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

Abstract

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.