Exploring the Boundaries of N2-Fixation in Cereals and Grasses: A Hypothetical and Experimental Framework

K.E. Giller, R. Merckx

Research output: Contribution to journalArticleAcademicpeer-review

28 Citations (Scopus)


Despite more than 40 years of research on free-living and endophytic bacteria associated with cereals and grasses, conclusive examples of impacts of non-symbiotic N2-fixation in agriculture are lacking. All available methods for measurement of N2-fixation associated with cereals and grasses have been employed, and N2-fixation has been demonstrated to occur under controlled conditions, but this is insufficient evidence to prove a significant role for N2-fixation by heterotrophic bacteria in the field. Recently attention has focused on endophytic N2-fixation with claims of major inputs of N from the atmosphere, particularly in sugar cane for which the largest body of information exists, although some methodological concerns remain unanswered. Here we analyse the evidence for N2-fixation in the rhizosphere and within graminaceous plants. We provide guidelines suggesting the type of information required to allow a critical analysis of the amounts of non-symbiotic N2-fixation contributed in agriculture. Given the substantial availability of C as a substrate for N2-fixation in sugar cane it remains the most likely candidate plant to benefit from non-symbiotic N2-fixation. Measurements of elevated respiration in zones of high bacterial colonization may give an indication of whether these really are focal points of the rapid metabolic activity required to support significant N2-fixation activity. The only conclusive evidence for a major role of non-symbiotic N2-fixation in the field will come from long-term N balance studies in which ALL processes of potential gain and loss are measured. We conclude that evidence for a large contribution from heterotrophic, non-symbiotic N2-fixation in both natural and agricultural systems is weak. Although amounts of non-symbiotic N2-fixation in natural systems may be small (
Original languageEnglish
Pages (from-to)3-17
Issue number1-3
Publication statusPublished - 2003


  • biological nitrogen-fixation
  • acetylene-reduction assay
  • azospirillum-brasilense
  • azotobacter-paspali
  • spirillum-lipoferum
  • dinitrogen fixation
  • fixing bacterium
  • isotope-dilution
  • fixed nitrogen
  • n-15 dilution

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