AN INTER-LABORATORY COMPARISON OF 10 DIFFERENT WAYS OF MEASURING SOILMICROBIAL BIOMASS-C

Ten versions of three methods for estimating soil microbial biomass, f umigation-incubation (FI), fumigation-extraction (FE) and substrate-in duced respiration (SIR) were compared in seven different laboratories, using a variety of methodological and analytical procedures. Twenty d ifferent soil samples were analyzed from a range of arable and forest sites. The 10 different measurements gave almost identical ranking for microbial biomass C in the 20 soil samples. However, comparison of da ta obtained by different methods and by different laboratories is hamp ered by soil-to-soil variation between the methods and by systematic e ffects on the biomass measurements that cause over- or underestimation s. Each variant of the SIR method should have its own calibration fact or. The relationship between the basic FI method and the other methods was mainly affected by the respiration rate of nonfumigated soil. The FI method is less suitable for the calibration of the FE and SIR meth ods in forest soils than in arable soils. Unintentional variation in t he experimental procedures, such as in the duration of any conditionin g incubation, may also contribute to the observed differences between the methods. (C) 1997 Elsevier Science Ltd.