WHY IS THE STRENGTH OF RELATIONSHIPS BETWEEN PAIRS OF METHODS FOR ESTIMATING SOIL MICROBIAL BIOMASS OFTEN SO VARIABLE

Physiological and biochemical methods for estimating soil microbial bi omass are usually calibrated against other methods and parameters. How ever, while calibrations are usually made over soils with a very wide range of microbial biomass values (across a wide geographical range) t hey are often used to assess relatively small differences in microbial biomass across a narrower range of microbial biomass values (across a smaller geographical area, e.g. within a single field), where their r eliability may be considerably less. We investigated the abilities of three methods of quantifying microbial biomass, i.e. substrate-induced respiration (SIR), fumigation-incubation (FI) and fumigation-extracti on (FE) to serve as predictors of each other across two geographical s cales, i.e. across 12 sites over an area of approximate to 100 x 100 k m; and within each of these sites (12 samples per site) over an area o f 0.3 ha each. Over the larger scale, relationships between pairs of m ethods were strong, with R(2) values always > 0.90. However, over the smaller scale, correlations between pairs of methods were variable and only significant for those sites in which spatial variability in soil organic matter was relatively high. Uncertain relationships between S IR and the fumigation-based methods may be expected because they apply to different subsets of the soil biomass (i.e. glucose-responsive vs chloroform-sensitive). However, we suggest that FI and FE are sometime s weakly correlated because the FI decomposition constant k(C) and the FE constant k(EC) vary differently relative to each other across unde rlying gradients. Calibration equations for estimating microbial bioma ss are most accurate when restricted to situations where the range of biomass values is comparable to that from which the calibration was fi rst derived, and to similar soil types. In our study, it appears that calibration equations for predicting microbial biomass are only likely to provide reliable relative estimates in situations where the coeffi cient of variation (standard deviation/mean) of soil organic C is > 15 %.