ARS | Publication request: SOIL C EXTRACTED WITH WATER OR K2SO4: PH EFFECT ON DETERMINATION OF MICROBIAL BIOMASS?

Submitted to: Canadian Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: June 25, 1999
Publication Date: N/A

Interpretive Summary: Soil microbial biomass is the biological agent responsible for decomposing organic amendments and cycling of nutrients in soil. Microbial biomass measurement techniques are numerous, but rely on indirect biochemical procedures that are not specific. One common approach is to fumigate soil with chloroform to kill microorganisms and then extract the primary element tof cell constituents (i.e., carbon) to determine the quantity of microorganisms present. We observed that the commonly used extractant of 0.5 M K2SO4 released much smaller amounts of carbon into solution than water at low soil pH, but similar amounts at high soil pH. Our results put into question the widespread adaptability of using chloroform fumigation-extraction to estimate microbial biomass carbon. Much more work is needed to test the validity of this method across a wide range of soils.

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