Biogeochemistry 55: 179–194, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. Nitrogen fixation rates of Stereocaulon vulcani on young Hawaiian lava flows LIANNE M. KURINA 1,2,∗ & PETER M. VITOUSEK 2 1 Unit of Health-Care Epidemiology, Department of Public Health, University of Oxford, Old Road, Headington, OX3 7LF, U.K.; 2 Department of Biological Sciences, Stanford University, Stanford, CA 94305-5020, U.S.A. ( ∗ Author for correspondence, e-mail: lianne.kurina@uhce.ox.ac.uk) Key words: nonsymbiotic nitrogen fixation, primary succession, tropics Abstract. Previous research has suggested that nitrogen-fixing lichens can play an important role in the nitrogen cycle of early primary successional systems and other extreme environ- ments. In this study, we estimate rates of nitrogen fixation by a nitrogen-fixing lichen, Stereocaulon vulcani, at 1500 m on the northeast slope of Mauna Loa volcano. Using micro- climate measurements and a climate-driven model of nitrogen fixation, we estimate that S. vulcani fixes between 0.2 and 0.45 kg N ha -1 yr -1 . We calculate that S. vulcani could have derived 40% of its nitrogen content from biological fixation. Introduction Nitrogen-fixing cyanobacteria are among the first colonists in most primary successional seres – whether in free-living form, in loose associations with algae, or in lichen symbioses (Eggler 1963; Henriksson et al. 1972; Sheridan 1991; Sprent 1993). It has been suggested that nitrogen-fixing organisms should have a competitive advantage early in succession, since most new substrates contain almost no nitrogen (Vitousek & Walker 1987). Moreover, colonization by nitrogen-fixers could increase nitrogen availability and facil- itate colonization by non-nitrogen fixing plants (Vitousek & Howarth 1991). Research addressing the role of nitrogen-fixing lichens in the nitrogen cycle of early primary successional systems, and in extreme environments more generally, suggests that while lichens’ rates of nitrogen fixation are low relative to those of nodulated vascular plants, their contribution of nitrogen is important, given the profound nitrogen limitation of their environment, and the absence of nodulated fixers in many such systems (Snyder & Wullstein 1973; Alexander et al. 1978; Skujins & Klubek 1978; Rhoades 1981; Van Cleve & Alexander 1981; Chapin & Bledsoe 1992; Line 1992).