| Summary: | Nitrate leaching is of great environmental concern, particularly with potatoes grown on sandy soils. This 3-year study evaluated the effect of three N rates (100, 150, and 200 kg ha<sup>−1</sup>) of single applications of polymer-coated urea (PCU) and a 75% PCU + 25% urea mixture, plus a conventional split application of 200 kg N ha<sup>−1</sup> of a 50% ammonium sulfate + 50% calcium ammonium nitrate mixture (CONV) on NO<sub>3</sub><sup>−</sup>-N leaching, potato yield, and N uptake under irrigated and non-irrigated conditions on a sandy soil in Quebec (Canada). Fertilizer N application increased growing season NO<sub>3</sub><sup>−</sup>-N leaching only under irrigation. On average, irrigation increased seasonal NO<sub>3</sub><sup>−</sup>-N leaching by 52%. Under irrigated conditions, PCU reduced NO<sub>3</sub><sup>−</sup>-N leaching compared to PCU + urea. However, both PCU and PCU + urea significantly increased NO<sub>3</sub><sup>−</sup>-N leaching compared to the CONV at the equivalent N rate of 200 kg N ha<sup>−1</sup>. This was attributed to the timing of soil N availability and deep-water percolation. Total (TY) and marketable (MY) yields in the CONV were similar to those in the PCU applied at the equivalent N rate of 200 kg N ha<sup>−1</sup>. Despite lower plant N uptake, PCU resulted in greater TY and MY compared to PCU + urea. Residual soil inorganic N was greater for PCU and PCU + urea compared to the CONV, providing evidence that PCU products have the potential to increase NO<sub>3</sub><sup>−</sup>-N leaching after the growing season. In this study, PCU was an agronomically and environmentally better choice than PCU + urea. The results also showed that the efficiency of PCU to reduce seasonal NO<sub>3</sub><sup>−</sup>-N leaching may vary according to the timing of precipitation and irrigation.
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