Nitrate leaching in temperate agroecosystems: sources, factors and mitigating strategies

Nitrate (NO3-) leaching from agricultural production systems is blamed for the rising concentrations of NO3- in ground- and surface-waters around the world. This paper reviews the evidence of NO3- leaching losses from various land use systems, including cut grassland, grazed pastures, arable cropping, mixed cropping with pasture leys, organic farming, horticultural systems, and forest ecosystems. Soil, climatic and management factors which affect NO3- leaching are discussed. Nitrate leaching occurs when there is an accumulation of NO3- in the soil profile that coincides with or is followed by a period of high drainage. Therefore, excessive nitrogen (N) fertilizer or waste effluent application rates or N applications at the wrong time (e. g. late autumn) of the year, ploughing pasture leys early in the autumn, or long periods of fallow ground, can all potentially lead to high NO3- leaching losses. N returns in animal urine have a major impact on NO3- leaching in grazed pastures. Of the land use systems considered in this paper, the potential for causing NO3- leaching typically follow the order: forest < cut grassland < grazed pastures, arable cropping < ploughing of pasture < market gardens. A range of management options to mitigate NO3- leaching is described, including reducing N application rates, synchronizing N supply to plant demand, use of cover crops, better timing of ploughing pasture leys, improved stock management, precision farming, and regulatory measures. This is followed by a discussion of future research needs to improve our ability to predict and mitigate NO3- leaching.