Soil gas and solute diffusivity as predictors of N2O emissions after spring fertilization: An incubation study with intact soil

Agricultural soils are the main source of anthropogenic N₂O emissions to the atmosphere. To effectively mitigate these emissions there is a need to understand the interactions between potential drivers under field conditions. We conducted an incubation experiment with intact soil cores adjusted to one of four matric potentials (−15, −30, −50 or −100 hPa). We measured the relative gas diffusivity (Dp/Do) and effective air-filled porosity (ε_eff) before incubation with four surface-applied fertiliser treatments: NS26-13 (NS) at 50 kg N ha⁻¹ (NS50; starter); NS at 50 + 100 kg N ha⁻¹ (NS150); NS at 50 + 100 kg N ha⁻¹, the main dose of 100 kg N ha⁻¹ containing the nitrification inhibitor 3,4-dimethylpyrazol phosphate (NS150 + DMPP); and an unfertilized Control. We monitored N₂O fluxes for 35 days and found a positive relationship with matric potential and a weak inverse relationship with Dp/Do and ε_eff.. After 35 days, the cores were sectioned for determination of mineral N pools and potential ammonia oxidation (PAO). A strong NH₄⁺ gradient, declining with depth, was observed which indicated that nitrification took place at some distance from the surface-applied fertiliser, and this was consistent with PAO and pH distributions. The flux of NH₄⁺ in the soil solution between 0–2 and 2–4 cm depth after 35 days showed a consistent relationship with N₂O emissions at −30 to −100 hPa indicating nitrification was the main source of N₂O in this range. The results of this study suggest that both gas and solute diffusivity have to be considered for predicting N₂O emissions from surface-applied fertilisers.