• Energy Research

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.

Nitrous oxide (N₂O) is a greenhouse gas emitted from grazed pasture systems. The influence of phosphorus (P) fertility on these emissions is not understood. This study examined if fertiliser P affected N₂O emissions following nitrate application to soil from the Winchmore long-term P fertiliser trial. We hypothesised increasing P fertility would enhance soil carbon (C) supply and N₂O emissions via denitrification. Using mesocosms, N₂O and CO₂ fluxes were measured over 29 days following KNO3-15N and glucose additions, along with soil chemical and biological variables. Microbial biomass P increased (P < 0.001; R 2 = 98.2%) with increasing Olsen-P. Fertiliser P enhanced net N mineralisation. Cumulative CO₂ fluxes were unaffected by P treatment regardless of KNO₃ addition. Fluxes of N₂O increased one day after KNO₃ addition with higher emissions at 250 kg ha¯¹ of superphosphate. Relatively low N₂O-¹⁵N enrichment indicated minor denitrification contributions to the N₂O flux. Subsequently, glucose addition enhanced N₂O-¹⁵N enrichment and denitrification in the KNO₃ treatment. Following glucose addition, the emission factor increased with P fertiliser. Denitrification derived N₂O emissions will increase with P fertilisation but only if C limitation is overcome.