The Soils and Their Nutrients 



237 



800 



S. 600- 





u> 

 J> 



o 



E 

 c 



« 



c 



— 



UJ 



200 400 



Moisture, % dw 



40 60 



Percent Oxygen 



600 



FIGURE 7-9. Response of 

 nitrogen fixation rates of 

 Peltigera aphthosa to mois- 

 ture (A) and oxygen concen- 

 tration (B), and of Nostoc 

 commune to oxygen concen- 

 tration (C). (After Alexan- 

 der et al. 1974 and Alexan- 

 der 1978.) 



than in the wet summer of 1973 (1 19 mg N m"'), the difference being due 

 primarily to a higher rate of fixation in the relatively dry areas during the 

 wetter year. These differences between years are considerably less than 

 the differences between specific microtopographic units. For example, in 

 the wetter summer seasonal input on the dry rim of a low-centered poly- 

 gon was only 6.7 mg N m"% whereas in a nearby low, mossy area it was 

 150.4 mg N m'^ 



The response of Nostoc commune to oxygen tension is of special 

 ecological interest, since the greatest nitrogenase activity of this organism 

 occurs in wet, mossy areas, where these algae exist in extremely close as- 

 sociation with mosses. In the water associated with the mosses, oxygen 

 saturation may range from 5 to 24% over 24 hours (Alexander et al. 1974). 



