Removal of Fecal Indicator Bacteria from Manured Soils Under Simulated Rainfall Events Spaced in Time.

Information about the release of bacteria from manure under rainfall is obtained predominantly from experiments that used freshly applied manure. In reality, manure may stay on fields for some time and be exposed to multiple rainfall events before being incorporated into the soil. The first rainfall following manure deposition may lead to the release of bacteria both from manure and from the thin mixing layer (0-1cm) on the soil surface where bacteria may accumulate following the first rainfall. It is not known whether the numbers of fecal indicator organisms released during subsequent rainfalls are larger or smaller than the numbers released during the first rainfall. Two phenomena seem to affect subsequent release. Firstly, it was found that E. coli can experience substantial growth in field-applied manure during the first week or two following deposition. Secondly, manure left open to elements ages over time and may become crusted which hampers release1. We designed laboratory experiments to observe both phenomena in detail. Soil-filled boxes were treated with either dairy cow slurry or solid manure and then received simulated rainfall. Runoff and infiltration samples were collected at 5-minute intervals for one hour. Soil boxes were then stored under cover for either one or two weeks before receiving a subsequent rainfall application. Soil cores were taken before and after rainfall at depths of 0-1, 1-2, 2-5, and 5-10 cm and then for one month following the final rainfall event. Preliminary results indicate a near 10-fold reduction in concentrations of bacteria removed in surface runoff in the event following the initial rainfall event. E. coli concentrations in infiltration waters appear similar between the 1st and 2nd release events. Differences in release patterns were observed for slurry and solid manures. However, with both manures, concentrations of E. coli in the soil grew between rainfall events whereas enterococci populations experienced immediate exponential decline. Aging appears to be a factor leading to decreased quantities of bacteria being removed from manured areas despite growth of E. coli in the soil.