Manure Consistency and Weathering Affect the Removal of Fecal Indicator Bacteria from Grass-Covered Soil

Abstract

Animal manures contain bacteria which may be used to indicate the potential microbial pollution of irrigation and recreation water sources. The objective of this work was to research how the release of these indicators to runoff is affected by the concurrent influence of the manure consistency and the duration of manure weathering between rainfall events. Liquid and solid dairy manure was applied on grassed soil boxes. The percentage of manure-borne bacteria removed in runoff over 1 hour was determined after the initial rainfall immediately after manure application and after rainfalls that occurred 1 week and 2 weeks after the initial rainfall. The runoff delay was linearly related to the time between rainfalls and was significantly affected by both weathering duration and manure consistency (P = 0.017 and 0.038, respectively). The runoff delay was dependent on the antecedent gravimetric soil water content (R2 = 0.92 and 0.99 for liquid and solid manure treatments, respectively). Manure weathering caused a substantial decrease in the removal of fecal bacteria with runoff in the 2-week weathering compared with the 1-week weathering treatments for both of E. coli and enterococci (P = 0.063 and 0.049, respectively). Every day of weathering decreased the percentage of the released E. coli and enterococci by about 1.2 and 1.5 times, respectively, for both manure consistencies. Manure consistency was not found to significantly affect the percentages of E. coli or enterococci removed with runoff for any level of weathering. Results indicate the need of correcting existing manure bacteria removal models by accounting for manure consistency and weathering duration.