The fumigation-extraction method to estimate soil microbial biomass: Calibration of the k(EC) value

The k(EC) value (=extractable part of microbial biomass C) of the fumigation-extraction (FE) method was assessed on the basis of 153 soils (94 arable, 46 grassland and 13 forest soils) by indirect calibration using the fumigation-incubation (FI) method. Sixty-six soils were investigated for the first time and the data on a further 87 soils were obtained from the literature. The single k(EC) values ranged from 0.23 to 0.84. A split according to the form of land use resulted in a significantly (Scheffe, P = 0.05) lower k(EC) value for the arable soils (0.42; n = 94) in comparison to those for the grassland (0.49; n = 46) and the forest soils (0.51; n = 13). This difference is mainly due to the significant effects of the respiration rate measured in non-fumigated control samples of the FI method which was used for calibration of the k(EC) value. For that reason, I investigated the effects of incubation temperature (22 degrees, 25 degrees and 28 degrees C) on biomass C data obtained by the FI method, and thus on the k(EC) value of the FE method, and discuss further problems of direct and indirect calibration. Based on experimental and literature data, I conclude that the k(EC) values of Vance et al. (Soil Biology & Biochemistry 19, 703-707, 1987) and Wu et al. (Soil Biology & Biochemistry 22, 1167-1169, 1990) remain valid. A k(EC) value of 0.38 can be recommended for C analysis by dichromate consumption and a k(EC) value of 0.45 for that by UV-persulfate or oven oxidation.