Crop residues contribute minimally to spring-thaw nitrous oxide emissions under contrasting tillage and crop rotations

Crop residues are sources of carbon and nitrogen (N) after harvest, releasing inorganic N through mineralization or protecting soil N through immobilization. Inorganic N controls nitrous oxide (N₂O) emissions, a potent greenhouse gas (GHG) from agriculture. Hence, crop residues are accounted for as N₂O sources in national GHG inventories. For locations where post-harvest N₂O emissions occurs due to freeze-thaw, it is not known if crop residues contribute to emissions, and if tillage or residue type impact this contribution. This is of concern since crop residue and freeze-thaw emission factors (EF) may be ‘double-counting’ N₂O sources. We conducted an experiment over two non-growing seasons (NGS) in a long-term corn, soybean and winter wheat trial to (i) compare N₂O emissions for different crop residues within simple or diverse crop rotations under no-tillage (NT) or conventional tillage (CT); (ii) determine the importance of above- and below-ground residue addition to spring-thaw N₂O emissions as affected by rotation and tillage. A¹⁵N residue enrichment study was used to directly trace above- and below-ground residue ¹⁵N into ¹⁵N₂O fluxes and derive EF. Higher N₂O emissions were observed for CT than NT, regardless of rotation. Soybeans induced higher N₂O emissions than corn residue and the same crop residue (e.g. corn or soybean) showed trends of higher N₂O in the long-term diverse rotation. In all cases, crop residues contributed minimally to spring-thaw N₂O emissions (<2%), meaning differences in N₂O emissions were due to tillage and rotation effects on soil N availability, rather than by affecting crop residue N release for N₂O production. The NGS 6-month EF for crop residues never surpassed 0.05%, a minimal fraction of the annual 0.6% recommended EF. Refinement in emission inventories for cold climates should focus on freeze-thaw substrate release for N₂O production from other sources than crop residue.