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Modified Calibration Procedure for the Measurement of Microbial Sulfur in Soil

Dep. of Soil Science, 202 Strand Agriculture Hall, Oregon State Univ., Corvallis, OR 97331

* Corresponding author.

ABSTRACT

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.

Oregon Agric. Exp. Stn. Journal no. 9135.

Received for publication January 31, 1990.

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