Frequency of soybean in rotation and persistence of Rhizobia in Manitoba soils

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Ordonez, Patricia Ann C.
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Soybean (Glycine max), along with canola and wheat, are some of Canada’s top agricultural exports. It is also one of the few crops that does not require industrial nitrogen fertilizer. Soybean gets nitrogen from forming a symbiotic relationship with a soil bacterium called Bradyrhizobium japonicum. B. japonicum is not native in Manitoba soils and must be inoculated into soybean fields. The objectives of this thesis are to examine the persistence of the B. japonicum inoculant in the soil, as well as to observe whether varying the crop rotation will affect its abundance. The overall bacterial communities will also be examined for compositional changes between crop rotation treatments (continuous soybean, canola–soybean, corn–soybean, diversified), timepoints (before planting, emergence, pod fill, and full maturity), and sites (Carman, Kelburn, Melita). The bacterial communities were analyzed using 16S rRNA sequencing, and B. japonicum quantification in the soil was measured using qPCR. B. japonicum was shown to persist in the soil years after the initial inoculation. It was also observed that there are native species of Bradyrhizobium present in Manitoba soils that cannot nodulate soybean. The crop rotation effect on B. japonicum, as well as the bacterial community, was minimal. The principal reason for observed differences seem to be the site locations and their soil properties, such as soil type and pH. Carman had significantly higher bacterial diversity, as measured by the Shannon index, than Kelburn and Melita. However, in all three locations, the majority of the bacteria were from three phyla: Proteobacteria, Actinobacteria, and Acidobacteria. It is the subdivisions within the phyla that varied greatly depending on the location. Observations made in this study can lead to a better understanding of the complex plant–microbe and microbe–microbe relationships for future research and applications.
Rhizobia, Soil microbiome, Soybean, Bradyrhizobium japonicum