The fumigation-extraction method to estimate soil microbial biomass: Calibration of the kEN value

The kEN value (= extractable part of microbial biomass N after fumigation) of the fumigation-extraction method was assessed using the C-to-N ratio of the organic matter which was rendered extractable by CHCl3 fumigation. The data for this calibration approach was obtained from 51 arable and 23 grassland soils. The second calibration approach was to compare the relationship between N rendered extractable by CHCl3 fumigation and the C-to-N ratio measured in the flush of the fumigation-incubation method by recalculating data obtained from the literature. On the basis of these two approaches, we recommend using a kEN value of 0.54 as originally proposed by Brookes et al. (Soil Biology & Biochemistry17, 837–842, 1985).