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 (rho(b)) and matric potential (psi) on Dp/Do and the associated N2O fluxes in the presence of denitrification substrates. The interaction between soil rho(b) and psi 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 psi and 5 levels of soil rho(b). After equilibration (4 d) N2O fluxes, Dp/Do, inorganic-N concentrations, and soil physical characteristics were determined. Emissions of N2O peaked at increasingly lower levels of psi (-1.5 to -6.0 kPa) as soil rho(b) increased (1.1 to 1.5 Mg m(-3)) due to increasing microporosity. Peak N2O emissions occurred across a relatively wide range of water-filled pore space (WFPS) and volumetric water content. A Gaussian fit of N2O-N fluxes against psi showed maximum fluxes were related to the soil's air-entry potential (r(2) = 0.96). Maximum N2O emissions occurred at a Dp/Do value of 0.006 that was independent of soil rho(b). Log N2O-N flux was a function of log Dp/Do (r(2) = 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.