268 F. L. Bunnell et al. 



of mycorrhizal and wood-rotting fungi. Influences of high moisture, low 

 aeration and associated low pH are ubiquitous among broad taxa: Actin- 

 omycetes are sharply reduced, gram negative bacteria dominate, plank- 

 tonic algae are common, as are aquatic fungi, while yeasts and higher 

 fungi are common only in better-aerated areas. The moist, acid environ- 

 ment supports a taxonomic structure with restricted potential for bacter- 

 ial fixation of nitrogen, abundant denitrifiers and a similarly restricted 

 potential for fungal decomposition of larger molecules. The presence of 

 permafrost and impeded drainage maintains a zone of the soil with re- 

 stricted potential for decomposition leading to accumulation of carbon. 

 In the better-aerated microtopographic units where fungi are more abun- 

 dant, levels of available nutrients in the soil are often higher and primary 

 production greater. 



MICROFLORA BIOMASS: ITS DISTRIBUTION IN THE 

 ENVIRONMENT AND CHANGES THROUGH TIME 



Although microbial biomass itself is of limited applicability in 

 assessing kinetics of decomposition, and many assumptions must be used 

 to calculate it, measures of biomass are useful in establishing broad rela- 

 tionships between the microflora and its environment (Table 8-1). Be- 

 cause of the marked differences in bulk density and organic matter with- 

 in profiles of tundra soil, estimates of microbial biomass are computed 

 on a volume basis as well as per gram of soil. Throughout the following 

 discussion the term density refers to numbers of bacteria or meters of 

 hyphae per gram substrate; the term biomass refers to weight of organ- 

 isms per volume of substrate. 



Spatial Distribution of Microbial Biomass 



Differences Among Microtopographic Units 



Within the soil the seasonal average of fungal biomass is highest on 

 the rims of low-centered polygons, and lowest in polygon troughs (Table 

 8-1, Figure 8-4) (Laursen 1975). Algal biomass, in contrast, is highest in 

 the basins of low-centered polygons and lowest in polygon troughs and 

 wet meadows (Table 8-1). Unfortunately, direct counts of the bacterial 

 component were not available when fungal data were collected. The 

 direct counts obtained from wet meadow soils in 1971 suggest that the 

 upper 7 cm of those soils contained about 16.5 g m"' of microbial 

 biomass of which, for a seasonal average, about 75<yo was bacterial. If 

 the plate to direct count ratios observed in wet meadows hold in other 



