Rhizobium Leguminosarum Strain Combination Effects on Nodulation and Biological Nitrogen Fixation in Cover Crop Vicia Villosa.

Hairy Vetch (Vicia villosa) is an important leguminous cover crop used to improve soil fertility in organic cropping systems. The soil bacterium Rhizobium leguminosarum specifically nodulates roots of V. villosa and fixes atmospheric nitrogen for assimilation into plant tissue. Many organic producers inoculate seeds with rhizobia to enhance biological nitrogen fixation, but utilization of efficient inoculant strains is limited by competition from native rhizobia. Improved biological nitrogen fixation in cover crop legumes could enhance nitrogen fertility and reduce fertilizer requirements in organic systems. We evaluated the ability of four rhizobia strains to nodulate and improve V. villosa growth as individual and combined inoculants compared to controls with no rhizobia added. Plants were inoculated with equal ratios of one to four strains and grown under controlled conditions for 45 days. After harvest, biomass, nodule number, nodule mass, and nitrogen content were measured. Rep-PCR generated DNA fingerprints of rhizobia strains found in nodules were used to determine nodule occupancy of each strain. Nodulation was variable across all treatments, with root nodule count ranging from 21 to 197. Mean plant biomass N from BNF of inoculant strains C10 and NCSU478 was 43.4 and 44.2 mg, respectively, and greater than strain NCSU435 which yielded 23.4 mg (p<0.05). Uninoculated controls without N fertilization averaged less plant biomass N than other treatments at 6.3 mg (p<0.001). Results indicate that individual rhizobia strains potentially differ in their ability to improve plant growth, but treatments containing multiple inoculant strains did not improve root nodulation or plant biomass N over single inoculant treatments. Furthermore, nodule occupancy determined by rep-PCR was variable with no dominant nodulating strain within the multiple inoculant strain treatments. Overall, the inoculant R. leguminosarum strain combinations showed no differences in BNF, and did not demonstrate competitive nodulation.