Spatial and temporal distribution of subsurface nitrous oxide concentrations and surface fluxes in a clay soil under...

In cold-climate agriculture, spring-thaw periods dominate annual nitrous oxide (N₂O) emissions, yet the subsurface processes that govern these fluxes remain poorly characterized. Crucially, it remains unknown how management practices influence the depth distribution of soil N₂O and its relationship to surface emissions. A field study was conducted from 2021 to 2023 on a clay soil in Southern Manitoba, Canada, to determine the effects of a fall rye cover crop and enhanced-efficiency nitrogen fertilizers (EENFs) on the spatial and temporal dynamics of N₂O in the soil profile. Soil N₂O concentrations at four depths (5, 15, 30, and 60 cm) were determined using silicone diffusive equilibrium samplers and compared to field-scale N₂O fluxes measured with a flux-gradient micrometeorological system. During the spring thaw, soil N₂O concentrations peaked at 30 cm depth, reaching 30.0-71.0 μL N₂O L^-1, relative to generally below 10.0 μL N₂O L^-1 during the post-harvest period. These results suggest increased N₂O production during spring thaw was driven by increased soil moisture and temperature that stimulate microbial denitrification. Compared to the non-cover crop control fields, the cover crop fields maintained low N₂O concentrations at shallow depths during the post-harvest. Compared to conventional urea, use of SuperU as EENF in 2023 reduced growing season cumulative emissions by 77% and soil N₂O concentrations in the 0-15 cm layer by 25% during the growing season. Critically, no significant correlation was found between surface flux and subsurface concentration at any depth. These findings demonstrate that soil N₂O dynamics are controlled by depth-specific processes modulated by management. Our study indicates that the soil profile functions as a spatially complex reactor for N₂O, not merely a source. Our findings demonstrate that integrating soil profile measurements with surface flux monitoring is therefore essential for accurate emission accounting and effective mitigation.