MODIFIED CALIBRATION PROCEDURE FOR THE MEASUREMENT OF MICROBIAL SULFUR IN SOIL

To better estimate S biomass in soils, an alternative calibration technique for the measurement of microbial biomass S in soil was developed. Rather than using pure cultures of bacteria and fungi to determine the biomass-S calibration constant (K(s)), cultures of bacteria and fungi were grown using soil dilutions as primary inoculum. These microorganisms were harvested and washed, and dense suspensions of bacteria or fungi (in 0.15 M NaCl matrix solution) were prepared. The suspensions were used as inoculum for the calibration procedure, where bacteria or fungi were each grown at three S concentrations (1, 4, or 16 mg SO4-S L-1) and subsequently added to soil and fumigated with CHCl3. The mean value of 39.3% was calculated for the proportion of microbial S extracted with calcium phosphate [8 mM Ca(H2PO4)2] from a Woodburn silt loam (a fine-silty, mixed, mesic Aquultic Argixeroll) following CHCl3 fumigation. This procedure may better reflect the soil microbial biomass than using pure cultures, because soils have mixed microbial populations.