Changes in Relative Gas Diffusivity Explain Soil Nitrous Oxide Flux Dynamics.

Nitrous oxide (N₂O) is a greenhouse gas and the main anthropogenic emission contributing to stratospheric ozone depletion. Agricultural soils dominate anthropogenic N₂O emissions but there is very limited information specifically relating relative soil gas diffusivity (Dp/Do) to N₂O emissions. This study was conducted to determine the effects of soil bulk density (ρ_b) and matric potential (ψ) on Dp/Do and the associated N₂O fluxes in the presence of unlimited denitrification substrate. The interaction between soil ρ_b and ψ on Dp/Do and N₂O fluxes was investigated using 880 repacked soil cores that were saturated with a nitrate (NO₃^-) solution and placed on tension tables at 11 levels of ψ and 5 levels of soil ρ_b. After equilibration (4 days) N₂O fluxes, Dp/Do, inorganic-N concentrations, and soil physical characteristics were determined. Emissions of N₂O peaked at increasingly lower levels of ψ (-1.5 to -6.0 kPa) as soil ρ_b increased (1.1 to 1.5 Mg m^-3) due to increasing microporosity. Peak N₂O emissions occurred across a relatively wide range of WFPS and volumetric water content. A Gaussian fit of N₂O-N fluxes against ψ showed maximum fluxes were related to the soil’s air-entry potential (r² = 0.96). Maximum N₂O emissions occurred at a Dp/Do value of 0.006 that was independent of soil ρ_b. Log N₂O-N flux was a function of log Dp/Do (r² = 0.82) when Dp/Do was > 0.006. Soil Dp/Do is a key indicator of N₂O emission potential and needs to be further explored as a predictor of N₂O emissions in a range of soil textures and denitrification substrates.