Microflora Activities and Decomposition 307 



peratures are lower. Here they can accumulate and thereby affect the 

 long-term carbon balance of the system. Under anaerobic conditions 

 these aromatic products may change their chemical structure, repolymer- 

 ize and become highly recalcitrant organic matter. 



However, decomposition occurs throughout the active layer (Figure 

 9-6) and the numbers of facultative and anaerobic bacteria appear suffi- 

 cient to prevent rapid accumulation of carbon in the wetter microtopo- 

 graphic units. Roots that follow the thaw front down presumably can ex- 

 ploit nutrients mobilized by microbial activity. 



Moisture and Microflora/ Metabolism 



Because of the interaction between moisture and temperature, a gen- 

 eral optimal moisture level for microbial respiration in organic residues is 

 indeterminable, but an optimal moisture range is determinable at opti- 

 mal temperatures (Douglas and Tedrow 1959, Flanagan and Veum 

 1974). When moisture is less than 20% of the dry weight residues it is not 

 possible to measure respiration at any temperature; thus moisture levels 

 above 20% dry weight seem necessary to initiate microbial metabolism. 

 The metabohc rates continue to increase throughout the moisture range 

 20 to 500% of dry weight. High moisture levels (> 500% dw) may depress 

 microbial respiration. In a Gilson respirometer the depressant effect can 

 be eliminated by increasing available oxygen (Flanagan and Veum 1974) 

 and is likely associated with reduced rates of supply of oxygen. As tem- 

 peratures increase, respiration in some substrates shows saturation of 

 metabolic moisture demand at amounts of moisture less than 400% dw. 

 Temperature-moisture interactions, as demonstrated by studies of 

 microbial respiration from plant remains, may reflect differences in oxy- 

 gen diffusivity in water or changes in microbial oxygen demand as tem- 

 perature varies. 



Oxygen and Microflora! Metabolism 



Oxygen is critical in microbial metabolism because it serves as an 

 electron acceptor in the breakdown of organic matter. In the soils at Bar- 

 row the potential alternate electron acceptors such as nitrate and sulfate 

 are present in such low concentrations (< 1 ppm) that they can support 

 little activity. Diffusion of oxygen into the soil is often below levels re- 

 quired for optimal microbial activity, and in very wet soils pore spaces 

 become filled with moisture. The effect is decreased microbial activity 

 among the aerobic component; the cause is not too much moisture, but 

 apparently too little oxygen. It is equally plausible that carbon dioxide 



