Remediation and recovery measures to expedite planting or replanting of vegetables following soil contamination by Salmonella enterica.

Producers of fresh fruits and vegetables need practical and sustainable methods to minimize the survival of human pathogens, such as Salmonella, in production soil. Much of the research effort to date has focused on long intervals following manure application and process controls for composting and thermal pasteurization treatments during pelletizing. Despite best intent in setting composting standards, contaminated compost applied to production fields remains a significant problem. Contaminated soil has resulted in hundreds of acres of abandoned crop due to Salmonella in consecutive years, especially with lettuce and salad greens. Remediation and soil recovery treatments are needed to effectively shorten the time interval before replanting of such high value vegetables without fear of losing another crop to preventable sources of contamination. This research, conducted in Australia and California will focus on optimizing the existing knowledge in low residue cover cropping, solarization, and field flooding for remediation of soils contaminated with chicken manure known to harbor Salmonella. The anticipate outcome is a set of grower options for integrated management of contaminated soil that may be extended to other pathogens and sources of contamination such as flooding, domestic animal grazing of crop residues, and large numbers of animal intrusion to croplands.

Producers of fresh fruits and vegetables need practical and sustainable methods to minimize the survival of human pathogens, such as Salmonella, in production soil. Contaminated soil has resulted in hundreds of acres of abandoned crop due to pathogens such as Salmonella enterica, especially with lettuce and salad greens. Destruction of multiple fields per year, occasional hundreds of acres, is damaging economically but also may negate initiatives of sustainability for the operation. The proposed research, conducted in parallel and coordinated efforts in Australia and California, will focus on optimizing existing agricultural practices and knowledge for predicting both inherent characteristic survival and options for remediation of soils contaminated with chicken manure known to harbor Salmonella. The central hypothesis is that single or sequential strategies involving short‐duration, low‐residue cover crops, field or seed‐bed solarization, and flooding will be effective in the practical elimination of residual Salmonella enterica contamination. Soil contamination by Salmonella enterica will be remediated by the allelopathic potential of cover crops, the physicochemical properties and suppressive microbial community that results from the C:N ratio within each system. Replicated 5 x 5 m plots in Year 1 and 2 will be managed to repeat a sequence of (1) chicken manure‐ litter amendment, (2) baby lettuce‐baby spinach crop, (3) ’contaminated’ crop incorporation, (4) 30 d and 45 d cover crop screening, and (5)/replant baby lettuce‐baby spinach. A targeted subset of plant (cover crop) and soil samples will be used to estimate the concentration of glucosinolates and phenolic compounds present at time 0, 30 and 45 days from cover crop planting and 15 days after disking. A similar targeted subset of samples will be used for microbial community analysis using Next Generation Sequencing technologies to describe soil microbial communities. Additional replicated plots will follow a sequence of (1) chicken manure‐litter amendment, (2) baby lettuce/baby spinach crop, (3) ’contaminated’ crop incorporation, (3a) field and bed solarization for 6‐10 weeks or (3b) flooding for 3‐5 weeks, and (4) replant baby lettuce baby spinach. While not the primary focus of the research, we feel it will be critical to determine the potential for the target remediation treatments to elevate populations of Listeria monocytogenes in soil. The primary purpose of the proposed research is, ultimately, to provide alternative solutions for soil remediation of human pathogens that growers across the United States and Australia could use irrespective of the size of the operation. Furthermore, the outcomes of this research will provide science‐based information that could be used to modify current public health regulatory and enforcement laws mandated through the respective country agencies and authorities.

1. Determine the optimal low‐residue cover crop that will enhance die‐off of Salmonella enterica in contrasting soils in Australia and California. 

2. Determine which single or combined cover crop‐solarization interval‐flooding combination will facilitate die‐off of Salmonella enterica in soil so that there is no re‐contamination associated with the re‐planting of leafy greens. 

3. Assess the potential for increase of Listeria sp. and Listeria monocytogenes in cover crop amended soils in laboratory and research (Listeria innocua) and natural (L. monocytogenes) field conditions.

The outcome of the primary objective, cover cropped experimental field trials to assess the effectiveness of treatments on the rate of applied S. enterica die-off, was greatly limited by the accelerated loss of viability in fallow controls, very similar to plots with cover crop residues. Achieving the Expected Measurable Outcomes in relation to developing recommendations for benefits from limited duration crop growth periods for cover crops as a remediation treatment was therefore largely unresolved. In contrast, solarization trial outcomes were more generally satisfactory in that the goals proposed in this project were realized by successfully eliminating S. enterica from contaminated soils prior to the industry standard 60-day no-crop period recommended by Best Practices guidance. However, repetition of solarization trials should be conducted under different field conditions and soil types to develop accurate recommendations for remediation treatments. There are many factors contributing to S. enterica elimination in solarized soil. Thermal inactivation of pathogens is the most important physical mechanism during solarization, in which soil is heated to lethal temperatures for bacteria, plant, soil pathogens, pests, etc. It has been suggested that solarized soil lethal temperatures should be higher than 39–40°C (Stapleton and DeVay, 1995; Wu et al., 2009; D’Addabbo et al., 2010). Wu et al. (2009) successfully inactivated E. coli from soil after 4 weeks of solarization with 40°C or higher temperatures. In-vitro solarization results from the present study are in accord with suggested lethal temperature above 40°C. When soil cups were incubated in temperatures below threshold, S. enterica survival was prolonged for up to 13 and 21 days for 29 and 37°C, respectively, whereas elimination at 55 and 48°C only needed 4 and 96 h, respectively. Based on 2014 solarization results, solarized soil temperature during the summer will be maintained at temperatures above 40°C for 6 h per day. Consequently, 94 consecutive hours at 48°C of in-vitro solarization would represent 16 days of field solarization in above 37°C weather. Polyethylene covers increase soil temperature by trapping solar radiation that passes through the cover and converting it to longer wavelength infrared energy, thus producing a “greenhouse” effect. The highest temperatures in soil profiles are found in the upper 15 cm and decrease with depth to sub-lethal temperatures (Stapleton 2000, D’Addabbo et al., 2010). During solarization trials, temperatures dropped on two occasions, both of them after rainstorm events with cumulative precipitation levels of 21.7 and 11.8 mm, respectively. Thermal soil effects, together with moisture retention and an increase of soluble mineral nutrient availability, have strong implications in soil microflora population shifts. While mesophilic populations rapidly decrease, thermotolerant and thermophilic bacteria thrive and re-colonize the soil (Katan, 1987; Stapleton and DeVay, 1995). A limitation in the execution of this objective was the presence of high populations of American crow birds, which were attracted to feed on the chicken manure pellets and thereby damaged the plastic tarp, letting heat escape. This problem was especially acute in Year 2. Evidence of severe bird damage in half of the solarized plots could explain the increase in population counts at the last time point. Although there was no significant difference (P > 0.05) in soil temperature between damaged and intact plots, heat penetration might not be deep enough to inactivate S. enterica in the soil top layer.