Attachment of Escherichia coli to Soil Aggregates as Affected by Aggregate Water Content and Presence of Manure Constituents

Many soils contain relatively large structural units that do not slack when soil is being wetted. Soil aggregates, obtained from dry soil samples by sieving, present a model media to study the interactions of intact soils with dissolved or suspended contaminants. Land-applied manures may contain various contaminants that cause water pollution and concomitant health problems. Some of these pollutants are bacteria, and Echerichia coli is widely used as an indicator of bacterial contamination. The objective of this work was to test the hypothesis that Echerichia coli attachment to soil aggregates is affected by aggregate size, aggregate water content, and presence of suspended manure colloids and dissolved organic compounds. Three aggregate fractions of 3.4-4.8 mm, 4.8-7.9 mm and 7.9-9.5 mm diameters were obtained by dry sieving of a loam soil. Air-dry and water-saturated aggregates were submerged in bacteria-water and bacteria-manure suspensions at four E. coli concentrations for 24 h. Amounts of attached E. coli were calculated from the difference between the amount applied and the amount remained in the suspension. Significant differences in E. coli attachment to air-dry and saturated aggregates were found. Both increase in water content and the presence of manure significantly decreased the Echerichia coli attachment to all aggregate fractions regardless of the aggregate size. Because E. coli transport in soil generally occurs through large pores between structural units when rainfall follows manure application, the decrease in bacteria attachment as a result of soil saturation and presence of suspended or dissolved manure components can enhance bacterial mobility and increase risk of ground water contamination.