Relationship between SIR and FE estimates of microbial biomass C in deciduous forest soils at different pH

Two methods to estimate soil microbial biomass, viz. substrate-induced respiration (SIR) and fumigation-extraction (FE), were compared using 40 beech (Fagus sylvatica L.) forest soils. Soil chemical properties, microbial biomass and activity indices differed over a wide range but generally fitted a normal distribution. Microbial biomass C estimates by SIR and FE were significantly correlated with r = 0.94. Microbial biomass C was significantly correlated with soil pH, soil organic C, K2SO4-extractable C and especially with the basal respiration rate, irrespective of the method used. As expected from the basic principles of an extraction and a physiological method, FE was more affected than was SIR by soil organic matter, with SIR being more affected by soil pH and basal respiration. The development of different microbial community structures at different pH values affects the SIR and FE methods in reverse directions, increasing the differences between the results of the two methods. Estimates of microbial biomass C-to-soil organic C ratio by SIR and FE were significantly interrelated, with r = 0.89 (P < 0.0001) and were closely connected with soil pH. Estimates of the metabolic quotient ent qCO2 by SIR and FE were also significantly correlated, but with r = only 0.54 (P < 0.001).