Soil moisture dynamics, fertiliser timing and fertiliser nitrogen form (nitrate: ammonium ratio) drive nitrous oxide (N2O) emission patterns in a temperate grassland.

Although both straight N and NPK (nitrogen, phosphorus, potassium) compound fertilisers are widely used, the influence of nitrate (NO3--N) to ammonium (NH4+-N) ratios in these fertilisers on nitrous oxide (N2O) emission dynamics remains poorly quantified. This study examined how fertilisers with contrasting NO3--N to NH4+-N ratios influence N2O emissions under varying environmental conditions, over a growing year in an Irish grassland soil. A randomised block field experiment was conducted on a moderately drained luvisol with a perennial ryegrass (Lolium perenne) sward. Fertilisers with varying NO3--N to NH4+-N ratios, calcium nitrate (CN) (100:0), NPK 27-2.5-5 (80:20), NPK 18-6-12 (53:47) and ammonium sulphate (AS) (0:100), were applied at a rate of 200 kg N ha-1 yr-1 in five equal splits. Nitrous oxide emissions were measured continuously from March to December 2023 using high-frequency automatic chambers, together with concurrent measurements of environmental variables. Daily N2O fluxes ranged from -2.07 to 271.34 g N2O-N ha-1 d-1. Higher NO3--N fertilisers (CN and NPK 27-2.5-5) produced short-lived peaks under high water-filled pore space (WFPS) following the first (early spring) and fourth (late summer) applications, likely due to readily available NO3- promoting denitrification. Conversely, higher NH4+-N fertilisers (AS and NPK 18-6-12) produced greater peaks after the fifth application (early autumn), when repeated NH4+ inputs coincided with warm, drying-rewetting conditions. Cumulative N2O-N emissions (1.8 to 2.4 kg N2O-N ha-1) and partial emission factors (0.81 to 1.08 %) did not differ significantly among fertiliser formulations. Overall, the response of N2O emissions to fertilisers with varying NO3--N to NH4+-N ratios was driven primarily by soil moisture dynamics. The results suggest that compound fertilisers with lower NO3--N to NH4+-N ratios may be suitable for early-season applications when soils are typically wet, while NO3--N fertilisers may be better targeted to periods when soils are expected to remain relatively dry. Additionally, ammonium accumulation in soils receiving repeated applications might be a significant source of delayed N2O emissions late in the year when soil conditions are conducive; this may have implications for mitigation strategies and for monitoring of emissions from NH4+-N based fertilisers, which might be underestimated by short-term monitoring.