A statistical evaluation of equations for predicting total microbial biomass carbon using physiological and biochemical methods

The proposed calibration equations for quantifying microbial biomass carbon were considered and variability associated with these methods was assessed., The fumigation-incubation (FI) method depends on a k c factor for biomass C determination. Three k c are commonly in use, i.e. 0.41, 0.45 and 0.50, but these all have an associated degree of variability. However, values of k c may also vary between certain soil treatments and between soil types. Substrate-induced respiration (SIR) has been calibrated against FI but the strength of the relationship between the two methods is uncertain in most instances because SIR measures glucose-responsive microbial biomass while FI measures chloroform-susceptible biomass. ATP and ninhydrin-extractable N analysis may show strong relationships with FI but only in specific circumstances. Fumigation-extraction (FE) demonstrates uncertain relationships when calibrated against SIR, possibly because FE measures chloroform-susceptible biomass. Calibrations of FE against FI also appear to be uncertain probably because k c and k ec values vary differently across soil types.