Microflora Activities and Decomposition 305 



respiration in its minimum-optimum temperature range, e.g. CCS (Fig- 

 ure 9-2). 



Together, the two hypotheses address extension of the lower temper- 

 ature Hmits and the form of response with changing temperature. The 

 microflora appears to have extended its capacity to degrade substrates, 

 respire and grow at low temperatures. Respiration continues to -7.5 °C, 

 growth is positive at 0°C, and oxidation of cellulose and phenols prob- 

 ably continues well below +5°C, possibly to -7.5 °C, because psychro- 

 philic and psychrophilic thermotolerant organisms show a propensity for 

 utilizing these substrates at low temperature. Both interspecific dif- 

 ferences and individual responses serve to increase the effective meta- 

 bolic range of the microflora, by broadening the near-optimal metabolic 

 range or form of response with changing temperature. Bacteria appear to 

 increase their effective metabolic range by shifting to compounds of 

 lower molecular weight (6-C compounds) at lower temperature, while 

 fungi continue oxidation of cellulose and phenols. In terms of growth 

 and respiration, evidence from tests of individual soil fungi and the cal- 

 culation of the percentage of psychrophiles in soil as compared to above 

 ground (7.597o in litter, 15.6% in soil) strongly suggests that soil organ- 

 isms at Barrow are cold-adapted, while aboveground populations with 

 wider ranges of near-optimal temperature occur. Adaptation to cold 

 rather than acclimation is indicated. Progressing from aboveground to 

 belowground environments, relations between temperature and respira- 

 tion tend from exponential to linear, suggesting another form of adapta- 

 tion to cold. Computed values of Q,o from soils of the Biome research 

 area are typically lower (closer to linearity) than are values from temper- 

 ate soils (Macfadyen 1970, de Boois 1974). Neither the flrst nor the se- 

 cond hypothesis can be rejected. 



Responses to Moisture and Oxygen 



As with temperature, moisture levels for microbial activity assume 

 minimum and maximum thresholds and optima. These cardinal moisture 

 levels differ between organisms and within a single organism for differ- 

 ent processes. While there are some notable exceptions (Pitt and Chris- 

 tian 1968), most microbial activities in soil are hmited by moisture poten- 

 tials below -100 bars. The lower limit for bacteria generally is believed to 

 be higher than that for fungi (Dommergues 1962, Griffln 1972). It is un- 

 likely that soils of the Barrow research area were dry enough to restrict 

 the activities of microorganisms, but in the standing dead canopy decom- 

 position may be limited by lack of moisture. Within the soils, moisture 

 effects are more likely to be indirect and associated with reduced flux of 

 oxygen (Chapter 8). 



