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Agronomy Journal Abstract -

Allocation and Cycling of Nitrogen in an Alfalfa-Bromegrass Sward


This article in AJ

  1. Vol. 88 No. 5, p. 834-843
    Received: June 27, 1995

    * Corresponding author(s): walley@sask.usask.ca
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  1. Frances L. Walley ,
  2. Gilberto O. Tomm,
  3. Alejandro Matus,
  4. Alfred E. Slinkard and
  5. Chris van Kessel
  1. D ep. of Soil Science, 51 Campus Dr., Saskatoon, SK S7N 5A8, Canada
    F ellow of the Brazilian Natl. Science Council (CNPq), EMBRAPA, P.O. Box 569, 99001-970 Passo Fundo, RS, Brazil
    D ep. of Crop Science and Plant Ecology, Univ. of Saskatchewan, 51 Campus Dr., Saskatoon, SK S7N 5A8, Canada



The N requirements of grasses grown in legume-grass mixtures can be met in part via transfer of symbiotically fixed N from the legume to the nonlegume. The objectives of this study were to determine above- and belowground biomass production and N accumulation of alfalfa (Medicago sativa L.) and meadow bromegrass (Bromus riparius Rhem.) in a mixed stand and to investigate N cycling between these species. Available soil inorganic N was monitored and biomass production and N accumulation were determined over 3 yr. Symbiotic N2 fixation and N transfer were estimated using 15N-enriched isotope dilution, natural 15N abundance, and N-difference methods. Both species depleted available soil N to comparable levels (less than 5 mg kg−1 soil). Following the establishment year, the majority of alfalfa N was allocated to aboveground plant production, whereas N-stressed meadow bromegrass favored belowground allocation. Estimates of the proportion of N derived from fixation in alfalfa using 15N-enriched isotope dilution and N-difference methods ranged from 74 to 89%, and 76 to 93%, respectively, after the establishment year. Use of the natural 15N abundance method resulted in consistently lower estimates of N2 fixation as compared with the 15N-enriched isotope dilution and N-difference methods. The average potential annual net N input was 86 kg N ha−1 in the mixed sward and 168 kg N ha−1 in monocropped alfalfa. Maximumn et transfer of N from alfalfa to meadow bromegrass, estimated using 15N-enriched isotope dilution, was 55 kg ha−1. Evidence suggests that N transfer largely occurred indirectly via net N mineralization of belowground plant components.

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