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Nitrogen Transfer from Forage Legumes to Grass in a Systematic Planting Design¹

Alfalfa (Medicago sativa L.) is the most important forage legume in the USA, but N transfer from this or other perennial forage species to companion grasses growing in a mixed sward is poorly understood. Furthermore, interplant distances and legume/grass ratios have seldom been controlled in studies of N transfer from legumes to non-legumes. The objectives of this field study were: (i) to determine the amount of N transferred from alfalfa and birdsfoot trefoil (Lotus corniculatus L.) to reed canarygrass (Phalaris arundinacea L.); (ii) to define conditions of distance and species ratio under which N transfer occurs; (iii) to compare the two legumes for N transfer; and (iv) to determine the effect of grass proximity on legume N₂ fixation. Legumes were interplanted with grass in single-species rows within 1-m² plots on a Typic Hapludoll soil labeled with ¹⁵N. Analysis of herbage showed significantly lower ¹⁵N concentration in grass near legumes than in grass grown alone, and significantly lower ¹⁵N concentration in legumes near grass than legumes in monoculture. Calculations using isotope dilution methods showed that grass derived a maximum of 68% of its N from alfalfa and 79% from trefoil. This N represented 13% of the N₂ fixed by trefoil and 17% of that fixed by alfalfa. The results indicated that N transfer occurred over a distance of 20 cm with maximum N transfer in areas of high legume/grass ratio. At third harvest, N derived from symbiosis was significantly higher (95% in alfalfa, 92% in trefoil) for legumes grown in mixture with grass than for legumes grown in monoculture (86% in alfalfa, 80% in trefoil). These results indicate that significant N transfer occurred, and that the amount of N transferred was dependent on interspecies distance and legume/grass ratio.

Key Words: Dinitrogen fixation • Nitrogen-15 • Isotope dilution • Nitrogen cycling • Medicago sativa L. • Lotus corniculatus L. • Phalaris arundinacea L.

² Graduate fellow and research assistant, Dep. of Agronomy and Plant Genetics (present address: Dep. of Botany and Plant Pathology, Oregon State Univ., Corvallis, OR 97331-2902); plant physiologist, USDA-ARS, Dep. of Agronomy and Plant Genetics; and soil scientist, USDA-ARS, U.S. Dairy Forage Res. Ctr., Dep. of Soil Science, Univ. of Minnesota, 1991 Upper Buford Circle, St. Paul, MN 55108.

Received for publication July 14, 1986.

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