Nitrogen Contributions from Winter Annual Cover Crops in the Upper Midwest.

Legume cover crops can play a valuable role in maintaining and increasing soil quality and nitrogen availability, but face unique challenges in the Upper Midwest, such as short growing seasons, cold, wet springs, and harsh winters. This study was performed to assess the viability of winter annual legume species in northern climates as a source of nitrogen fertility. Treatments included red clover (Trifolium pratense), two cold-hardy ecotypes of hairy vetch (Vicia villosa), a winter rye-hairy vetch biculture (Secale cereale L., Vicia villosa), and winter rye as a non-legume control. In Y1, legumes were split into rhizobia inoculated and non-inoculated treatments. In Y2, inoculation treatments were dropped due to no significant findings in biomass production and nodulation between inoculated and non-inoculated treatments. Cover crops were planted in fall 2015 and 2016 at the University of Minnesota Research and Outreach Centers located in Grand Rapids, MN and Lamberton, MN in a randomized complete block design. Total biomass, total nitrogen, and natural ¹⁵N abundance were determined for all site years, with nodule weight and number determined in Y1 only. The rye monoculture and biculture produced the most biomass at all site-years ranging from 1.9-3.7 Mg ha^-1, with the exception of Y1 Grand Rapids, where a vetch ecotype performed equally well as the rye monoculture and biculture. Both vetch ecotypes contributed the most nitrogen in Y1, while clover contributed equally to vetch at Grand Rapids in Y2, and there were no significant differences in nitrogen contributions among all treatments in Lamberton in Y2. Hairy vetch contributed up to 85 kg N ha^-1 from aboveground biomass. Preliminary data from natural abundance isotopic approaches indicated that 78% of vetch tissue nitrogen in Grand Rapids and 53% of vetch tissue nitrogen in Lamberton was derived from atmospheric N fixation, with equal or higher fixation of vetch in biculture at all site-years.