Hairy Vetch Cultivation History Affects Nodulation and Biological Nitrogen Fixation Across Host Genotypes.

Hairy vetch (Vicia villosa Roth.) cultivation has multiple agronomic benefits, including biological nitrogen fixation (BNF) through mutual symbiosis with Rhizobium leguminosarum biovar viciae (Rlv). The use of legume cover crops can help reduce leaching and runoff losses by providing biologically-fixed N from slowly-decomposing cover crop residue, necessitating improved understanding of agricultural practices on legume-rhizobia interactions. We evaluated the effect of cultivation history on nodulation and BNF of ten hairy vetch cover crop genotypes using soil resident Rlv. Five groups of hairy vetch genotypes were inoculated with soil dilutions from six fields, three with hairy vetch cultivation history (HV+) and three without history (HV-). Nodule number, mass, and total plant nitrogen were assessed. Plants inoculated with HV+ soil dilutions averaged 60% greater nodule number and 70% greater nodule mass.  Such plants also had greater plant biomass and total plant tissue N than those inoculated with soil dilutions from HV- fields, except in one site where no difference in N was found perhaps as a result of rhizobia population mixing between the HV+ and HV- fields. Host genotype also had a significant effect on nodulation, where some genotypes produced mean nodule numbers and mass that were 60% and 70% higher, respectively, when compared to the least nodulated genotypes. Plant biomass and plant tissue nitrogen were linearly correlated to nodule mass (r² = 0.80 and 0.56 respectively). Variation in nodulation and BNF between host genotypes suggests that distinct genotypes may be used to breed cultivars with high symbiotic capacity with resident Rlv populations. Results also suggest that past hairy vetch cultivation may enhance populations of effective rhizobia capable of high nodulation and N fixation.