Separating NO production and consumption in intact agricultural soil cores at different moisture contents and depths.

Agricultural soils are a major source of the potent greenhouse gas and ozone depleting substance, NO. To implement management practices that minimize microbial NO 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 NO flux measurement techniques provide little insight into subsurface mechanisms. In addition, denitrification studies are often conducted on sieved soil in altered O environments which relate poorly to field conditions. Here, we developed a novel incubation system with headspaces both above and below the soil cores and field-relevant O concentrations to better represent 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. N-NO pool dilution and an SF tracer were injected below the cores to determine the relative diffusivity and the net NO emission and gross NO 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 NO 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 NO consuming process and simultaneously occurring denitrification and nitrification both producing NO in the drier cores. We provide further evidence of substantial NO consumption in drier soil but without net negative NO emissions. The results from this study are important for the future application of the N-NO pool dilution method and N budgeting and modelling, as required for improving management to minimize NO losses.