Microbial quotient

Fig. 1. Effects of long-term land use on organic C, extractable C, microbial biomass C, microbial quotient, basal respiration and the metabolic quotient of soils (0-10 cm). Bars are S.E. (From Haynes, unpublished, 2005). 

Calculation of the microbial quotient can make treatment effects much more obvious. For example, in Fig. 2 the microbial quotient was greater in the row than inter-row and greater under residue return than burning, both to a depth of 30 cm. In addition, values declined with depth as soil organic matter content declined and the proportion of organic C present in labile form declined. 

Fig. 2. Distribution of organic C and microbial biomass C in the soil profile under sugarcane annually preharvest burnt or green cane harvested with retention of crop residues on the soil surface. Soils were sampled from below the plant row or in the centre of the inter-row area. Values for microbial quotient are shown in parenthesis. LSD (P<0.05) shown. (From Haynes, unpublished, 2005).

From the measurements of microbial biomass C and basal respiration the metabolic quotient (qC02) can be calculated. This is a measure of microbial... 

It has been suggested (Insam and Haselwandter 1989 Anderson and Domsch 1990) that the metabolic quotient can be used as an index of ecosystem development. As an ecosystem reaches maturity, selection pressure towards efficient use of available resources results in a larger microbial biomass with a lower metabolic quotient. Nonetheless, Wardle and Ghani (1995) concluded that qC02 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 wherever stress increases along successional gradients. 

Certainly, calculation of the metabolic quotient can reveal trends very different from those of basal respiration. As shown in Fig. 1, for the 7 land uses, trends in basal respiration were broadly similar to those for microbial biomass C and organic C. However, when the metabolic quotient was calculated, trends with land use were very different. Values were greater under sugarcane, maize and to a lesser extent annual ryegrass, than the other treatments. This suggests that the microbial community under these arable land uses is under more stress and/or has a different composition to that under the others. The most likely microbial stress under these land uses is likely to be a shortage of available substrate C. 

Metabolic quotient Variable often in the range of 2-40pg C02-C mg-1 biomass day-1 Basal respiration per unit of microbial biomass C. Provides a useful measure of the efficiency with which microbes are using substrate C. Often used as an index of the degree of microbial stress. 

Anderson JPE, Domsch KH (1978) A physiological method for the quantitative measurement of microbial biomass in soils. Soil Biol Biochem 10 215-221 Anderson TH, Domsch KH (1990) Application of eco-physiological quotients (qC02 and qD) on microbial biomasses from soils of different cropping histories. Soil Biol Biochem 22 251-255... 

Anderson TH, Domsch KH (1993) The metabolic quotient for C02 (qC02) as a specific activity parameter to assess the effects of environmental conditions, such as pH, on the microbial biomass of the soil. Soil Biol Biochem 25 393-395... 

Wagai R, Sollins P (2002) Biodegradation and regeneration of water-soluble carbon in a forest soil leaching column study. Biol Fertil Soils 35 18-26 Wardle DA, Ghani A (1995) A critique of the microbial metabolic quotient (qC02) as a bioindicator of disturbance and ecosystem development. Soil Biol Biochem 27 1601-1610... 

Respiratory quotient (qC02) Calculation from soil respiration divided microbial biomass carbon Indicates activity of microbial biomass determined in nonstandard laboratory with specialized equipment when high, considered an indication of stress on microbial biomass Rice et al. (1996)... 

Dixon, A.J. and Bridge, B.J., 1968. Respiratory quotient arising from microbial activity in relation to matrix suction and air-filled pore space of soil. Nature, 218 961. 

Actinomycetes Microbial biomass Metabolic quotient Substrate-induced respiration Microbial biomass Soil organic carbon Dehydrogenase activity... 

The metabolic quotient, or specific respiration rate (qCO-, is the ratio of the basal respiration rate (as CO2-C) per unit MBC, and provides an indication of efficiency of microbial in utilization of available substrates (Anderson and Domsch, 1993). Low C02 in detritus of the impacted site suggests poor efficiency of microbes in utilization of available substrates (DeBusk and Reddy, 1998 Wright and Reddy, 2001b). Minimal differences in qC02 were observed for 0-10 cm soil of impacted and reference sites. Consumption of electron acceptors such as oxygen, nitrate, and sulfate during... 

Mammalian growth usually shows respiratory quotients close to 1, that is, carbon dioxide formation (CER) equals oxygen uptake rate (OUR). With respect to the relatively poor mass transfer conditions installed in cell culture reactors (compared to microbial conditions), dissolved CO2 (dCOj) levels can accumulate during fed-batch processes. While optimum dC02 concentrations appear to be cell-specific, growth inhibiting partial pressures of approximately 100 mbar had been identified for, for example, hybridoma cells [81]. 

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