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Perennial forage legumes are potentially important sources of N in cropping systems, but little information is available on the amounts of N 2 fixed during long‐term stands. We conducted field experiments with the 15 N isotope dilution method over a stand life of 4 years to determine die inter‐ and intraannual fixation of N 2 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 N 2 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 N 2 fixation increased from an average of 126 to 198 kg N/ha because of the greater dependence of older stands on N 2 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.
Heichel et al. (Sun,) studied this question.