REDUCING NITRATE LOSSES FROM SIMULATED GRAZING ON GRASSLAND LYSIMETERS IN IRELAND USING A NITRIFICATION INHIBITOR (DICYANDIAMIDE)

Abstract

Nitrate ( ${\mathrm{N}\mathrm{O}}_{3}^{-}$ ) pollution of water is a serious environmental problem, as ${\mathrm{N}\mathrm{O}}_{3}^{-}$ 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 ${\mathrm{N}\mathrm{O}}_{3}^{-}$ pollution, with much of this ${\mathrm{N}\mathrm{O}}_{3}^{-}$ originating from urine deposited by grazing animals. The research objective was to determine the effectiveness of a nitrification inhibitor, dicyandiamide (DCD), in reducing ${\mathrm{N}\mathrm{O}}_{3}^{-}$ 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 ${\mathrm{N}\mathrm{O}}_{3}^{-}-\mathrm{N}$ losses from urine patches on three soils ranged from 16kg to 204kg ${\mathrm{N}\mathrm{O}}_{3}^{-}-\mathrm{N}\mathrm{h}{\mathrm{a}}^{-1}$ . Peak concentrations on the non-urine treatments did not exceed 6.1mg ${\mathrm{N}\mathrm{O}}_{3}^{-}-\mathrm{N}{1}^{-1}$ , but reached 17.1—148.6mg ${\mathrm{N}\mathrm{O}}_{3}^{-}-\mathrm{N}{1}^{-1}$ when urine was applied. DCD reduced the total ${\mathrm{N}\mathrm{O}}_{3}^{-}-\mathrm{N}$ losses from urine treatments on the lighter soils by 38%—42%, and reduced peak ${\mathrm{N}\mathrm{O}}_{3}^{-}-\mathrm{N}$ concentrations by over 50%. This trial has shown that DCD has the potential to significantly reduce ${\mathrm{N}\mathrm{O}}_{3}^{-}$ leaching losses from grazed grassland in Ireland.