Bacterial composition in the rhizosphere of nodulating and non-nodulating soybean

Little is known about the particular species of bacteria that are favored for growth in the rhizosphere. Obtaining this information will facilitate a better understanding of root colonization and competitiveness of rhizosphere organisms. We used fatty acid methyl ester (FAME) analysis to identify bacteria in two different soils and in the rhizosphere of nodulating and non-nodulating 'Lee' soybean [Glycine max (L.) Merr.]. In a light-room experiment, bacterial isolates from each soil and soybean rhizosphere were selected at 3 (emergence), 6 (V1 stage) and 15 (V2-V3 stage) d after planting (DAP). Of 1131 bacterial isolates examined, 572 (50.6%) were identified. The majority of the identified isolates were Bacillus (324 isolates), Pseudomonas (121 isolates), and Arthrobacter (48 isolates). Of the 49 total bacterial species identified, 29 were detected in only one soil or the other. Numbers of Bacillus spp. were lower in the rhizosphere of nodulating and non-nodulating soybean than in root-free soil, and these lower numbers were usually offset by higher numbers of Gram-negative bacteria, especially Pseudomonas spp. Principal components analysis showed a distinct clustering in each of the two soils. The percentages of isolates identified from each habitat, and the clusters derived from the FAME analysis, showed differences between soybean rhizosphere and root-free soil and, to a Lesser extent, between nodulating and non-nodulating soybean rhizosphere. These results suggest that each soil offered specific populations of bacteria and from these populations, soybean roots favored growth of certain bacteria. To a lesser extent, this specificity was also apparent between nodulating and nonnodulating soybean.