Effects of Cover Crop Species and Season on Population Dynamics of Escherichia coli and Listeria innocua in Soil

Abstract

dCover crops provide several ecosystem services, but their impact on enteric bacterial survival remains unexplored. The influence of cover cropping on foodborne pathogen indicator bacteria was assessed in five cover crop/green manure systems: cereal rye, hairy vetch, crimson clover, hairy vetch-rye and crimson clover-rye mixtures, and bare ground. Cover crop plots were inoculated with Escherichia coli and Listeria innocua in the fall of 2013 and 2014 and tilled into the soil in the spring to form green manure. Soil samples were collected and the bacteria enumerated. Time was a factor for all bacterial populations studied in all fields (P < 0.001). E. coli levels declined when soil temperatures dipped to <5°C and were detected only sporadically the following spring. L. innocua diminished somewhat but persisted, independently of season. In an organic field, the cover crop was a factor for E. coli in year 1 (P ‫؍‬ 0.004) and for L. innocua in year 2 (P ‫؍‬ 0.011). In year 1, E. coli levels were highest in the rye and hairy vetch-rye plots. In year 2, L. innocua levels were higher in hairy vetch-rye (P ‫؍‬ 0.01) and hairy vetch (P ‫؍‬ 0.03) plots than in the rye plot. Bacterial populations grew (P < 0.05) or remained the same 4 weeks after green manure incorporation, although initial reductions in L. innocua numbers were observed after tilling (P < 0.05). Green manure type was a factor only for L. innocua abundance in a transitional field (P < 0.05). Overall, the impacts of cover crops/green manures on bacterial population dynamics in soil varied, being influenced by bacterial species, time from inoculation, soil temperature, rainfall, and tillage; this reveals the need for long-term studies. In agricultural environments, soil can serve as a reservoir and route of transmission for foodborne pathogens (1). Since fresh produce is often consumed raw without a "kill step" to inactivate human-pathogenic microorganisms, it is important to prevent the contamination of these foods during production (2). Growers may implement good agricultural practices (GAPs) to minimize the risk of produce contamination at the preharvest level (3), as this program focuses on on-farm risk factors, including animalbased fertilizers, irrigation water quality, farm worker training and hygiene, and wildlife exclusion. Outbreaks and contamination events, however, continue to occur, emphasizing the need to evaluate the role of other agricultural practices, which thus far have received less attention, on enteric pathogen dynamics on a farm.One such practice is cover cropping, the establishment of a crop, typically a small grain or legume, in between cultivations of a cash crop. In recent years, economic and environmental considerations have renewed interest in this old practice for improving crop productivity and soil health and maintaining the sustainability of agroecosystems (4). Cover cropping brings a variety of ecosystem services to agricultural systems, including seasonal protection of soil from erosion, weed suppression, soil improvement, a...

Persistence of Escherichia coli in the soil of an organic mixed crop‐livestock farm that integrates sheep grazing within vegetable fields

“…Lower mean generic E. coli MPN/g values at the start of sampling for field B (DPS 1 and 7) were most likely less than 1.0 log 10 MPN/g because the sheep manure and residual cover crop were not tilled into the soil until DPS 10. Similar studies have hypothesised that tilling in cover crops in the spring in organic fields may contribute to increased bacteria populations in the soil (Reed‐Jones et al, ). And Hutchison et al () found that delaying incorporation of raw manure into the soil led to significantly faster decline rates for pathogens.…”

“…Weekly farm management practices and environmental parameters were not tracked in detail over the entire period of this current study. However, a more cohesive collection of these parameters, as well as projects conducted throughout different seasons, might illuminate other associations related to the peaks of generic E. coli MPN/g in the soil, as suggested by various authors (Fraser et al, 2013;Gorski et al, 2011;Ongeng et al, 2014;Reed-Jones et al, 2016;Salaheen et al, 2015). Warmer temperatures and precipitation occurring in the spring and summer have been shown in past studies to be correlated with increased incidence and survival of foodborne pathogens in the environment (Fraser et al, 2013;Gorski et al, 2011).…”

“…Foodborne pathogens can be transmitted to humans directly through contact with faeces, or indirectly through contaminated water or produce and they can exist in the soil for extended periods of time (Franz, Schijven, Husman, & Blaak, 2014;Fraser, MacRae, Ogden, Forbes, & Strachan, 2013;Harris et al, 2013;Moriarty, Mackenzie, Karki, & Sinton, 2011;Ongeng et al, 2015). STEC is usually introduced into agricultural environments by livestock and/or wildlife faeces (Jay-Russell, 2013;Reed-Jones, Marine, Everts, & Micallef, 2016). Sheep are asymptomatic reservoirs of STEC and faecal shedding from ruminants may increase during periods of stress (Cooley et al, 2013;Edrington et al, 2009;Moynihan, Richards, Ritz, Tyrrel, & Brennan, 2013;Ongeng et al, 2014;Schilling et al, 2012).…”

Effects of cover crops and tillage in a muskmelon production system

“…In July and Aug. of 2015, no L. innocua was detected in any rye or rye-vetch plots. In contrast, Reed-Jones et al (2016) found that L. innocua in rye and rye-vetch plots can persist for the several weeks after inoculation, while a vetch monoculture and bare ground plots can show an immediate decline. In a different year of the same study, vetch monoculture and rye-vetch plots had significantly higher populations of soilborne L. innocua than rye monoculture plots.…”

“…During the winter in Iowa, soilborne L. innocua was capable of surviving, and was not affected by cover crop management. In Maryland, USA when organic fields were populated with L. innocua in the fall, the bacteria persisted and were recoverable the following spring (Reed-Jones et al, 2016). Contaminated water and animal manure are likely sources of human pathogens in produce fields.…”