Microflora Activities and Decomposition 311 



T = temperature, °C 

 M = moisture, percent dry weight 

 a,,...,ai = substrate specific parameters. 

 The rationale of the gresp function has been presented elsewhere (Bun- 

 nell and Tait 1974, Bunnell et al. 1977a) and only a summary is repeated 

 here. Microbial respiration is assumed to be related to the moisture po- 

 tential of the substrate via two saturation processes. The first process is 

 related to the metabolic water requirements of decomposer organisms 

 and embodies a convention of soil mycologists, that is, the expression of 

 water content on a relative basis, or as a percentage of the value when the 

 soil is saturated (Griffin 1966). This process is expressed as M/{a, +M), 

 where M represents the percent water content on a dry weight basis and 

 fir, represents the percent water content at which the substrate is "half- 

 saturated" with water or respiratory activity is at half its optimal level. 

 The second saturation process occurs at high moisture levels. It is 

 assumed to represent the effect of water on gas exchange with the atmos- 

 phere either of oxygen, carbon dioxide or both. The simplest formula- 

 tion is employed (Bunnell and Tait 1974). Since the degree to which gas 

 exchange is inhibited can be expressed as M/(a2 + M), the degree to which 

 it is not inhibited can be expressed as: 



l-[M/(a2-hM)] or 02/(02+ M). 



Again M represents the moisture content, and 02 represents the percent 

 water content at which gas exchange is limited to half its optimal value. 

 The third and fourth factors, temperature and substrate characteris- 

 tics, are treated as a substrate-specific Qio relationship: 



where a^ is the substrate specific respiration rate that occurs at 10 °C 

 when neither moisture nor oxygen are limiting and a^ is the Qio coeffi- 

 cient. Alternative formulations for both moisture and temperature influ- 

 ences on rates of nutrient supply are discussed by Bunnell et al. (1977a); 

 the treatment of substrate characteristics is pursued later in this chapter. 

 According to the gresp function any one of the major determinants of 

 rates of respiration (moisture, oxygen, temperature and substrate) can 

 effectively reduce the rate of microbial respiration independently of the 

 other factors. Thus, the rate determinants are combined multiplicatively. 

 Evaluation of the complex hypothesis represented by the ores? 

 function indicates that it predicts carbon dioxide evolution more accu- 

 rately from aboveground substrates than from tundra soils. In the some- 

 what more aerobic soils of the taiga the model accounts for 78 to 84% of 

 the variability in respiration rates (Bunnell et al. 1977a). The generality 



