334 P. W. Flanagan and F. L. Bunnell 



Moisture levels above 20% dry weight are necessary to initiate 

 microbial metabolism, while levels much above 400% dry weight attenu- 

 ate microbial activity. Oxygen, carbon dioxide, and temperature rela- 

 tions interact with moisture levels to obscure definition of optimal mois- 

 ture levels for decomposition. Shifting bacterial: fungal ratios along oxy- 

 gen gradients indicate that oxygen availability and/or moisture alters the 

 numbers and character of participants in decomposition. Large numbers 

 of facultative anaerobic bacteria in litter and soil reflect a commonly oc- 

 curring niche. Although present, obligate anaerobes are major contribu- 

 tors to decomposition only in heated soils. 



Perhaps because of microbial adaptations to low temperatures, or- 

 ganic matter does not appear to be accumulating in some microtopo- 

 graphic units. Only the fungi are capable of degrading the larger com- 

 pounds, particularly at low temperatures. The fungi, however, are re- 

 stricted by high levels of moisture to the upper 7 to 10 cm of soil, and or- 

 ganic matter may accumulate at depth. The nutrient dynamics within the 

 soil also suggest that phosphorus, but not nitrogen, immobilized within 

 the standing crop of microbial tissue may be a factor limiting nutrient 

 availability to vascular plants. 



