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 quantity and source of nitrogen credits from winter annual legume species that may address these challenges.  Treatments included three cold-hardy ecotypes of hairy vetch (Vicia villosa), red clover (Trifolium pratense), a rye-Austrian winter pea biculture (2014; Secale cereale L., Pisum sativum arvense), a rye-hairy vetch biculture (2015; Secale cereale L., Vicia villosa) and rye as a non-legume control.  Legumes were further divided into rhizobia inoculated and non-inoculated treatments planted in a randomized 4-block design.  Cover crops were planted in fall 2014 and 2015 at the University of Minnesota Research and Outreach Centers located in Grand Rapids, MN and Lamberton, MN.  In 2014, poor establishment was observed at both locations with low biomass harvested in June 2015 immediately prior to termination. Good establishment was observed in fall 2015 with ample biomass in spring 2016.  The two top performing varieties of vetch at both sites yielded between 1-1847 kg ha^-1 in 2015 and 1100-2730 kg ha^-1 in 2016; clover ranged from 0-93 kg ha^-1 in 2015 and 300-1600 kg ha^-1 in 2016; and rye ranged from 885-3074 kg ha^-1 in 2015 and 2030-4238 kg ha^-1 in 2016.  Inoculation treatment was found to be insignificant at all site-years with regards to biomass production.  Preliminary data from Lamberton suggest the two top performing varieties of hairy vetch in 2015 yielded between 11-63 kg N ha^-1 and rye 11-43 kg N ha^-1.  Natural abundance isotopic approaches indicated that 60-81% of vetch tissue nitrogen in Lamberton was derived from atmospheric N fixation.  Nitrogen results of the 2015-2016 trial are pending.