Ten versions of three methods for estimating soil microbial biomass, fumigation-incubation (FI), fumigation-extraction (FE) and substrate-induced respiration (SIR) were compared in seven different laboratories, using a variety of methodological and analytical procedures. Twenty different 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 data obtained by different methods and by different laboratories is hampered by soil-to-soil variation between the methods and by systematic effects on the biomass measurements that cause over- or under-estimations. Each variant of the SIR method should have its own calibration factor. The relationship between the basic FI method and the other methods was mainly affected by the respiration rate of non-fumigated soil. The FI method is less suitable for the calibration of the FE and SIR methods in forest soils than in arable soils. Unintentional variation in the experimental procedures, such as in the duration of any conditioning incubation, may also contribute to the observed differences between the methods.

An inter-laboratory comparison of ten different ways of measuring soil microbial biomass C