The effect of nitrogen fertiliser and soil type on nitrate and ammonium leached losses from grazed temperate grassland in Ireland

Nitrogen (N) leached losses from temperate grassland systems can negatively impact the water quality of associated water bodies. Consequently, strategies are needed to help farmers manage and reduce losses. The present 3-yr study examined leached N losses on well, moderate and poorly drained intact soil lysimeters installed within a grazed field in Ireland. Lysimeters were grazed uniformly and received N fertiliser treatments of either urea early in the growing season followed by calcium ammonium nitrate (CAN) or urea + the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT) season long. No significant effect of fertiliser treatment on annual nitrate (NO 3 − -N) leached losses was observed. Leached losses of 11.1–73.6 kg NO 3 − -N ha −1 and NH 4 + -N losses of 1.1–18 kg ha −1 were measured. In a third year when no N fertiliser was applied and input of animal excreta was excluded but grass was removed, losses of 1.5–7.1 kg NO 3 − -N ha −1 and 0.1 kg NH 4 + -N ha −1 were recorded. The moderately drained soil had the highest NO 3 − -N losses and the poorly drained soil had the highest NH 4 + -N losses. These findings show that urea-CAN and urea + NBPT fertiliser programmes result in similar N leaching and that soil is an important factor, that differentially mediates the level of N loss that will occur at a given stocking rate and inorganic fertiliser input level.