A critique of the microbial metabolic quotient (qCO2) as a bioindicator of disturbance and ecosystem development

The microbial metabolic quotient (respiration-to-biomass ratio) or qCO2, conceptually based on Odum's theory of ecosystem succession, is increasingly being used as an index of ecosystem development (during which it supposedly declines) and disturbance (due to which it supposedly increases). We investigated the suitability of qCO2 as an bioindicator using: (1) data from the Franz Josef Glacier chronosequence, spanning over 22,000 years and (2) data recalculated from published studies. In the Franz Josef sequence, a detectable decline in qCO2 occurred in the first 250 years in both the L-layer and mineral soil layer. However, in the later phases of the succession there was a sharp increase in qCO2 indicating reduced microbial efficiency, which appeared to be related to stress (independent of disturbance) resulting from steady-state conditions. Calculation of qCO2 from three previous studies on disturbance and ecosystem development indicated that this index responds unpredictably and does not necessarily decline during succession. Plant litter decomposition studies demonstrate that while qCO2 usually declines initially, a significant increase in qCO2 can eventually follow on litter types resistant to decomposition. Correlation analysis of each of 24 previous studies demonstrated that qCO2 often declines with increasing pH, clay content and amounts of microbial biomass: these three soil properties are all indicative of varying stress rather than disturbance levels. Reanalysis of data from 16 previous studies indicated that some disturbances such as fertilization and liming can either increase or decrease qCO2 values depending on whether the disturbance alleviates stress (reducing qCO2) or is more extreme than the stress it alleviates (enhancing qCO2). Although cultivation represents a severe disturbance, qCO2 is not predictably enhanced by this perturbation. While qCO2 undoubtedly provides a useful measure of microbial efficiency, our data suggests it has limitations because it can be insensitive to disturbance and ecosystem development, fails to distinguish between effects of disturbance and stress, and does not decline predictably in response to ecosystem development whenever stress increases along successional gradients.