Organic carbon extraction efficiency in chloroform fumigated and non-fumigated soils

The chloroform-fumigation extraction (CFE) method is widely used to estimate soil microbial biomass. The relationship between extraction efficiency and the extractant-to-soil ratio has not been fully investigated in chloroform fumigated and non-fumigated soils. We analyzed the extraction efficiency of 0.5 M K₂SO₄ in five soils at six extractant-to-moist soil ratios (3:1, 5:1, 7.5:1, 10:1, 12.5:1, and 15:1). A positive relationship between extractant-to-soil ratio and the amount of extracted dissolved organic carbon (DOC) was observed for both non-fumigated and fumigated samples. The extractant-to-dry soil ratio required for 90% extraction averaged 12.0 for non-fumigated samples and 8.2 for fumigated samples. A minor, yet statistically significant, positive relationship between extractant-to-dry soil ratio and chloroform-labile carbon was observed in four of five soils, with a 5.6 average extractant-to-dry soil ratio required for 90% extraction. For analyses where DOC extracted from non-fumigated samples is a critical variable, an extractant-to-soil ratio greater than 5:1 should be considered.