Microflora Activities and Decomposition 333 



the available pool. These observations indicate a pool of available phos- 

 phorus which turns over very rapidly and/or possible limitation to plant 

 metabolism by microbial competition for and immobilization of soil 

 phosphorus. 



SUMMARY 



Field and laboratory measures have been combined to provide an 

 overall picture of decomposition. The ability of the tundra microflora to 

 utilize substrates varies spatially, with aerobic decomposers showing a 

 marked increase in capacity to degrade cellulose and phenols in the soil, 

 as compared to the phyllosphere. This gradient in potential utilization is 

 accompanied by an increase in anaerobes, and a marked decrease in both 

 zymogenous forms and general microbial biomass. Fungi are better able 

 to utilize cellulose and phenolic substrates than are bacteria. Unfortu- 

 nately, the enzyme capabilities of the anaerobic microflora remain un- 

 known, but no anaerobic decomposers of aromatic compounds are 

 known from other areas. 



Respiration rates of the microflora are governed in a predictable 

 fashion by temperature, moisture, and substrate chemistry. Respiration 

 rates are shown to be the dominant influence on weight loss from sub- 

 strates. Substrate chemistry establishes a potential maximum rate which 

 is modified by abiotic variables. Ethanol-soluble compounds generally 

 are respired 5 to 7 times more rapidly than non-ethanol-soluble com- 

 pounds, but some substrates (e.g. Dryas leaves) apparently contain sub- 

 stances inhibitory to microbial respiration. 



Both bacteria and fungi show adaptations to cold. Microbial respir- 

 ation continues to - 7.5 °C and fungal growth is still positive at 0°C, in- 

 dicating greater levels of activity at low temperatures than are observed 

 in vascular plants. Many microorganisms in colder strata of the environ- 

 ment show linear rather than exponential responses with increasing tem- 

 perature, suggesting adaptation to cold through more rapid response to 

 small increases in temperature. Cold-adapted microorganisms, especially 

 fungi, increase in numbers from the phyllosphere into soils, while in the 

 upper regions of the soil microbial populations display a wider range of 

 temperature optima and mesophilic forms are more prominent. Among 

 the fungi, psychrophiles, thermotolerant psychrophiles and cold-tolerant 

 mesophiles retain the capacity to utilize structural plant carbohydrates at 

 temperatures below 0°C, while aerobic bacteria are largely restricted to 

 non-structural plant components at 0°C. As bacteria can use the pro- 

 ducts of fungal decomposition of large molecules, it is possible that co- 

 evolution has permitted the development of different enzymatic re- 

 sponses to low temperatures. 



