Nitrogen fixation and productivity of winter annual legume cover crops in Upper Midwest organic cropping systems

Legume cover crops can play a valuable role in maintaining and increasing soil quality and nitrogen availability, but are infrequently grown in the Upper Midwest due to short growing seasons with minimal management windows; cold, wet springs; and harsh winters. This study was performed to assess the viability of winter annual legume species in northern climates as a potential source of nitrogen (N) fertility to a 75-day sweet corn (Zea mays convar. saccharata var. rugosa) cash crop in Lamberton and Grand Rapids, MN in 2016 and 2017. Treatments included medium red clover (Trifolium pratense), two cold-hardy ecotypes of hairy vetch (Vicia villosa Roth), a cereal rye-hairy vetch biculture (Secale cereale L., Vicia villosa Roth), cereal rye as a non-legume control, and a fallow weed-free control. Legumes were split into rhizobia inoculated and non-inoculated treatments. Inoculation had no effect on nodulation, biomass production, or N fixation likely due to competition with endogenous rhizobia strains. The rye monoculture and biculture produced the most biomass at all site-years averaging 7.7 and 7.0 Mg ha⁻¹ respectively while the two vetch ecotypes averaged 4.5 and 3.9 Mg ha⁻¹. Both vetch ecotypes contributed among the most nitrogen in all site-years, contributing up to 211 kg N ha⁻¹ from aboveground biomass. Data from natural abundance isotopic approaches indicate that 75% of vetch tissue N in Grand Rapids and 59% of vetch tissue N in Lamberton was derived from atmospheric N fixation, with equal or higher percent fixation of vetch in biculture at all site-years. More studies should be performed to better understand controls on N fixation of legume cover crops in cold climates.