In order to evaluate potato response to drip irrigation and nitrogen (N) fertigation; and also to improve N and water use efficiencies of potato and eventually reduce nitrate contamination of surface and ground water, 10 field experiments were carried out at three different locations in Cappodocia Region of Turkey over a 4-year period (1997-2000). Nitrogen as ammonium sulphate (NH₄)₂SO₄ was supplied through drip irrigation (fertigation) at rates of 0, 300, 600, and 900 kg N/ha. Also, as another treatment, N as ammonium sulphate in a granule fertilizer form was applied on to the soil surface and mixed in at the rate which was equivalent to the fertigation treatment of 600 kg N/ha (N_soil treatment). These five treatments were investigated in a completely randomized block design with four replications. Agria potato variety was used in all experiments and potato was planted in mid May and harvested at the end of October. ¹⁵N-labelled ammonium sulphate fertilizer were applied in isotope-sub plots within the macroplots for each treatment in order to determine the amount of N taken up by the plant, N use efficiency and the distribution of residual N at different depths in the soil profile. Each year during the growth period, 12 irrigations were done and 50 mm of water was applied at each irrigation. At harvest, plant samples (tuber and leaf+vein) and soil samples were taken and % N, % ¹⁵N atom excess (% ¹⁵N a.e.) and % Ndff were determined. Soil water contents at different soil depths were determined by soil moisture neutron probe at planting and at harvest so that water contents at different soil layers and water use efficiencies were calculated for each treatment.

The results obtained showed that 33500 kg/ha mean total marketable tuber yield was obtained with application of 600 mm irrigation water. Also, it was found that water did not move below 90 cm of soil layer in drip irrigation-fertigation system, suggesting that no N movement occurred beyond 90 cm soil depth. Although the % ¹⁵N a.e. values of the soil solutions taken from the tensionics were highest in those placed at the 65 cm soil depth (0.095-1.06 % ¹⁵N a.e.), no ¹⁵N was detected at the 85 cm depth for all locations and for every treatment. Tuber yields and % Ndff increased when N was applied with drip irrigation-fertigation system in comparison to the application to the soil and then drip irrigation. At harvest, more N was accumulated at 0-30 and 30-60 cm depths with fertigation treatments.

Keywords: Solanum tuberrosum L., drip irrigation, fertigation, ¹⁵N technique, neutron probe, percent nitrogen derived from fertilizer (% Ndff), percent nitrogen use efficiency (%NUE), fertilizer nitrogen residue in soil.