Reducing Nitrate Losses From Simulated Grazing on Grassland Lysimeters in Ireland Using a Nitrification Inhibitor (Dicyandiamide)

Nitrate ( NO₃⁻ ) pollution of water is a serious environmental problem, as NO₃⁻ can contribute to the eutrophication of surface waters, and high levels may cause methaemoglobinaemia in formula-fed infants. Grassland agriculture is a major source of diffuse NO₃⁻ pollution, with much of this NO₃⁻ originating from urine deposited by grazing animals. The research objective was to determine the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in reducing NO₃⁻ leaching under Irish dairy farming conditions. Urine was applied in autumn to undisturbed monolith lysimeters to replicate an average cattle urination (2l of 8.6g N l -1 urine), or 344kg N ha -1 equivalent. Nitrogen (N) fertiliser was applied at 141kg ha -1 and 291kg ha -1 . DCD was applied in autumn and spring. The total annual NO₃⁻-N losses from urine patches on three soils ranged from 16kg to 204kg NO₃⁻-Nha⁻¹ . Peak concentrations on the non-urine treatments did not exceed 6.1mg NO₃⁻-N1⁻¹ , but reached 17.1—148.6mg NO₃⁻-N1⁻¹ when urine was applied. DCD reduced the total NO₃⁻-N losses from urine treatments on the lighter soils by 38%—42%, and reduced peak NO₃⁻-N concentrations by over 50%. This trial has shown that DCD has the potential to significantly reduce NO₃⁻ leaching losses from grazed grassland in Ireland.