Molar concentration of K2SO4 and soil pH affect estimation of extractable C with chloroform fumigation–extraction

Methods of determining soil microbial biomass need to be reliable and produce consistent results across soils with a wide range of properties. We investigated the effect of extractant molarity (distilled water and 0.001, 0.01, 0.1, and 0.5 M K2SO4) on the flush of C (i.e. the difference between fumigated and unfumigated subsamples) with the chloroform fumigation–extraction method in soils of different pH. Extraction efficiency of 0.5 M K2SO4 relative to water was dependent upon soil pH. The ratio of extractable C in water to that in 0.5 M K2SO4 for five acidic soils was 1.5±0.3 in unfumigated controls, 1.4±0.2 in fumigated samples, and 1.8±0.7 in fumigated minus control flushes, respectively. Ratios in six alkaline soils were 1.0±0.2, 0.9±0.2, and 0.8±0.2, respectively. Flocculation/dispersion of organic colloids and changes in the diffuse double layer surrounding clay particles are possible reasons for differences in extractable C with changes in extractant molarity and soil pH. Chloroform fumigation–extraction with any of the extractants was less related to soil organic C and potential C and N mineralization during 50 days of incubation (r2=0.51±0.11) than was chloroform fumigation–incubation without subtraction of a control (r2=0.74±0.08). Changes in microbial biomass estimates with changes in extractant molarity and soil pH suggest that chloroform fumigation–extraction may not be reliable in a wide range of soils.