Modification of the original chloroform fumigation extraction technique to allow measurement of δ13C of soil microbial biomass carbon

In studies of the soil microbial biomass C by the chloroform fumigation extraction (CFE) technique, biomass C is routinely extracted using 0.5 M K2SO4 solution. The excessive amounts of salts contained in the extracting solution pause a significant challenge in using 13C isotope techniques to study the nature of C in the soil microbial biomass. This is because the salts can affect the oxidation process and therefore hamper accurate mass spectromic analysis of dried extracts. In spite of this, no standard protocol exists for preparing the K2SO4 extracts for 13C isotope analysis. We have modified the original CFE method to allow measurement of the δ13C of soil microbial biomass C by using 2 M KCl instead of the usual 0.5 M K2SO4 solution to extract biomass C. Excess salts were removed by dialysis in 100 molecular weight cut off membranes, after which the extracts were freeze-dried and their δ13C measured using a mass spectrometer. The soil microbial biomass C and δ13C of 2 M KCl extracts were compared with those of 0.5 M K2SO4 extracts. There was excellent agreement between organic C and δ13C estimates for dialyzed 2 M KCl and 0.5 M K2SO4 extracts, but the speed of dialysis for the latter was very slow, making use of the former more rapid. These results suggest that in procedures where oxidation with potassium dichromate is not critical to analysis of soluble C, 2 M KCl may be used in place of 0.5 M K2SO4 to extract soil microbial biomass C for δ13C measurements. The new procedure is relatively easy and rapid for obtaining indices for both pool sizes and turnover rates of soil microbial biomass C and provides a promising approach to study soil organic C.