Diversity of rhizobia in soybean [Glycine max (Vinton)] nodules varies under organic and conventional management

Legume cultivation is an important nitrogen source for organic farmers, yet we understand little about how organic agriculture affects rhizobia diversity in the field. We investigated how the suite of practices common in certified organic management affect diversity of rhizobia occupying soybean nodules in the field. We used molecular analysis to evaluate genetic diversity and taxonomic relationships among rhizobia strains across two organically and two conventionally managed fields. Nodule presence of inoculant isolates added at planting was also assessed. We found that the organically managed soybean fields contained a greater diversity of rhizobia isolates than conventionally managed fields, resulting in over 10 clusters containing isolates of over 70% similarity. Each of the two conventional fields was dominated by genetically similar rhizobia by field, with little crossover. One conventional field showed remarkable isolate similarity, with a majority of the isolates being more than 85% similar, and indicating that many of the field isolates were related to the inoculant. In contrast, isolates from organic fields did not group according to farm field from which they were isolated, but instead formed clusters containing a combination of isolates from organically managed soils from both fields, and did not match the inoculant strain genetic composition. This experiment provided evidence that organic management was a strong driver of the rhizobia genotype present in those fields, and further understanding of rhizobia ecology and function as related to specific organic management practices remains a priority.