Temperature and host plant ecotype drive nitrogen fixation, but not nodule community composition, in hairy

Hairy vetch (Vicia villosa Roth) is a commonly grown cover crop throughout the U.S., which can contribute nitrogen for subsequent cash crops through biological nitrogen fixation (BNF) in association with Rhizobium leguminosarum biovar viciae (Rlv) bacteria. Hairy vetch is one of the few cover crops sufficiently cold-tolerant to over-winter in the Upper Midwestern U.S. However, nitrogen contributions by hairy vetch vary across locations, potentially due to cold impacts on the legume/rhizobia symbiosis. The traditional route to improve BNF in legumes involves selecting superior rhizobia strains to create more effective inoculants to apply at planting, but inoculants often fail to compete and survive in agricultural soils. Instead, this study tested the effects of temperature and host plant ecotype on hairy vetch BNF and Rlv community composition in nodules, with the goal of potentially identifying vetch ecotypes able to select beneficial Rlv strains from the soil community. Four hairy vetch ecotypes trapped Rlv from three Minnesota soils, at warm or cold temperatures. Vetch ecotype was a key driver of BNF and nodule formation under warm and cold conditions. However, temperature and plant ecotype did not drive Rlv community composition in nodules, and Rlv community composition did not affect plant productivity. Taken together, these results suggest that the best strategy to improve BNF at low temperatures in hairy vetch likely depends on breeding for improved biomass accumulation and nitrogen fixation in host plants, rather than focusing on host plant selection of beneficial rhizobia.