Overcoming Tunnel Vision: Incorporating Cover Crops into High Tunnel Rotations to Improve Soil Health.

High tunnels are increasingly used for season extension in the Upper Midwest. However, intensive high tunnel planting, irrigation, and fertilization to optimize high-value growing space can lead to soil health problems, such as increased salinity and compaction, as well as organic matter loss. This project evaluated the use of cover crops as a management tool for high tunnel soil health improvement and nitrogen fertilization. Three cover crop treatments 1) Red clover monoculture, 2) Austrian winter pea/winter rye bi-culture, and 3) hairy vetch/tillage radish/winter rye tri-culture were planted in three replicates and repeated in three high tunnels across USDA hardiness zones in Minnesota, compared to a bare ground control. Treatments were evaluated for their effect on soil quality parameters including cover crop biomass carbon (C) and nitrogen (N), extractable N in soil, potentially mineralizable N, and labile soil carbon (Permanganate Oxidizable Carbon, POX). Cover crops were planted in late August 2015, terminated and incorporated in mid-May 2016, and followed by orange bell peppers. Cover crop biomass, weed biomass, and soils were sampled before cover crop termination/incorporation and soil was sampled again two weeks after termination/incorporation. At one site, biomass %N ranged from a low of 1.2 in rye to a high of 5.9 in vetch, biomass %C ranged from a low of 31.0 in radish to a high of 41.2 in rye, and C:N ranged from a low of 6.9 in vetch to a high of 34.2 in rye. For all treatments at all sites, extractable N ranged from 6.9-22.0 mg N/kg dry soil, potentially mineralizable N ranged from 3.7-29.9 mg N/kg dry soil, and POX carbon ranged from 727-1089 mg C/kg dry soil at cover crop termination and from 741-1137 mg C/kg dry soil two weeks after cover crop termination. Overall POX carbon increased from the first to second sampling.