Agricultural soils are a major source of the potent greenhouse gas and ozone depleting substance, N₂O. To implement management practices that minimize microbial N₂O production and maximize its consumption (i.e., complete denitrification), we must understand the interplay between simultaneously occurring biological and physical processes, especially how this changes with soil depth. Meaningfully disentangling of these processes is challenging and typical N₂O flux measurement techniques provide little insight into subsurface mechanisms. In addition, denitrification studies are often conducted on sieved soil in altered O₂ environments which relate poorly to in situ field conditions. Here, we developed a novel incubation system with headspaces both above and below the soil cores and field-relevant O₂ concentrations to better represent in situ conditions. We incubated intact sandy clay loam textured agricultural topsoil (0-10 cm) and subsoil (50-60 cm) cores for 3-4 days at 50% and 70% water-filled pore space, respectively. ¹⁵N-N₂O pool dilution and an SF₆ tracer were injected below the cores to determine the relative diffusivity and the net N₂O emission and gross N₂O emission and consumption fluxes. The relationship between calculated fluxes from the below and above soil core headspaces confirmed that the system performed well. Relative diffusivity did not vary with depth, likely due to the preservation of preferential flow pathways in the intact cores. Gross N₂O emission and uptake also did not differ with depth but were higher in the drier cores, contrary to expectation. We speculate this was due to aerobic denitrification being the primary N₂O consuming process and simultaneously occurring denitrification and nitrification both producing N₂O in the drier cores. We provide further evidence of substantial N₂O consumption in drier soil but without net negative N₂O emissions. The results from this study are important for the future application of the ¹⁵N-N₂O pool dilution method and N budgeting and modelling, as required for improving management to minimize N₂O losses.

Keywords: denitrification; diffusion coefficient; isotope pool dilution; nitrogen cycling; sulphur hexafluoride.

© 2023 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science.