, Volume 46, Issue 1, pp 179–202

Ecosystem constraints to symbiotic nitrogen fixers: A simple model and its implications

  • Peter M. Vitousek
  • Christopher B. Field

DOI: 10.1007/BF01007579

Cite this article as:
Vitousek, P.M. & Field, C.B. Biogeochemistry (1999) 46: 179. doi:10.1007/BF01007579


The widespread occurrence ofN limitation to net primary production (NPP) and other ecosystem processes, despite the ubiquitous occurrence ofN-fixing symbioses, remains a significant puzzle in terrestrial ecology. We describe a simple simulation model for an ecosystem containing a generic nonfixer and a symbioticN fixer, based on: (1) a higher cost forN acquisition byN fixers than nonfixers; (2) growth of fixers and fixation ofN only when lowN availability limits the growth of nonfixers, and other resources are available; and (3) losses of fixedN from the system only when the quantity of availableN exceeds plant and microbial demands. Despite the disadvantages faced by theN fixer under these conditions,N fixation and loss adjustN availability close to the availability of other resources, and biomass and NPP in this simple model can be substantially but only transientlyN limited. We then modify the model by adding: (1) losses ofN in forms other than excess availableN (e.g., dissolved organicN, trace gases produced by nitrification); and (2) constraints to the growth and activity ofN fixers imposed by differential effects of shading,P limitation, and grazing. The combination of these processes is sufficient to describe an open system, with input from both precipitation andN fixation, that is nevertheless stronglyN-limited at equilibrium. This model is useful for exploring causes and consequences of constraints toN fixation, and hence ofN limitation, and we believe it will also be useful for evaluating howN fixation and limitation interact with elevatedCO2 and other components of global enviromental change.

Key words

carbon dioxide grazing nitrogen fixation nitrogen limitation phosphorus shade 

Copyright information

© Kluwer Academic Publishers 1999

Authors and Affiliations

  • Peter M. Vitousek
    • 1
  • Christopher B. Field
    • 2
  1. 1.Department of Biological SciencesStanford UniversityStanfordUSA
  2. 2.Department of Plant BiologyCarnegie Institution of WashingtonStanfordUSA

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