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



/; 



y, = weight of substrate component /; 1 y, = Y 



1 = 1 

 k, = decay rate of substrate component /'. 



The above equation states that each substrate component decomposes at 

 a constant rate of decay specific for that substrate constituent and inde- 

 pendent of the amount of other substrate components present. The rela- 

 tion of decomposition to temperature and moisture discussed earlier sug- 

 gests that the above equation can be written 



RiT,M) = i, r,{T,M)y. 



where R{T,M) = respiration rate of the total substrate at temperature T 



and moisture M 

 r,{T,M) = respiration rate of substrate component /, a function 

 of temperature and moisture 

 y, = amount of substrate component / present, as before. 



As expressed in the above equation, the model of decomposition not 

 only accounts for the observed differences in response surfaces of res- 

 piration versus temperature and moisture for substrates of different 

 chemical composition, but also accounts for the observed differences in 

 rates of loss of different chemical components from a substrate in the 

 field. To document the relative contributions of different chemical con- 

 stituents to total respiration, the influences of temperature and moisture 

 must be reduced or removed. Bunnell and Tait (1974) proposed several 

 methods for separating the temperature and moisture effects from 

 chemical-specific effects. Three methods have been evaluated by Bunnell 

 et al. (1977b). Their evaluation suggests that dominating influences on 

 the pattern of respiration for any specific constituent are levels of tem- 

 perature and moisture. Only 1 to 4% of the variation in instantaneous 

 respiration rates is due to chemical composition. These observations are 

 not incompatible with the preceding sensitivity analyses, which suggest 

 that the overall response surface of microbial respiration versus tempera- 

 ture and moisture is sensitive to substrate quality. The general amplitude 

 of the response surface, and thus its overall shape, are sensitive to 

 substrate quality (Bunnell et al. 1977a). Variations in temperature and 

 moisture, however, account for more of the variation of the total surface 

 (Figure 9-8) as it rises to the amplitude set by substrate quality. When 

 summed over a year, even small differences associated with substrate 

 chemistry will produce distinctly different annual rates of loss. 



In their evaluation of the relationships expressed in the last equa- 

 tion, Bunnell et al. (1977b) initially treated five different substrate com- 

 ponents: ethanol-soluble cellulose, lignin, pectin, starch and volatiles. 



