Anaerobic soil disinfestation, amendment-type, and irrigation regimen influence Salmonella survival and die-off in agricultural soils

Abstract

Aims: This study investigated Salmonella concentrations following combinations of horticultural practices including anaerobic soil disinfestation (ASD), soil amendment type and irrigation regimen.

Methods and results: Sandy-loam soil was inoculated with a five-serovar Salmonella cocktail (5.5 ± 0.2 log CFU per gram) and subjected to one of six treatments: (i) no soil amendment, ASD (ASD control), (ii) no soil amendment, no-ASD (non-ASD control) and (iii-vi) soil amended with pelletized poultry litter, rye, rapeseed or hairy vetch with ASD. The effect of irrigation regimen was determined by collecting samples 3 and 7 days after irrigation. Twenty-five-gram soil samples were collected pre-ASD, post-soil saturation (i.e. ASD-process), and at 14 time-points post-ASD, and Salmonella levels enumerated. Log-linear models examined the effect of amendment type and irrigation regimen on Salmonella die-off during and post-ASD. During ASD, Salmonella concentrations significantly decreased in all treatments (range: -0.2 to -2.7 log CFU per gram), albeit the smallest decrease (-0.2 log CFU per gram observed in the pelletized poultry litter) was of negligible magnitude. Salmonella die-off rates varied by amendment with an average post-ASD rate of -0.05 log CFU per gram day (CI = -0.05, -0.04). Salmonella concentrations remained highest over the 42 days post-ASD in pelletized poultry litter, followed by rapeseed, and hairy vetch treatments. Findings suggested ASD was not able to eliminate Salmonella in soil, and certain soil amendments facilitated enhanced Salmonella survival. Salmonella serovar distribution differed by treatment with pelletized poultry litter supporting S. Newport survival, compared with other serovars. Irrigation appeared to assist Salmonella survival with concentrations being 0.14 log CFU per gram (CI = 0.05, 0.23) greater 3 days, compared with 7 days post-irrigation.

Conclusions: ASD does not eliminate Salmonella in soil, and may in fact, depending on the soil amendment used, facilitate Salmonella survival.

Significance and impact of the study: Synergistic and antagonistic effects on food safety hazards of implementing horticultural practices should be considered.

Keywords: Salmonella; anaerobic soil disinfection; die-off; fumigation; horticulture; irrigation; preharvest; soil amendment.

Figure 1.

Change in observed Salmonella concentration (A) during each step of ASD (pre-ASD, post-soil saturation, post-ASD at d 0) and (B) between 0 and 42 days post-ASD (0, 1, 2, 3, 7, 10, 14, 21, 24, 28, 31, 35, 38 and 42 d post-ASD) for each treatment [red = ASD, non-amended; Pink = non-ASD, non-amended; blue = ASD, rye amended; teal = ASD, rapeseed amended; yellow = ASD, hairy vetch amended; green = ASD, pelletized poultry litter amended soil].The post-ASD plot (B) shows Loess smoothed trendlines with 95% confidence intervals (gray shading). n = 24 for each treatment and time-point.

Figure 2.

Prevalence of each inoculation strain at each time point (A) and in each treatment (B) [AC = ASD, non-amended; NAC = non-ASD, non-amended; RY = ASD, rye amended; RS = ASD, rapeseed amended; HV = ASD, hairy vetch amended; PL = ASD, pelletized poultry litter amended soil]. Serovars: Braendrup (red), Javiana (yellow), Newport (teal), Poona (blue) and Saintpaul (pink).