Pollution of ground water caused by excessive and uncontrolled use of nitrogen fertilizer is worrying. A recent example of such pollution has been observed in an agricultural basin in the province of Nevsehir, Turkey, where up to 900 kg ha(-1) nitrogen fertilizer is used for growing potatoes in sandy soils under irrigation. Using nitrogen fertilizer in amounts that guarantee large yields without polluting ground water is essential. We present results of field experiments and numerical simulations involving N-15-labelled nitrogen fertilizer leaching. In the field, we monitored the movement of water and the distributions of nitrogen species within the soil-water-plant continuum. The detailed dynamics of the nitrogen cycle within the system were simulated. Simulations included calibration and validation of the nitrogen version of the LEACHM model (LEACHN, version 3) and long-term applications of the model. The model's predictions of nitrogen fluxes under long-term use of fertilizer and irrigation were analysed. Nearly half of the applied ammonium-N was converted to nitrate-N during the growing season. With increasing additions of N the rate of plant uptake declined, while leaching increased significantly, and the fraction of nitrogen remaining in the soil profile increased only moderately. In long-term applications, a significant fraction of the applied fertilizer tended to accumulate after the first year in soil as the residual nitrogen not taken up by the crop. Accumulated residual nitrogen is converted to nitrate-N and leached rapidly from the soil profile during the wet season following the harvest. To reduce leaching of the residual nitrate, the rates, frequencies and timings of fertilizer application and irrigation must be scheduled in accordance with the plant growth periods and the hydraulic regime of the soil.