Onions are valued as food and medicine primarily for the flavor and medicinal properties of their sulfur compounds. A field study was conducted at KSU Research Farm. The soil in five plots was mixed with sewage sludge, five plots were mixed with yard waste compost, five plots were mixed with laying hen manure each at 15t acre-1, and five unamended plots that never received soil amendments were used for comparison purposes. Plots were planted with onion, Allium cepa L. var. Super Star-F1. The objectives of this investigation were to: i) determine the concentrations of organosulfur compounds (dipropyl disulfide and dipropyl trisulfide) in onion bulbs and ii) investigate the effect of mixing soil with three amendments (sewage sludge, yard waste, and chicken manure) on the concentration of sulfur compounds in onion bulbs. Gas chromatographic/mass spetrometric (GC/MS) analyses of onion oil in chloroform extracts revealed the presence of two major fragment ions that correspond to dipropyl disulfide and -trisulfide. Concentration of these two organic sulfur compounds was greatest (1.5 and 0.8 mg 100 g-1 fresh weight, respectively) in onion bulbs of plants grown in chicken manure and lowest (0.4 and 0.07 mg 100 g-1 fresh weight, respectively) in onion bulbs of plants grown in yard waste compost treatments. We concluded that chicken manure could be exploited in growing onions with health-promoting properties.
Why Study Chicken Manure and Onions?
The objectives of this investigation were to: i) determine the concentrations of organosulfur compounds (dipropyl disulfide and dipropyl trisulfide) in onion bulbs and ii) investigate the effect of mixing soil with three amendments (sewage sludge, yard waste, and chicken manure) on the concentration of sulfur compounds in onion bulbs.
What Did We Do?
A field study was conducted on a Lowell silty-loam soil (2.0% organic matter, pH 7) at Kentucky State University Research Farm, Franklin County, KY. Plots (n=20) of 2 ×10m each were separated using grass strips to prevent cross contamination between adjacent treatments. The soil in five plots was mixed with sewage sludge (SS) from Metropolitan Sewer District, Louisville, KY at 15 t acre-1 (on dry weight basis). Five plots were mixed yard waste (YW) compost made from yard and lawn trimmings, and vegetable remains (Con Robinson Co., Lexington, KY) at 15 t acre-1 (on dry weight basis), and five plots were mixed with chicken manure (CM) obtained from University of Kentucky Poultry Research Facility, Lexington, KY at 15 t acre-1. The native soil in five plots was used as a no-mulch (NM) control treatment (roto-tilled bare soil) for comparison purposes. Amendments were incorporated into the topsoil with a plowing depth of 15 cm. Seedlings of onion, Allium cepa var. Super Star F1 were planted under the four soil management practices. At harvest, onion bulbs were blended with chloroform. After cleaning-up the extract, one μL (n=3) of the filtrate was injected into a gas chromatograph (GC) model 5890A equipped with a mass spectrometric detector (GC/MSD) model 5971A operated in total ion monitoring with electron impact ionization (EI) mode and 70 eV electron energy for identification and confirmation of individual peak. Concentrations of the two sulfur compounds (dipropyl disulfide and dipropyl trisulfide) in onion bulbs grown under four soil management practices were statistically analyzed using ANOVA. Means were compared using Duncan’s multiple range test.
What Have We Learned?
Mass spectrometric analysis of purified chloroform extracts prepared from onion bulbs revealed the presence of two organosulfur compounds, dipropyl disulfide and dipropyl trisulfide (Figure 1). Analyses of onion bulbs of plants grown under three soil management practices and plants grown in no-mulch treatments showed fragments with identical molecular ions at m/z 150 (Figure 2) and at m/z 182 (Figure 3), along with other characteristic fragment ions which are consistent with the assignment of the molecular formula of dipropyl disulfide (C6H14 S2) and dipropyl trisulfide (C6H14 S3), respectively, known as major constituents of onion bulbs. Dipropyl disulfide spectral data, which showed a molecular ion peak (M+) at m/z 150, were in agreement with those previously reported in onion bulbs.  Dipropyl trisulfide was also assigned on the basis of its mass fragmentation pattern, which gave a molecular ion peak (M+) at m/z 182, along with other characteristic fragment ion peaks as determined by GC/MSD in total ion mode. Figures 2 and 3 also indicated that the two sulfur compounds have a common fragment ion at m/z 48 that can be used for monitoring dipropyl disulfide and –trisulfide in onion extracts. Concentrations of dipropyl disulfide and dipropyl trisulfide were significantly higher (P< 0.05) in onion bulbs of plants grown in soil amended with chicken manure compared to other soil treatments (Figure 4). This response may be due to improved soil porosity, soil water holding capacity and nutrient retention in chicken manure treated soil. There were about 1.5 mg of the disulfide and 0.8 mg of the trisulfide per 100 g onion bulb. These results are in agreement with the results reported by the onion oil manufacturer and wholesaler in India (Shiva Exports India-2007),  who reported that out of all the compounds present in onion, dipropyl disulfide comprises the largest amount of the sulfur compounds present in onion oil.
Chicken manure used in this study was obtained from laying hens fed diets containing 19% crude protein and approximately 0.8% sulfur amino acids, which provided approximately 0.2% sulfur to the diet. Other minor sources of sulfur in the diet were sulfate salts of several trace minerals. The diets were formulated to meet or exceed the requirements published by the National Research Council  and commercial feeding recommendations. To the best of our knowledge, no reports in the literature have documented the importance and value of chicken manure in increasing organic sulfur compounds, having health-promoting activity. >
Figure 1. Gas chromatographic-mass spectrometric (GC/MS) chromatogram of onion, Allium cepa L. var. Super Star-F1 extract prepared in chloroform.
Figure 2. Electron impact mass spectrum of dipropyl disulfide (C6H14S2) extracted from onion, Allium cepa L. var. Super Star F1 bulbs indicating the molecular ion m/z 150.
Figure 3. Electron impact mass spectrum of dipropyl trisulfide (C6H14S3) extracted from onion, Allium cepa L. var. Super Star F1 bulbs indicating the molecular ion m/z 182, along with other characteristic fragments ions.
Figure 4.Concentrations of dipropyl disulfide (upper graph) and dipropyl trisulfide (lower graph) in onion bulbs of plants grown in soil amended with chicken manure (CM), sewage sludge (SS), yard waste (YW), and no-mulch (NM) soil. Statistical comparisons were carried out between soil treatments. Bars accompanied by different letter are significantly different (P< 0.05) from each other, using Duncan’s multiple range test.
Future objectives will include the impact of soil amendments on the nutritional composition of field-grown vegetables and monitoring bioaccumulation of trace-elements in edible plants.
Eric T. Turley, Co-Investigator, Kentucky State University-College of Agriculture, Food Science, and Sustainable Systems- Division of Environmental Studies and Sustainable Systems, Frankfort, KY 40601, USA, email@example.com
Regina R. Hill, Research Assistant, Kentucky State University-College of Agriculture, Food Science, and Sustainable Systems- Division of Environmental Studies and Sustainable Systems, Frankfort, KY 40601, USA
George F. Antonious, Professor, Kentucky State University –College of Agriculture, Food Science, and Sustainable Systems- Division of Environmental Studies and Sustainable Systems, Frankfort, KY 40601, USA
We thank Jami Rogers for her assistance in preparing onion extracts. This investigation was supported by a grant from USDA/CSREES to Kentucky State University under agreement No KYX-10-08-43P.
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