References

134 O. Dilly
extensive root densities frequently present in grassland soils may stimulate
the development of the microbial biomass via exudation of readily available
C compounds and root litter (Cheng et al. 1996; Grayston et al. 1996). In
contrast, careless cultivation and stress factors reduce microbial biomass
more rapidly than organic matter content, leading to low Cmic/Corgvalues
(Sparling 1997).
The Cmic/Corgratio, named ‘microbial quotient’ by Sparling (1992), is
considered an indicator for biological activity and accumulation of organic
matter in soil (Sparling 1992; Swift and Wommer 1993; both in Elliott 1994).
Thus, the soil microbial biomass content is regulated by the site-, soiland
management-specific quantity and quality of the organic substrate
(Sparling 1992). The Cmic/Corgratio integrates the quality of soil properties
looking at the extent of microbial colonisation. High values indicate that
the biotope favours the establishment and energetic metabolism of many
microorganisms.
The efficiency of soil microorganisms for transforming energy sources
controls microbial growth for which the qCO2 is used as an indirect easily
determinable indicator. This indicator evaluates, with the Cmic/Corg ratio,
the specific C mineralisation rate and the eco-physiological status of
the soil microbiota (Insam et al. 1996), the succession stage (Insam and
Haselwandter 1989), reflects the current energetic maintenance requirement
and catabolic metabolism (Anderson 1994), and refers so far to the
efficiency of the microbial metabolism (Wardle and Ghiani 1995). The term
‘eco-physiology’ is used here to evaluate the microbial biomass as a single
organism with reference to its environment.
The maintenance requirement of actively metabolising microbial communities
ranged in a similar order of magnitude to the qCO2 value (Anderson
and Domsch 1985a). In contrast, the maintenance requirement of
dormant organisms is more than ten-fold lower (Anderson and Domsch
1985b). Therefore, the qCO2 is used for the estimation of the maintenance
requirement of the soil microbiota (Joergensen 1995) although there may
be some variation as to the precise maintenance carbon requirement (Anderson
and Domsch 1985a).
Under unfavourable conditions, the organisms require more energy to
sustain the biomass, therefore, qCO2 values are enhanced and the carbon is
lost. High qCO2 values indicate stress such as high heavy metal availability
(Fließbach et al. 1995). The qCO2 valueisalsoenhancedwhentheSOM
contains high amounts of readily available compounds (Fig. 3; Cheng et al.
1996; Dilly and Munch 1996). Consequently, the qCO2/Corg ratio refers to
the interrelationship between C-use efficiency and quality of the available
organic matter in soil.
Figure 6 demonstrates the concept for the use of the qCO2/Corg ratio: the
value increases and simultaneously the C-use efficiency declines when half