Microflora Activities and Decomposition 309 



tures and approximate the rates expected, given the respiratory Q,o of 

 1.78 measured from unheated soils and a 4° to 6°C increase in mean 

 daily soil temperature. The heat treatment documents the in situ poten- 

 tial for anaerobic decomposition present in tundra soils suggested by the 

 high number of facultative bacteria relative to strictly aerobic species 

 (Chapter 8). 



DECOMPOSER ACTIVITIES AND DECOMPOSITION 



Two general approaches are used in analyzing the response of de- 

 composer organisms to temperature, moisture and other environmental 

 phenomena. The first approach employs integrative measures of all mi- 

 crobial groups. The evolution of carbon dioxide, the rates of weight loss 

 from selected substrates, and the patterns of nutrient concentration are 

 each considered direct or indirect functions of the activities of all micro- 

 bial groups. The second, more direct, approach examines the individual 

 activities of specific microbial groups. The specific activities suggest the 

 contribution particular groups make to the general integrative measures 

 such as carbon dioxide evolution. We discussed some specific responses 

 earlier in this chapter without relating them to patterns of weight loss or 

 carbon dioxide evolution measured in the field. The present discussion il- 

 lustrates how these activities are enacted in the changing environment to 

 produce the observed patterns of weight loss or decomposition. 



Decomposition of aboveground parts of graminoids begins at the 

 time when necrotic patches appear on the leaves and stem bases. The ne- 

 croses are most apparent from mid-August onwards. Before or concur- 

 rent with visual signs of senescence the leaves lose up to \2% of their 

 weight as green healthy tissue (Figure 9-7). The initial loss, which occurs 

 prior to substantial microbial ingress, and whether or not rain has fallen, 

 is apparently caused by translocation to belowground parts. The freeze- 

 thaw cycle and the physical throughput of water remove up to 18% of 

 the dry weight of overwintering leaves by the spring. During and prior to 

 the period of leaching substantial microbial activity may take place, con- 

 tributing to overall weight loss, but microbial contributions to weight 

 loss in this and the previous phase of aboveground plant weight losses are 

 undetermined. During the period from mid-August to the end of spring 

 runoff, up to 30% dry weight may be lost from leaves of graminoids 

 (Figure 9-7, Table 9-5). 



Three simulation models relate microbial activities quantitatively 

 and unambiguously to the environmental phenomena that govern them. 

 One model, gresp (Bunnell et al. 1977a), relates the respiratory response 

 of microbial populations to changing temperature and moisture. The sec- 

 ond, DECOMP (Bunnell et al. 1977b), expresses the respiration rate as a 



