<p>Reducing the over-use of nitrogen (N) fertiliser and mitigating leaching in highly cultivated agricultural areas have become the top priorities for potato researchers and the industry in the USA. In response, field trials were conducted in 2022 and 2023 at the University of Wisconsin Hancock Agricultural Research Station to explore the effects of different N fertilisation treatments on the in-season N status and at-harvest yield and size distribution of three potato (<i>Solanum tuberosum </i>L.) cultivars (Colomba, Snowden, Goldrush) grown in irrigated sandy soil. The study was performed in two field sites with varied background irrigation groundwater nitrate–N concentrations. The findings revealed that (1) N treatment effects were season- and cultivar-dependent; (2) the high N rate at 350&#xa0;kg N ha⁻¹ significantly increased petiole nitrate–N during the latter part of tuber bulking, yet this increase was not always associated with higher total and marketable tuber yields at harvest; (3) in a year when high N inputs came from the background nitrate in irrigation water, high yields were observed even in the control treatment with no supplemental N added; (4) nitrate–N concentrations in irrigation could be confounding variables when assessing potato responses to N fertilisation. This study provides new insights into sustainable N management for potato production in irrigated sandy soil, emphasising that different nitrogen sources—including fertiliser N and nitrate in irrigation groundwater—should be accounted for when making N application recommendations.</p>