Nitrogen dynamics of hairy vetch (Vicia villosa) and red clover (Trifolium pretense L.) legume cover crops in organicallymanaged agroecosystems in the northern United States

Cover crops, non-harvested crops grown between cash crop production, are an agroecologicalapproach that can increase ecosystem service provisioning. Legume cover crops in particularare valuable for their contribution of nitrogen (N) via the process of biological nitrogenfixation. Organic producers in the United States are prohibited from utilizing synthetic nitrogenfertilizer, and thus employ legume cover crops to provide available N for their non-legume cashcrops. This study evaluates the potential of legume cover crops to contribute to soil N pools.We assessed four winter annual cover crop treatments at two sites in the state of Minnesota inthe northern United States, including a rye non-legume control (Secale cereal, RYE), red clover(Trifolium pretense L., CLO), hairy vetch ecotype 1 (Vicia villosa, V1), hairy vetch ecotype2 (V2), and a hairy vetch – rye bi-culture (V2 MIX). Additional treatments of with-rhizobia inoculationand without-rhizobia inoculation were imposed to determine the need for inoculation.Results showed that cover crop legumes contributed from 30-75 kg N ha yr-1, with 50-70% ofplant biomass N derived from the atmosphere via biological nitrogen fixation. An increase inavailable soil N was observed following spring cover crop termination. No differences wereobserved in nitrogen fixation or nodulation parameters, suggesting that sufficient native soilrhizobia populations were present to carry out nitrogen fixation.