The Microflora 277 



during the season and provides further circumstantial evidence concern- 

 ing the nature of the rapid early-season growth. In both litter and surface 

 soil, some members of the Mucorales and common molds are more evi- 

 dent early in the season and become less evident as the season progresses. 

 These species are cold-tolerant (Latter and Heal 1971, Flanagan and 

 Veum 1974), fast-growing and can be termed "soft" decomposers since 

 they are better able to exploit readily leachable compounds than cellulose 

 or lignin (Chapter 9). 



In deeper soil layers the fungal population includes "hard" decom- 

 posers, fungi with a relatively greater capacity to utilize cellulose and lig- 

 nin than have the mucors and common molds. Many of these are basidi- 

 omycetes and ascomycetes, although zygomycetes are present at inter- 

 mediate depths. The "hard" decomposers generally fluctuate less rapidly 

 than the "soft" decomposers of the Utter and surface soil layers, and 

 tend to increase slowly in biomass over the season as soil temperatures in- 

 crease and buried organic soils become drier (Laursen and Miller 1977), 



A third broad component of the soil fungi are the mycorrhizal form- 

 ers. Profiles of these fungi with depth approximate the profiles of roots 

 of dicotyledonous plants, particularly Salix, Cassiope and Vaccinium 

 species. On the rims of low-centered polygons, fungal biomass increases 

 at depths of 2 to 4 cm and the average proportion of basidiomycete cells 

 increases from well below 20% in the surface layers to 29% at 1 to 2 cm 

 depth and 42% at 2 to 3 cm depth, declines to 37% at 3 to 4 cm, and sub- 

 sequently drops sharply below 4 cm. The temporal pattern of mycorrhi- 

 zal formers is marked by rather stable biomass levels which increase 

 steadily from early season levels until fruiting, during July and early 

 August (Miller and Laursen 1978). 



In general, yeasts are most abundant in early, frozen samples. Num- 

 bers of yeasts decline as the soil thaws and bacterial biomass increases 

 early in the season. The seasonal decrease in abundance is greater in soils 

 with deeper thaw — about ten times greater from 26 June to 5 August in 

 1973. Yeast numbers thus appear negatively correlated with bacterial 

 numbers, but the data are sparse. Differences in average population 

 levels between years suggest that the differential response of yeast and 

 bacteria is associated with amounts of moisture. In the moist summer of 

 1973, yeast counts in the upper 2 cm of wet meadow soil were less than 

 20% of the levels observed in 1971. At depths of 5 cm, counts in 1973 

 were at least 100-fold lower than in 1971, less than 10^ (gdw soil)"'. 



Bacteria 



In the surface soil, bacteria appear to follow the same seasonal pat- 

 tern as fungi and may be responding similarly to changes in substrate. 



