Nitrate leaching on Irish grassland dairy farms: A review

Pasture-based dairy farming is a source of nitrate (NO3- ) leaching to Irish water bodies. Nitrate leaching can lead to excess nitrogen in water bodies that causes eutrophication leading to a reduction of oxygen levels, which ultimately can have a negative human health and environmental impacts. The objectives of this study were to review relevant literature pertaining to nitrate leaching and methods of leaching measurement in relation to Irish grassland dairy farming and to identify the main factors associated with leaching. Data from ceramic cup, lysimeter, hydrologically isolated drainage plots and groundwater borehole studies were evaluated. Recorded leaching from ceramic cup studies was found to be highly variable. Lysimeter studies did not account for paddock scale grazing and growth effects. Drainage studies provided consistent data but were not suitable for well-drained soils. Borehole studies were found to be the most reliable method of assessing leaching to groundwater on welldrained soils. Factors that influence leaching such as soil type, climate, nutrient load and stocking rate were analysed. Farm nitrogen surplus per hectare and its interaction with climate were found to be important factors that affect leaching. More long term studies that combine root zone and groundwater measurements methods across a range of spatial scales (farm - catchment), which account for climate, nitrogen surplus, soil type and grassland management effects were recommended. Targeted precision grassland management strategies are recommended to reduce nitrogen surplus and leaching while increasing nitrogen use efficiency.