Depth-dependent inactivation of Escherichia coli and Enterococcus faecalis in soil after manure application and simulated rainfall

Authors: Stocker (ctr), Matthew; Pachepsky, Yakov; HILL, ROBERT - University Of Maryland; Shelton, Daniel

Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/2/2015
Publication Date: 5/2/2015
Citation: Stocker, M.D., Pachepsky, Y.A., Hill, R., Shelton, D.R. 2015. Depth-dependent inactivation of Escherichia coli and Enterococcus faecalis in soil after manure application and simulated rainfall. Applied and Environmental Microbiology. 81(14):4801-4808.

Interpretive Summary: E.coli and Enterococcus serve as important microbial water quality indicator organisms. Rainfall action on manured fields and pastures releases these organisms into soil with infiltrating water. They can then be released back to runoff during subsequent rainfall or irrigation events as soil solution interacts with runoff. Therefore, fate and transport of indicator organisms in surface soil layers is of substantial interest; yet relatively little quantitative information is available for solid manures. The objective of this study was to evaluate the survival of both E. coli and Enterococcus in the near-surface layer of soil after the microorganisms were released from solid manure under simulated rainfall. We found that most of released organisms were in a very thin (1 cm) layer near the soil surface. The population of E. coli grew substantially during the first week after rainfall, independent of the amount of rainfall, followed by a very slow decline. In contrast, enterococci populations declined precipitously after the initial rainfall. The results of this work will be useful to environmental professionals involved in microbial water quality monitoring in that it provides a scientific basis for sampling soils for indicator microorganisms, and interpretation of their occurrence in surface waters used for recreation and irrigation.

Technical Abstract: E.coli and Enterococcus serve as important water quality indicator organisms. Rainfall action on manured fields and pastures releases these organisms into soil with infiltrating water. They can then be released back to runoff during subsequent rainfall or irrigation events as soil solution interacts with runoff and fine soil particles become suspended. Therefore fate and transport of indicator organisms in surface soil layers is of substantial interest, and yet relatively little quantitative information is available on this matter. The objective of this study was to evaluate the survival of both E.coli and Enterococcus in the near-surface layer of soil that the organisms were released into from manure under simulated rainfall. Manure application and rainfall simulations were performed in triplicate on 18 soil-filled boxes with grass cover for one hour with rainfall depths of 30, 60, and 90 mm from simulated rainstorm events. The boxes were then kept in a hoop house at an average daily temperature of 16C for one month and watered weekly with 7 mm of water. Soil samples were collected weekly from three different locations across the box in the depth ranges 0-1, 1-2, 2-5, and 5-10 cm. While total numbers of enterococci declined over time in the soil at all three rainfall intensity treatments, E.coli experienced an initial growth stage during the first week in the soil and then began to gradually decline. Rainfall intensity was found to have a significant impact on initial concentrations of fecal indicator bacteria in the soil. Proportions of indicator bacteria were greater in the top 0 – 1 cm depth than in the three lower depths combined in most observations. Results of this work indicate the need to revisit the bacteria survival patterns that are assumed in water quality models and recommendations of manure application for irrigated crops.