Abstract
Abstract. The application of nitrogen (N) fertilisers to crops grown on tile-drained fields is required to sustain most modern crop production, but it poses a risk to the aquatic environment since tile drains facilitate rapid transport pathways with no significant reduction in nitrate. To maintain the water quality of the aquatic environment and the provision of food from highly efficient agriculture in line with the EU's Water Framework Directive and Nitrates Directive, field-scale knowledge is essential for introducing water management actions on-field or off-field and producing an optimal differentiated N-regulation in future. This study strives to provide such knowledge by evaluating on 11 years of nitrate-N concentration measurements in drainage from three subsurface-drained clayey till fields (1.3–2.3 ha) representing approximately 71 % of the surface sediments in Denmark dominated by clay. The fields differ in their inherent hydrogeological field settings (e.g. soil-type, geology, climate, drainage and groundwater table) and the agricultural management of the fields (e.g. crop type, type of N fertilisers and agricultural practices). The evaluation revealed three types of clayey till fields characterised by: (i) low net precipitation, high concentration of nitrate-N, and short-term low intensity drainage at air temperatures often below 5 °C; (ii) medium net precipitation, medium concentration of nitrate-N, and short-term medium-intensity drainage at air temperatures often above 5 °C; and (iii) high net precipitation, low concentration of nitrate-N and long-term high intensity drainage at air temperatures above 5 °C. For each type, on-field water management actions, such as the selection of crop types and introduction of catch crops, appeared relevant, whereas off-field actions only seemed relevant for the latter two field types given the temperature-dependent reduction potential of nitrate off-field. This initial well-documented field-scale knowledge from fields that are representative of large areas in Denmark is a first step towards establishing a differentiated N-regulation for clayey till areas. Additionally, it provides a unique starting point by identifying important parameters for future mapping of catchment-scale variations in nitrate concentrations and fluxes.
Highlights
Future regulations covering aquatic environments under the Nitrates Directive (EEC, 1991), the EU Water Framework Directive (EC, 2000), the EU Groundwater Directive (EC, 2006) and additional national laws and regulations (DanishEPA, 2012) call for long-term monitoring data to describe the complex interaction between soil, geology, geochemistry and hydrology at a local level as well on larger scales
All three fields are located in a temperate climate with average air temperatures between 8.8 and 9.0 ◦C for 2001– 2011
The annual average air temperature and evaporation were around the same in all fields, but the average annual precipitation varied between 685 mm yr−1 (Faardrup) at the lowest to 1089 mm yr−1 at the highest (Estrup)
Summary
Future regulations covering aquatic environments under the Nitrates Directive (EEC, 1991), the EU Water Framework Directive (EC, 2000), the EU Groundwater Directive (EC, 2006) and additional national laws and regulations (DanishEPA, 2012) call for long-term monitoring data to describe the complex interaction between soil, geology, geochemistry and hydrology at a local level as well on larger scales. The Danish Commission on Nature and Agriculture issued a report (Commission on Nature and Agriculture, 2013) that recommended that nitrogen (N) regulations should be adapted locally in future and, if possible, at field scale. As an integrated part of such a policy, there is a need to develop tools to identify fields that are vulnerable or non-vulnerable to nitrate leaching. The agricultural sector, has been identified as the largest
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