Measuring microbial biomass carbon by direct extraction – Comparison with chloroform fumigation-extraction

Soil microbial biomass, a small and highly dynamic organic matter pool, plays a critical role in soil fertility. Therefore it is important to have an accurate and rapid method to measure microbial biomass carbon (C). The chloroform fumigation extraction (CFE) method is used by most researchers, but it is quite time-consuming. The direct extraction method where the chloroform exposure and extraction steps are combined is quicker but not often used because it is not clear if it is as reliable as the CFE method. Using 20 Australian soils with a wide range in soil properties, we measured microbial biomass C with the CFE and the direct method. Chloroform labile C extracted by the two methods was correlated (r = 0.87), but it was significantly (p < 0.05) higher with the direct extraction method compared to CFE. Chloroform labile C extracted by both methods was significantly (p < 0.05) positively correlated with clay content, but the correlation coefficient was higher with the direct extraction method. The coefficient of variation for chloroform labile C was greater with the CFE than with the direct extraction method. Chloroform labile C extracted by the direct extraction method did not change between 0.5 and 4 h of shaking with K2SO4 solution and chloroform. We conclude that the chloroform labile C concentrations measured with the CFE method are comparable with those determined by the direct extraction method which is quicker and has a lower variability among replicates.

► Soil microbial biomass C was measured with the chloroform fumigation extraction (CFE) and the direct extraction method.
► The chloroform labile C concentration was higher with the direct method.
► The correlation coefficient between chloroform labile C and clay was higher with the direct method.
► The coefficient of variation for chloroform labile C among replicates was higher with the CFE method.
► The direct extraction method requires less time than the CFE method.