Dinitrogen fixation in alpine tundra, Niwot Ridge, Front Range, Colorado, USA.

The highest overall acetylene reduction rates are found in the wet Caltha meadow and moist Salix shrub tundra sites (5.8-7.8 mu mol ethylene produced in m^-2 h^-1); lowest activities are found in the soils of the xeric sites, Kobresia meadow and fellfield tundras (0.13-0.33 mu mol ethylene m^-2 h^-1). Soil moisture is the primary environmental factor influencing in situ rates of acetylene reduction. Acetylene reduction activity was associated with the vascular plants Dryas octopetala ssp. hookeriana and Trifolium dasyphyllum (0.13-5.4 mu mol ethylene m^-2 h^-1), and the lichens Peltigera aphthosa and Stereocaulon alpinum (0.05-0.42 mu mol ethylene g^-1 h^-1). Low activity associated with the moss Pohlia is attributed to epiphytic cyanobacteria. The contribution of N by dinitrogen fixation, estimated to be 5 mg N m^-2 annually, does not apparently constitute the major source of N input to this tundra system. - from Authors