278 F. L. Bunnell et al. 



Both groups have early- and late-season peaks. While the peak early in 

 the summer may be associated with the release of organic material from 

 both aboveground and belowground material, the second peak is more 

 probably associated with the senescence and death of roots. Because the 

 second peak appears to coincide with the first frost, this increase may 

 also be associated with subsequent leaching of aboveground vegetation. 

 Further evidence for the concept of substrate abundance influencing 

 bacterial abundance is apparent in the differences with depth. Before the 

 Recovery of bacterial numbers in late August or early September, plate 

 counts from meadow soils show a gradual decline from 10' to lO*" in the 

 upper 2 cm and a more marked decline from 10* to 10'' at depths of 2 to 7 

 cm (Benoit, unpubl.). Direct counts from the surface soil are also 

 depressed in mid-season, but show no mid-season decrease in numbers in 

 the deeper soil. The lower soil layers depend more upon root growth and 

 exudation for replenishment of potential substrate, and less upon 

 leaching from aboveground organic matter. Root activity at mid-season 

 is apparently sufficient to maintain the bacterial population in the deeper 



soil. 



Although bacterial biomass and ratios of fungi to bacteria differ 

 among microtopographic units (Table 8-1), seasonal courses of bacterial 

 plate counts do not. Composition of the biomass does show some taxo- 

 nomic shift and chromogenic bacteria appear relatively more abundant 

 in late season. The consequences of such a shift to decomposition are 

 unknown. 



Estimates of Minimal Production 



The estimates of production discussed below consist simply of the 

 sum of the positive changes in measured biomass sensu Ivanov (1955) 

 and are thus minimal and frequently confused by the variability of the 

 microflora. 



Although biomass varied considerably among sample points on a 

 specific microtopographic unit, both fungal productivity and fungal den- 

 sity showed broad differences among these units (Figure 8-8). Fungal 

 density was lower in the wetter summer of 1973. The difference in density 

 between years was most consistent in the wetter microtopographic units, 

 suggesting that the decrease was due to higher moisture levels. Although 

 minimal fungal productivity is broadly correlated with average mycelial 

 denshy, the nature of the relationship apparently shifts between years. 

 Despite lower mean density, the total fungal growth in 1973 was general- 

 ly higher than in 1972 for drier microtopographic units, in particular the 

 rims of low-centered polygons, and for basins of low-centered polygons. 

 Much of this growth occurred in late July to early August (Figure 8-7) 



