Reducing Nitrate Leaching to Groundwater in an Intensive Dairy Farming System

Abstract

Dairy farming is one of the main contributors to nitrate leaching to groundwater, particularly on soils that are susceptible to leaching, such as light well-drained sandy soils. In the Netherlands, as in many other European countries, these soils are predominantly used for dairy farming. A prototype dairy farming system that has been implemented in practice in 1989 has continuously been adapted since then to meet environmental standards (i.e. the EU-standard of 50 mg NO3− l−1) without reducing milk production intensity (11900 kg ha−1). After an initial decline in nitrate concentration from 193 mg l−1 to 63 upon implementation, it subsequently ‘stabilized’ at a level higher than the environmental standard: 55 mg l−1. The goal of this paper is to examine causes of excessive nitrate leaching. This was done by relating measured nitrate concentrations with management characteristics such as N balances, cropping patterns and grazing intensities. Special attention was paid to aspects that were supposed to be conducive for leaching: crop rotation of grass and maize and grazing. No evidence was found for enhanced nitrate leaching due to the rotation of grass with maize compared to permanent cultivation. This could be ascribed to the reduction in fertilization levels in first and second year maize with 90 and 45 kg N ha−1, respectively to account for the expected N release from the ploughed-in grass sod. Triticale was found to lead to higher leaching than grass or maize which is attributed to its poor growth in the period that it should function as catch crop in maize. Grazing contributed to a nitrate increase of about 30 mg NO3− l−1 on grassland. As grazing management and intensity is already strictly optimized in order to restrict nitrate leaching, this result underpins the need to develop sustainable grazing methods on soil that is susceptible to nitrate leaching.