Soil C extracted with water or K2SO4:: pH effect on determination of microbial biomass

Routine determination of soil microbial biomass C has shifted during the past decade from chloroform fumigation-incubation to chloroform fumigation-extraction using 0.5 M K2SO4 as extractant. We compared extractable C with water and 0.5 M K2SO4 in eight soils ranging in pH from 5.4 to 8.3. In unfumigated soils with low pH, extractable C was 0.8- to 1.2-fold greater with water than with 0.5 M K2SO4. However, in unfumigated soils with pH > 7.7, extractable C, although not statistically significant, was 11 to 19% less with water than with 0.5 M K2SO4. In fumigated soils, no difference in extractable C between water and 0.5 M K2SO4 was detected among soils with pH < 7.7, but extractable C was 13 to 17% less with water than with 0.5 M K2SO4 with pH > 7.7. Our results suggest that 0.5 M K2SO4 (1) may flocculate soil and cause adsorption of solubilized C onto colloids at pH < 7.7, but (2) may disperse calcareous soils at pH > 7.7, thereby differentially affecting the fate of solubilized C depending upon soil pH. Our results put into question the widespread adaptability of using chloroform fumigation-extraction to estimate microbial biomass C.