Soil type, bulk density and drainage effects on relative gas diffusivity and N₂O emissions

Abstract

Nitrous oxide (N₂O), a greenhouse gas, contributes to stratospheric ozone depletion. Agricultural fertiliser use and animal excreta dominate anthropogenic N₂O emissions. Soil relative gas diffusivity (Dp/Do) has been used to predict the likelihood of soil N₂O emissions, but limited information exists about how soil N₂O emissions vary with soil type in relation to Dp/Do. It was hypothesised that, regardless of soil type, the N₂O emissions would peak at the previously reported Dp/Do value of 0.006. Four pasture soils, sieved and repacked to three different bulk densities, were held at nine different soil matric potentials between near saturation and field capacity. Soil nitrate and dissolved organic matter concentrations were adequate for denitrification at all soil matric potentials. Increasing soil bulk density and soil matric potential caused Dp/Do to decline. As Dp/Do declined to a value of 0.006, the N₂O fluxes increased, peaking at Dp/Do ≤ 0.006. This study shows that the elevation of N₂O fluxes as a Dp/Do threshold of 0.006 is approached, holds across soil types. However, the variability in the magnitude of the N₂O flux as Dp/Do declines is not explained by Dp/Do and is likely to be dependent on factors affecting the N₂O: (N₂O + N₂) ratio.