Scenario based modelling approach to inform the spatial refinement of nitrogen management strategies for improving nitrogen use efficiency in Irish grasslands

Current national nitrogen (N) management policies in Ireland, such as the Green Book (GB) and the Fifth Nitrates Action Programme (NAP), do not explicitly consider geographical factors (e.g. weather, soils) that influence nitrogen uptake and loss and which could lead to improved on-farm sustainability, in line with the goals of 4R Nutrient Stewardship (4RNS). One approach to address this is to classify the landscape by soil and climatic indicators of yield and N loss (Group 1 Variables -regional); this could be refined with more targeted monitoring in zones where a reduction of yield and N loss occurs under a reduced N input regime (Group 2 Variables - locale specific), required to identify more local context-specific factors. Here, we employed the DNDC (DeNitrification DeComposition) model to identify Group 1 and Group 2 Variables for grass yield and N loss, through scenario analysis for three Irish grassland sites for year 2019, with local model parameterisation and a suite of minimum inputs required for reliable performance - identified from existing literature. The study sites were a sandy loam (JCSL) and a loam soil (JCL) site at Johnstown Castle (lower average annual rainfall and daily temperature) and a sandy loam (MP) site at Moorepark. While both MP and JCSL had sandy loam soil, MP a higher bulk density (BD), pH and water filled pore spaces (WFPS) at field capacity (FC) and wilting point (WP) but lower soil organic carbon (SOC) and clay content than JCSL. Based on the scenarios simulated, the NAP resulted in a lower simulated yield (-4.98 % to -15.62 %) and a reduction in ammonia (NH₃) volatilisation (-29.40 % to -30.35 %), nitrous oxide (N₂O) emissions (-30.49 to -35.15 %) and nitrate (NO₃^-) leaching (-55.51 % to -61.38 %). Both Group 1 and 2 Variables for annual yield and N₂O emissions were soil sand content, BD, SOC and for NH₃ volatilisation, annual rainfall and average annual temperature, for both GB and NAP. The Group 2 Variables for NO₃^- leaching were soil sand and clay content, pH, annual rainfall and average annual temperature; corresponding Group 1 Variables were found to vary under both GB and NAP.