290 F. L. Bunnell et al. 



early summer appears to be associated with release of organic material at 

 thaw, the late peak may be associated with senescence and death of plant 

 parts. This underlying pattern generated by availability of substrate is 

 subsequently modified by the amounts of moisture present. Depending 

 upon the microtopography and climate, fungi in the soil show minimal 

 turnover rates for biomass of two to six times per season. 



Although low temperatures may Hmit amounts of microbial bio- 

 mass, temperature-related differences between years or among microtop- 

 ographic units were not observed. Rates of activity (Chapter 9) and 

 growth, however, are positively related to temperature. The organic sub- 

 strate seldom governs the biomass of the microflora, but low concentra- 

 tions of inorganic nutrients, particularly phosphorus, are limiting to 

 growth rates in some microtopographic units. The major control on 

 composition and biomass of the microflora appears to be moisture and 

 associated aeration. These factors are indirectly controlled by tempera- 

 ture through the impeded drainage associated with permafrost, and are 

 much modified by physical characteristics of the soils such as bulk 

 density. 



