Influence of Cover Crops and Winter Warming on Soil N2O Content and Surface Fluxes During Freeze-Thaw

Abstract

Overwinter cover crops alter nitrogen dynamics and soil temperatures, potentially mitigating nitrous oxide (N₂O) emissions during freeze-thaw (FT). Meanwhile, increasing winter temperatures can remove the insulating snow layer intensifying FT cycles and N₂O fluxes. Comparing the patterns of soil N₂O content against surface fluxes under different management and environmental conditions can improve the understanding of what mechanisms enhance N₂O fluxes at thaw. Soil profile (0–140 cm) N₂O gas concentrations and fluxes were measured from December to April in large-scale lysimeters with two dominant soils in Ontario, Canada (silt loam and loamy sand) over two years. The simultaneous heat and water model was used to simulate liquid water and ice content during freezing conditions, needed for total N₂O content estimations (i.e., aqueous + gaseous). During year 1, the peak soil N₂O content ranged from 23.6 to 79.0 mg N₂O m⁻², and two significant emissions events occurred (9.6–41 g N₂O-N ha⁻¹ d⁻¹). In year 2, no significant N₂O profile accumulation or emissions were observed due to warm winter conditions. Difference in soil physical conditions impacted the response of soil N₂O content to cover crops, with N₂O content decreasing by 42% in the loam soil and increasing 101% in sand. Intermittent heating caused colder soil conditions in year 1, increasing soil N₂O content in loam soil while reducing it in sand. Despite changes in soil N₂O content, the N₂O surface flux was not impacted, indicating that alternative nitrogen loss pathways are likely responsible for reducing N₂O content during FT events not surface fluxes.