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



ature and moisture is characterized by the physical and chemical nature 

 of the substrate. The model gresp closely mimics this response surface 

 for a variety of substrates (Figure 9-8, Table 9-2). Ignoring hysteresis ef- 

 fects and microbial succession, the assumption can further be made that 

 given the initial characterization for a specific substrate one can predict 

 the instantaneous respiration rate under any temperature and moisture 

 condition during a year. Unfortunately, in situ measures of respiration 

 from the soil are confused by plant and invertebrate activity, and there 

 are no measurements of the pattern of weight loss during a specific an- 

 nual cycle. However, annual measures of weight loss from a variety of 

 substrates under markedly different meteorological conditions are avail- 

 able. These data were collected from different International Tundra 

 Biome research sites. 



Bunnell et al. (1977a) and Bunnell and Scoullar (198I)have compared 

 measures of total weight loss from different litters with losses due to 

 microbial respiration as simulated by the model. To project simulated 

 weight losses due to microbial respiration they first estimated coefficients 

 a^ through a^ (p. 310-311) from respirometry data collected at the specific 

 site using the non-linear optimization techniques described by Bunnell et 

 al. (1977a). The gresp model was then used to project rates of microbial 

 respiration during the year. Temperature and moisture data used in the 

 projection were those collected from the appropriate site. Computations 

 of weight loss assume that the substrate is 45*Vo carbon and has a respira- 

 tory quotient of I.O. 



The annual weight losses computed from the simulated daily respi- 

 ration rates are compared with the rates of weight loss as measured by lit- 

 ter bags (Table 9-3). Bunnell and Scoullar (1981) discuss the implications 

 of computed coefficients a, through a^ for each substrate and site. The 

 hypothesis relating microbial respiration to measures of temperature and 

 moisture accounts for 71 to 98% of the variation in rates of respiration 

 measured from a variety of natural substrates and predicts annual loss 

 rates under a wide range of environmental conditions that are 70 to 90% 

 of the measured loss from litter bags. The values for loss of weight seem 

 reasonable, given the additional losses due to leaching and physical 

 reduction. 



The equation on page 320 explicitly encompasses the additional in- 

 fluence of substrate chemistry. In nature, rates of weight loss from 

 ethanol-soluble compounds and ethanol-insoluble compounds show a 

 ratio of 5.75:1. When the effects of temperature and moisture are incor- 

 porated according to the central equation of gresp, rates of respiration 

 from the two chemical groups show ratios ranging from 5.1:1 to 8.7:1 

 (Bunnell et al. 1977b). Thus the rates of microbial respiration associated 

 with the two chemical constituents show approximately the same ratio as 

 rates of weight loss from these constituents in the field. If the simpUfying 



