seasonal trends in N2 fixation and to assess the relationships among alfalfa dry matter (DM) yield, percentage alfalfa in the mixture, and N2 fixation. The trial was carried out in south‐central Iowa in 1979 and 1980 on Aquic Argiudoll and Typic Argiaquoll soils, 2 to 13% slope. A 15N‐dilution technique was used to estimate the percentage of alfalfa N derived from N2 fixation during individual harvest intervals. The percentage of alfalfa N derived from N2 fixation varied slightly among harvest dates and averaged 91%. As percentage alfalfa in the mixture and alfalfa DM yield increased, the percentage of alfalfa N derived from fixation tended to decrease. The amount of N2 fixed was highly positively correlated with percentage alfalfa and alfalfa DM yield in both years. The total annual amount of N2 fixed averaged 70 kg ha−1 with a range across microplots of 15 to 130 kg ha−1. In conclusion, alfalfa DM yield was the major determinant of the amount of N2 fixed. The coefficient 0.90 can be used under these conditions to estimate the proportion of harvestable alfalfa N derived from N2 fixation when growing with orchardgrass. The low variability associated with measurements of percentage of alfalfa N derived from fixation illustrated the usefulness of the 15N‐isotope dilution method in this study.

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