IMPACT OF EXCRETED NITROGEN BY GRAZING CATTLE ON NITRATE LEACHING

At De Marke experimental farm, data on water and nitrogen flows in the unsaturated zone were gathered on two grazed pastures on sandy soils during the years 1991 to 1994. These provided a basis for calibration and validation of simulation models. The different levels of nitrate-N concentrations of the two plots could largely be explained by differe nces in crop uptake and simulated denitrification as influenced by dif ferent groundwater levels. The irregular distribution of excreta was t aken into account by a simulation study quantifying the variability of nitrate-N concentrations under a grazed field. The resulting distribu tion of simulated nitrate-N concentrations explained the average and p eak values of the measured concentrations. Temporal variability of wea ther was used to assess the nitrate leaching risk under urine patches deposited in either July or September. At site A the probability of ex ceeding the EC-directive by drinking water (11.3 mg/l nitrate-N) under a urination deposited in either July or September was respectively 10 and 25%. The average field concentration at this site will hardly eve r be a high risk for the environment under the current farm management . At site B the EC-directive will be exceeded under any urine patch in almost 100% of the years, affecting the field average concentration. In field B careful grazing management would result in less nitrate lea ching, but the environmental goals would not be reached.