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 produ ce consistent results across soils with a wide range of properties. We inve stigated 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 fumi gated and unfumigated subsamples) with the chloroform fumigation-extraction method in soils of different pH. Extraction efficiency of 0.5 M K2SO4 rela tive to water was dependent upon soil pH. The ratio of extractable C in wat er to that in 0.5 M K2SO4 for five acidic soils was 1.5 +/- 0.3 in unfumiga ted controls, 1.4 +/- 0.2 in fumigated samples, and 1.8 +/- 0.7 in fumigate d 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/dispers ion 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-extractio n with any of the extractants was less related to soil organic C and potent ial C and N mineralization during 50 days of incubation (r(2) = 0.51 +/- 0. 11) than was chloroform fumigation-incubation without subtraction of a cont rol (r(2) = 0.74 +/- 0.08). Changes in microbial biomass estimates with cha nges in extractant molarity and soil pH suggest that chloroform fumigation- extraction may not be reliable in a wide range of soils. (C) 2001 Elsevier Science Ltd. All rights reserved.