The Microflora 287 



weight basis their potential influence on fungal biomass and bulk density 

 may be confused by strong covariance between both factors. It is note- 

 worthy that the form of the relationship between fungal biomass and 

 moisture is similar to that between microbial respiration and moisture 

 (Bunnell et al. 1977a). Relative growth rates of soil fungi show no rela- 

 tionship with soil moisture. These observations are consistent with the 

 fact that soil moisture is only rarely limiting in the horizons where fungi 

 are abundant. 



Although there are few data on amounts of oxygen, those available 

 (Figure 7-6), together with the documented relationships among fungal 

 biomass, organic matter, soil moisture and bulk density, suggest that the 

 low fungal biomasses of some microtopographic units are a function of 

 excessive moisture and oxygen depletion or excessive carbon dioxide. In- 

 creases in bulk density resulting from additional fine mineral particles 

 decrease total pore volume and the percentage of air-filled pores (Figure 

 7-3). With relatively high bulk densities, concentrations of oxygen de- 

 cline rapidly with depth, carbon dioxide presumably increases, and 

 amounts of fungal biomass are low despite the relatively large amounts 

 of organic substrate. The decline in fungal biomass is due not simply to 

 aeration, but also to available moisture (Bunnell et al. 1977a). In the 

 more mineral layers, field moisture contents decline to 0.55 to 0.60 g 

 cm"\ below the optimal value for fungi (Figure 8-14). Total bacterial 

 biomass is largely unaffected by the declining pore volume or oxygen 

 saturation and appears to respond more directly to available substrate 

 (Figure 8-4, Table 8-2). 



Dominant Controls 



The broad relations documented above plus the seasonal courses of 

 biomass indicate which environmental factors exert the greatest control 

 on microfloral biomass and productivity. Within the standing dead vege- 

 tation the relatively high quality of the substrate in June and, to a lesser 

 extent, in September permit high levels of microfloral biomass and pro- 

 ductivity despite relatively low temperatures. At these times, moisture is 

 not limiting. As temperatures increase during the growing season, mois- 

 ture becomes limiting, substrate quality declines, and microfloral bio- 

 mass declines. The limitation by moisture is more pronounced for micro- 

 topographic units more exposed to wind and drying, such as rims of low- 

 centered polygons and tops of high-centered polygons. The pattern is 

 similar in litter with only slight modiflcations. The early-season period of 

 positive influence by substrate quality lasts a few days longer in litter 

 than in standing dead, and the mid-season period of control by low 

 amounts of moisture is shorter, markedly so for concave microtopo- 



