Defining threshold values for residual soil N levels

Nitrogen (N) leaching in the Netherlands mainly takes place during the period of precipitation surplus from September 15-April 15. Soil mineral N after harvest is consequently the major source of nitrate pollution of groundwater. To reduce losses of nitrate to groundwater, a national leaching threshold value of 35 kg N ha(-1) yr(-1) is defined based on the maximum nitrate concentration in groundwater of 50 mg l(-1) and the average annual precipitation surplus of 300 mm. Based on local weather conditions in the study area in the Northern part of the country, the leaching limit should be set at 42 kg N ha(-1) yr(-1). in this paper, N profiles after harvest are defined which do not result in leaching quantities exceeding the national and local leaching thresholds. The spatial and temporal variability of N leaching is quantified using dynamic simulation for 65 soil profiles in a farmer's field. Leaching is simulated for five different N profiles at the end of the growing season for a 20-year period. For the given initial N range (15-120 kg N ha(-1) m(-1)), space-time relations for the wet period are linear. Different results among the 65 soil profiles in both space and time are shown. The required N profile at September 15 is calculated for the national and local threshold values of 35 and 42 kg N ha(-1) yr(-1) respectively, using three risk levels of exceeding these thresholds. Spatial interpolation of the required N profiles provided N target maps which can be used to focus and evaluate N fertiliser management. (C) 1998 Elsevier Science B.V. All rights reserved.