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N₂ Fixation, and N and Dry Matter Partitioning During a 4-Year Alfalfa Stand¹

Perennial forage legumes are potentially important sources of N in cropping systems, but little information is available on the amounts of N₂ fixed during long-term stands. We conducted field experiments with the ¹⁵N isotope dilution method over a stand life of 4 years to determine die inter- and intraannual fixation of N₂ and the partitioning of dry matter among plant components for two alfalfa (Medicago sativa L.) cultivars, Saranac and Agate, managed for hay production. Dinitrogen fixation varied with date of harvest within a year and with age of stand. Alfalfa fixed 160 to 177 kg N/ha during the seeding year, and up to 224 kg N/ha during the final year of a 4-year stand. Depending upon stage of growth and year of stand, fixed N₂ comprised 33 to nearly 80% of the total N in the crop. Herbage yields declined with successive harvests within each year of the stand. The highest seasonal total herbage yields (11 to 12 Mg oven dry matter/ha) were obtained in Year 2. Despite the progressive decline in annual yields from Years 2 through 4, the total annual N₂ fixation increased from an average of 126 to 198 kg N/ha because of the greater dependence of older stands on N₂ instead of soil N. For example, seeding year stands averaged 58% N from symbiosis, while those in Year 4 averaged 77%. The total phytomass in roots plus crown of Saranac at the end of the seeding year, and Years 2, 3, and 4 was about 2.0, 2.0, 2.8, and 1.8 Mg oven dry matter/ha, respectively. The N concentration of roots plus crown averaged only 0.55 that of herbage. The relatively low partitioning of dry matter to roots plus crowns and their low N concentration in comparison to herbage indicated that fall plowdown of a large amount of herbage in addition to incorporation of roots plus crowns is necessary for greatest return of symbiotically fixed, as well as total, N to the soil.

Key Words: Nitrogen cycling • N concentration • Root plus crown growth • ¹⁵N isotope dilution method • Perennial legume • Medicago sativa L.

² Plant physiologist, research geneticist, and plant physiologist respectively, USDA-ARS, and associate scientist in the Dep. of Agronomy and Plant Genetics; Univ. of Minnesota, 1509 Gortner Ave., St. Paul, MN 55108.

Received for publication October 20, 1983.