The Soils and Their Nutrients 235 



Peltigera canina, Lobaria linita. Nephroma spp., Solorina spp., Stereo- 

 caulon spp. and other Peltigera spp. also occur in significant amounts. 

 With ambient temperatures of about 15°C nitrogenase activity ranges 

 from 4.5 fig N gdw"' hr"' in Stereocaulon tomentosum to 41 .5 /ig N gdw"' 

 hr"' in Nostoc commune. Nitrogen fixation per unit of biomass in the 

 Peltigera species is high (8.8 to 25.8 /^g N gdw"' hr"' at 15°C), considering 

 the large proportion of its biomass contributed by fungus and thus not 

 directly involved in nitrogen fixation. 



Heterotrophic bacteria also may contribute to nitrogen fixation 

 (Chapter 8). Azotobacter was isolated from a mesic meadow, but the 

 numbers were low, 10^ to 10^ cells (gdw soil)"', and nitrogen fixation 

 within the soil was consistently less than 1 \xg N m"^ hr"'. If heterotrophic 

 bacteria are indeed active fixers of nitrogen in the soils, their activity is 

 very low compared with the free-living and symbiotic blue-green algae of 

 lichens. 



No significant nitrogen fixation was found to be associated with any 

 higher plants (Alexander and Schell 1973). Alpine tundra soils in central 

 Alaska, however, have a substantial input of nitrogen from vascular spe- 

 cies, including Dryas spp., Lupinus, Astragalus and Oxytropis spp., 

 which are abundant in the alpine sites and in the Prudhoe Bay region but 

 absent or rare in the coastal tundra at Barrow. 



Similar constellations of organisms have been found dominant in 

 the nitrogen-fixation regimes of other tundra sites (Alexander 1974, 

 1975, Jordan et al. 1978). Nostoc commune is a cosmopolitan species 

 that is important in nitrogen fixation in a variety of natural ecosystems 

 (Fogg et al. 1973). In particular, the Nostoc-moss association, which has 

 drawn much attention in circumpolar studies, also appears to be an im- 

 portant feature of the grassland ecosystem (Vlassak et al. 1973). 



Biomass of Nitrogen-Fixing Organisms 



Nitrogen fixation rates for any location on the tundra depend pri- 

 marily on the distribution and biomass of the nitrogen-fixing organisms 

 and secondarily on the various abiotic variables that influence nitrogen- 

 ase activity within organisms. The distribution of nitrogen-fixing organ- 

 isms is correlated with moisture regime and vegetation. Nostoc commune 

 occurs in wet environments, and is especially abundant in wet, low-lying 

 meadows, where it may occur as extensive mats floating over the moss 

 layer. One low-lying area contained 19.5 g Nostoc m'\ 10% of the stand- 

 ing crop (Williams et al. 1978). Additionally, Nostoc forms an epiphytic 

 or intercellular association with various genera of mosses. 



The nitrogen-fixing lichens, primarily Peltigera aphthosa, tend to 

 occur at intermediate moisture levels such as the slopes between troughs 



