236 P. L. Gersper et al. 



and rims, although P. aphthosa, P. canina and Lobaria linita are com- 

 mon in wet meadows, as well as in depressions between clumps of Erio- 

 phorum vaginatum in better-drained meadows (Williams et al. 1978). In 

 the microtopographic units that are more favorable for lichens, such as 

 the rims of low-centered polygons, a total lichen biomass as high as 180 g 

 m"^ has been observed; however, only about 2 g m"^ of this is capable of 

 nitrogen fixation. Thus the biomass of nitrogen-fixing organisms in- 

 creases from dry to wet areas, with the major fraction made up of free- 

 living or moss-associated Nostoc, which is confined to wet areas, and the 

 remainder composed of nitrogen-fixing lichens, which are relatively 

 more abundant in the mesic areas. 



Environmental Controls on Nitrogen Fixation 



In laboratory studies the principal environmental factors modifying 

 rates of nitrogenase activity in nitrogen-fixing organisms are tempera- 

 ture, moisture, light and oxygen tension. Response of Nostoc and Pelti- 

 gera to climatic factors, and to some inorganic nutrients, is described by 

 Alexander et al. (1978). Diurnal temperature fluctuations of 10 °C for the 

 rim of a low-centered polygon and 15°C for a polygon trough were re- 

 corded in July 1972. Thus rather high temperatures can be attained in the 

 immediate vicinity of the maximum algal biomass. Fluctuations in both 

 light and temperature appeared to exert a strong influence on field rates 

 of nitrogen fixation (Alexander et al. 1974). 



The most critical environmental factor in determining the rate of ni- 

 trogen fixation is moisture. The response of Peltigera to moisture (Figure 

 7-9) shows that saturation of nitrogenase activity does not occur until the 

 moisture content exceeds 250% of dry weight (Alexander et al. 1978), a 

 response similar to that shown by other nitrogen-fixing lichens (Kallio 

 1973). Although no similar data exist for Nostoc, it shows no activity at 

 all when dry, but rapidly resumes activity when moistened above 100% 

 dry weight. The nitrogen-fixing organisms appear to be well adapted to 

 handle periodic desiccation, and are able to make effective use of mois- 

 ture whenever it is available. 



On a season-long basis, highest inputs from nitrogen fixation occur 

 on wet, mossy areas. In drier areas, seasonal nitrogenase activity is limit- 

 ed by available moisture (Alexander et al. 1974), and overall rates are 

 somewhat lower during summers with low rainfall. Extremely wet areas 

 devoid of moss cover also have very low rates of nitrogen fixation. In dry 

 summers, such as 1972, there was a decline in fixation in moderately 

 moist areas as the season progressed and soil moisture declined. The total 

 seasonal input from nitrogen fixation, integrated over an area compris- 

 ing a variety of microtopographic units, was lower in 1972 (85 mg N m"^ 



