Managing Inter-Seeded Cover Crops and Tillage to Decrease Nitrate Leaching and Nitrous Oxide Emissions from Agricultural Soils.

Human alteration of the global nitrogen (N) cycle due to Haber-Bosch N-fixation and cultivation of N-fixing crops leads to increased nitrate (NO₃^-) leaching and nitrous oxide (N₂O) emissions from agricultural soils. Leaching contributes to ground water pollution and eutrophication in coastal waterways while N₂O emissions exacerbate climate change. These N losses result when soil N mineralization is not matched by plant demand, particularly post-harvest when fields are left fallow. As a result, decreasing N pollution depends on coupling N inputs to plant demand by immobilizing N between seasons. Inter-seeded cover crops when combined with reduced tillage offer a potential strategy to improve soil N retention by slowing N mineralization and maintaining constant demand for soil NO₃^- throughout the year. We compared four feed and forage cropping systems that utilize different tillage intensities and strategies for cover crop establishment. The system with minimum tillage that utilized both manure injection and cover crop inter-seeding maintained significantly lower soil NO₃^- levels throughout the summer and fall following spelt harvest. This system maintained soil NO₃^- below 5 kg NO₃-N ha^-1 whereas the traditionally managed system with broadcast manure and cover crops established with tillage increased soil NO₃^- to 30 kg NO₃-N ha^-1 until three months after spelt harvest. This corresponded to a decrease in NO₃^- leaching below 25 cm with 68 kg NO₃^- leached from the traditional system and 43 kg NO₃^- leached from the system with inter-seeding and manure injection. Spelt yield in each of these systems was not significantly different. Future N₂O measurements will allow us to examine tradeoffs between leaching and greenhouse gas emissions.  Results to date illustrate that novel techniques such as manure injection and cover crop inter-seeding can improve N retention without impacting cash crop yields.