Interactive effects of straw management, tillage, and a cover crop on nitrous oxide emissions and nitrate leaching from a sandy loam soil

Minimum tillage, residue recycling and the use of cover crops are key elements of conservation agriculture that play important roles in soil carbon (C) and nitrogen (N) dynamics. This study determined the long-term effects of tillage practice (conventional ploughing vs. direct seeding), straw management (retained vs. removed), and the presence of a cover crop (CC; fodder radish in this study) on nitrous oxide (N₂O) emissions, nitrate (NO₃⁻) leaching, and soil mineral N dynamics between October 2019 and June 2020. In the factorial experiment with eight treatment combinations, cumulative N₂O emissions ranged from 0.04 to 0.8 kg N ha⁻¹, whereas NO₃⁻ leaching varied between 4 and 28 kg N ha⁻¹. The study did not find effects of straw retention on NO₃⁻ leaching or N₂O emissions. No-till reduced N₂O emissions by on average 46% compared to ploughing. Fodder radish reduced NO₃⁻ leaching by 80–84%, and there was little N₂O emission in the presence of the cover crop; however, after termination in spring there was a flush of N₂O, cumulative N₂O-N averaged 0.1 and 0.5 kg N ha⁻¹ without and with a cover crop. With information about long-term soil C retention from straw and fodder radish, an overall greenhouse (GHG) balance was calculated for each system. Without straw retention after harvest, there was always a positive net GHG emission, and the indirect N₂O emission from NO₃⁻ leaching was similar to, or greater than direct N₂O emissions. However, in the presence of fodder radish, the direct N₂O emissions after termination were much more important than indirect emissions, and negated the C input from fodder radish. Direct seeding, straw retention and the use of a cover crop showed positive effects on N retention and/or GHG balance and could substantially improve the carbon footprint of agroecosystems on sandy soil in a wet temperate climate.