Plant and Soil

, Volume 175, Issue 1, pp 57–66 | Cite as

Estimates of seasonal nitrogen fixation of annual subterranean clover-based pastures using the15N natural abundance technique

  • T. P. Bolger
  • J. S. Pate
  • M. J. Unkovich
  • N. C. Turner
Research Article


Annual pasture legumes play a key role in ley farming systems of southern Australia, providing biologically fixed nitrogen (N) to drive the production of the pastures as well as subsequent crops grown in rotation. Seasonal inputs of biologically fixed N in shoot biomass of the subterranean clover (Trifolium subterraneum) component of grazed annual pastures were assessed using the15N natural abundance technique and appropriately timed sampling of herbage dry matter (DM) for N accumulation. At three study sites spanning a gradient across the Western Australian wheatbelt from 300 to 600 mm annual rainfall the performance of the clover and non-legume herbs and grasses was examined as paired comparisons involving two management treatments expected to give contrasting effects on pasture productivity, botanical composition and N2 fixation. The proportion of clover N derived from atmospheric N2 fixation (%Ndfa) ranged from 65 to 95% across sites, treatments and sampling times. Amounts of fixed N accumulated in clover shoot biomass ranged from 50 to 125 kg ha−1, and paralleled trends in clover production. Substantial increases in pasture production in high yielding treatments generally occurred without decrease in %Ndfa, suggesting that N2 fixation was essentially non-limiting to performance of the clover component. Seasonal profiles for accumulation of fixed N were skewed towards the late winter and spring period, particularly in low plant density pastures following a cereal crop. There were seasonal, site and treatment-specific effects on the proportion of clover and non-legume pasture components and consequently clover yield and N2 fixation were variably affected by competition from non-legume species.

Key words

competition crop rotation farming systems ley farming soil nitrogen Trifolium subterraneum 


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Copyright information

© Kluwer Academic Publishers 1995

Authors and Affiliations

  • T. P. Bolger
    • 1
  • J. S. Pate
    • 2
  • M. J. Unkovich
    • 2
  • N. C. Turner
    • 1
  1. 1.CSIRO Division of Plant IndustryWembleyAustralia
  2. 2.Botany Department and Centre for Legumes in Mediterranean AgricultureUniversity of Western AustraliaNedlandsAustralia

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