A dispersion and differential centrifugation technique for representatively sampling microorganisms from soil

A multi-stage dispersion and differential centrifugation technique for repesentatively sampling non-filamentous microorganisms from soil was developed. The individual steps of the method aimed to disperse soil aggregates and dissociate microorganisms from soil particles. The released microorganisms were then separated by low-speed centrifugation. The dispersion and dissociation efficiency of the individual steps and the method as a whole were tested turbidometrically using a clay loam, a sandy loam and a peat soil. The evaluation indicated that individual steps rarely achieved more than 50% dispersion and. that the sandy loam was more readily dispersed than the other two soils and that the peat soil was particularly difficult to disperse. The multi-stage method was very efficient at dispersing all three soils. The efficiency of the method at sampling microorganisms was assessed by determining the biomass recovered in extracts and residues compared to the soil at the outset. Biomass was determined using direct microscopic cell counts, ATP, phospholipid, lipopolysaccharide, ergoslerol contents and viable counts. The method yielded extracts which were enriched with microorganisms as determined by direct microscopic counts, ATP, phospholipid and lipopolysaccharidc contents and residues enriched with the fungal biomarker, ergosterol. The sandy loam soil provided samples of microorganisms that were the least contaminated with soil material. Difficulties in separating either the smaller mineral particles or the less dense organic particles resulted in lower recoveries from the clay and peat soils. Density gradient ccntrifugution of the soil extracts indicated that a large number of the cells counted by direct microscopy in the extracts were associated with relatively dense soil material.