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 mine ral N after harvest is consequently the major source of nitrate pollut ion of groundwater. To reduce losses of nitrate to groundwater, a nati onal leaching threshold value of 35 kg N ha(-1) yr(-1) is defined base d on the maximum nitrate concentration in groundwater of 50 mg l(-1) a nd the average annual precipitation surplus of 300 mm. Based on local weather conditions in the study area in the Northern part of the count ry, 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 le aching quantities exceeding the national and local leaching thresholds . The spatial and temporal variability of N leaching is quantified usi ng dynamic simulation for 65 soil profiles in a farmer's field. Leachi ng is simulated for five different N profiles at the end of the growin g season for a 20-year period. For the given initial N range (15-120 k g 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 a re 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) r espectively, using three risk levels of exceeding these thresholds. Sp atial interpolation of the required N profiles provided N target maps which can be used to focus and evaluate N fertiliser management. (C) 1 998 Elsevier Science B.V. All rights reserved.