Association of Fecal Coliforms With Soil Aggregates

Manure-borne microorganisms and organic particles can move in soils in large interconnected pores or in long macropores. These pathways in structured soils are located between soil aggregates or peds. Therefore, interaction with soil aggregates may substantially affect the ability of macropores to serve as bacterial conduits. The study tested the hypothesis that fecal coliform (FC) association with soil aggregates is affected by aggregate size, water content, and bovine manure application. Tyler loam soil aggregates were separated into three fractions. Air-dried and water-saturated aggregates were submerged in water-FC and water-manure-FC suspensions with concentrations of 103, 104, or 105 CFU mL−1 for 24 h. The maximum association of FC with aggregates was observed in the experiments with air-dried aggregates in the water-FC suspensions; no measurable FC amount was associated with the saturated aggregates in the same suspension. In the water-manure-FC suspension, about 2.5 times more FC were associated with the air-dried soil aggregates than with the water-saturated aggregates. The FC amount associated with air-dried aggregates in the water-manure-FC suspension was about 300 times less compared with the amount in the water-FC suspension. The FC association with the aggregates was not affected by aggregate size. Increased water content of soil aggregates and presence of manure in water-FC suspensions decreased FC association with soil aggregates. Because FC transport in soil generally occurs through interaggregate pores after rainfalls after manure applications, a decrease in bacteria-soil association with aggregates can enhance bacterial mobility and increase risk of groundwater contamination.