Diversity of Hairy Vetch (vicia villosa) Cover Crop Rhizobia Symbionts Varies Among Field Inoculated Plants and Naturalized Populations.

Biological nitrogen fixation (BNF) is the major source of new nitrogen entering organic farming systems, and many growers utilize legume cover crops to fix atmospheric N₂ through the plant-rhizobia symbiosis. Many legumes in organic systems are inoculated with commercial rhizobia in order to successfully nodulate and carry out BNF. Naturalized or native rhizobia can be beneficial, or problematic if they compete with inoculant strains and are less efficient in BNF. Our objectives were to characterize rhizobia isolated from inoculated cover crop hairy vetch (Vicia villosa) root nodules and un-inoculated adjacent soils to determine naturalized population diversity and inoculation effectiveness. Four varieties of hairy vetch were planted in three sites across North Carolina and inoculated with commercial inoculants. After 4 months of growth nodules were collected from field-inoculated plants and cultured. Rhizobia populations from non-inoculated adjacent soils and from commercial inoculants were trapped using seedlings grown in native soil or sterile growth pouches, respectively, and nodules removed and cultured. PCR amplification of the nifH gene region followed by digestion with restriction endonucleoases confirmed rhizobia identity. PCR of repetitive regions of the DNA (rep-PCR) for all cultured rhizobia using the BOX A1R primer was carried out and genomic patterns compared using the Bionumerics software package. Nodule numbers on non-inoculated plants differed significantly among field soils. No differences were found between rhizobia isolated from the four varieties of hairy vetch. Rhizobia occupancy varied among sites, with DNA of naturalized populations from soils diverging from that of rhizobia isolated from inoculated plants found at the same site. Inoculant strains were genetically similar to each other, but at only one site did inoculated plant rhizobia DNA cluster tightly with inoculant strains. Results suggesting ecological complexity of hairy vetch rhizobia populations warrant continued investigation of rhizobia management and function for improved nitrogen-fixation in organic systems.