Reassessement of the Kec coefficient of the fumigation–extraction method in a soil profile

We investigated whether the chloroform fumigation extraction (CFE) method could be used to determine the microbial biomass in subsurface soils. The structure of microbial communities may vary over depth and the distribution of their cellular components may result in variations in the value of the calibration coefficient Kec. To verify whether this is so, we adopted a procedure based on in situ labelling of the microbial biomass using 14C-labelled carbon substrates. This procedure was evaluated under different operating conditions, by changing the chemical type (glucose or acetate), and the amount and duration of contact with the tracer substrates. Experimental data support two main conclusions. First, realistic values of the Kec coefficient can be obtained for subsoil microflora as long as they are determined under nutrient conditions similar to those prevailing in the natural environment. Second, a relationship exists between the value of the Kec coefficient and the depth in the soil profile.