Cover Crop Cocktails Enhance Nitrogen Management.

Nitrogen (N) management is among the greatest challenges of contemporary agriculture. Cover crops are frequently used to provide N to a subsequent crop or retain N to mitigate losses to the environment, two ecosystem services essential to N management.  In recent years, farmers have expressed interest in using mixtures of cover crop species (“cocktails”) to provide multiple ecosystem services. To test if cocktails can simultaneously provide and retain N, we are conducting a field study using 17 cover crop treatments planted following oats and incorporated prior to corn.  Treatments include eight monocultures, seven four species cocktails, one eight species cocktail, and a no cover crop control. Four species cocktails containing winter hardy legumes [hairy vetch (Vicia villosa Roth.) and red clover (Trifolium pratense L.)] and the eight species cocktail resulted in corn yields (9.2 to 10.2 Mg ha^-1) above the county average [8.4 Mg ha^-1 (134 bu ac^-1)] and similar to those achieved following hairy vetch (9.8 Mg ha^-1) and red clover (9.7 Mg ha^-1) monocultures.  Winter hardy non-legume monocultures [canola (Brassica napus L.) and cereal rye (Secale cereale L.)] significantly reduced spring nitrate-N leaching compared to the control, as did four species cocktails containing these species and the eight species cocktail.  Cocktails containing winter hardy non-legumes reduced leaching 60-90% compared to the control and 55-85% compared to legume monocultures.  A four species cocktail of red clover, hairy vetch, canola, and cereal rye and the eight species cocktail led to both adequate N provision for the subsequent corn crop (yields of 9.2 and 9.9 Mg ha^-1, respectively) and reduced spring nitrate leaching (total losses of 3.9 and 11.0 kg nitrate-N ha^-1, respectively).  This research demonstrates that increasing biodiversity in agricultural ecosystems with cover crop cocktails can reduce N pollution while provisioning N to cash crops to meet yield goals.