Submitted to: Journal of Environmental Science and Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2010
Publication Date: 7/1/2010
Publication URL: http://www.ars.usda.gov/SP2UserFiles/Place/53102000/pdf_pubs/P2337.pdf
Citation: Ibekwe, A.M., Papiernik, S.K., Yang, C. 2010. Influence of Soil Fumigation by Methyl Bromide and Methyl Iodide on Rhizosphere and Phyllosphere Microbial Community Structure. Journal of Environmental Science and Health. 45(5):427-436. Interpretive Summary: Soil fumigation maybe a common practice used for leafy green production in certain parts of the country, but in others, different soil management practices may be used. Soil fumigation reduces the survival of many soil borne pathogens, but little research has been done on the effects of fumigants on total microbial composition on leafy green vegetables planted on fumigated soils. Our study showed that the effects of fumigants on root and leaf microbial composition were very little, since it took about two to three weeks from fumigation to planting. This study also showed that there were significant increases in bacterial groups with plant age in both the control and the fumigated treatments, confirming that the two fumigants had little impact on root and leaf surface bacterial populations. The findings from this study will be used by growers of leafy greens to manage their preplant fumigation practices since the practice may not have long term effects on soils used for the production of leafy greens.
Technical Abstract: Rhizosphere and phyllosphere microbial communities were evaluated on roots and leaves of growth chamber-grown lettuce (Lactuca sativa (L.) cv. Green Forest) plants by culture-dependent and -independent methods after soil fumigation. Denaturing gradient gel electrophoresis (DGGE) with 16S rRNA primers followed by cloning and sequencing was used to identify major rRNA bands from the rhizosphere and phyllosphere. Three weeks after fumigation, there were no differences (P=0.16) in rhizosphere microbial communities between the fumigated treatments and the control. The same effect was observed during week seven after fumigation (P=0.49). Also, no significant differences (P=0.49) were found in the phyllosphere microbial communities between the fumigated treatments and the control during the growth period of the plant. A majority of the bands in the rhizosphere were related to known bacterial sequences with a 96 to 100 % sequence similarity. Some of the derived sequences were related to Pseudomonas syringae pv. tomato DC300 and Bradyrhizobium japonicum USDA 110. A total of 23 isolates were identified from leaf surface by both culturedependent and independent methods, and only Photorhabdus luminescens was found on leaf surface using both techniques. All the Biolog isolates from phyllosphere were from the Proteobacteria phylum compared to the culture-independent bands from the leaves that were from different bacterial phyla. Based on our data, methyl bromide (MeBr) and methyl iodide (MeI) did not have any significant negative effects on rhizosphere and phyllosphere microbial communities throughout the growing period of lettuce.