Water and nitrogen (N) are major constraints for cotton growth and yield formation in arid regions. Irrigation and N application have been widely investigated to improve crop yield and water and N use efficiency (NUE). However, further optimization of water and N management is needed because the effects of N application on soil N balance and N loss under different irrigation levels remain unclear. In this study, a field experiment was conducted in 2020 and 2021 to investigate the effects of different irrigation amounts (full irrigation (100% ETc), moderate irrigation (80% ETc), low irrigation (60% ETc)) and N application rates (control (without N application, N0), low N (150 kg ha⁻¹, N150), medium N (225 kg ha⁻¹ and 300 kg ha⁻¹, N225 and N300), and high N (375 kg ha⁻¹, N375)) on cotton yield, NUE, and soil N balance. The 2-year results showed that under the 60% ETc treatment, cotton dry matter accumulation, N uptake, and yield were significantly enhanced by increasing N application. Under the 80% and 100% ETc treatments, these parameters peaked with the N300 treatment; the N375 treatment showed no significant difference or decrease compared to the N300 treatment. The 80% ETc N300 treatment had the highest cotton yield and NUE, which increased by 17.49–106.57% and 12.28–88.78% compared with other treatments, respectively. Residual soil N accumulation (RSN_min), apparent N loss (ANL), and apparent N surplus (ANS) increased as the N application rate increased under the 60% and 100% ETc treatments. While under the 80% ETc treatment, the ANS did not significantly differ between the N225 and N300 treatments, ANL significantly decreased by 43.51–88.56% when the N application rate increased from 150 to 225 and 300 kg ha⁻¹. The ANL of the 80% ETc N300 treatment was the lowest, but ANS did not significantly increase. The regression analysis and spatial analysis results showed that under irrigation of 336–348 mm and N application of 254–327 kg ha⁻¹, cotton yield, NUE, and WPI reached more than 80% of the maximum value, with less apparent N loss, thus maintaining the soil N balance in drip-irrigated cotton fields. This study helps to improve the utilization of water and N resources in cotton production. Future research on optimizing water and N management needs to more fully consider environmental pollution to achieve sustainable development of agricultural ecosystems.

Cotton production under drip irrigation is common in arid and semi-arid Northwest China. Drip line placement determines the location of fertilizer application; therefore, evaluating the appropriate drip line layout and N application rate is essential for maintaining yield and reducing N losses. A two-year field experiment (2024–2025) was conducted at a single site with three independent replicates per treatment in a randomized complete block design arranged as a 2 × 2 factorial: two drip line layouts (wide-row and narrow-row) crossed with two N rates (300 and 240 kg N ha⁻¹; W300, W240, N300, and N240). Under the wide-row layout, root length density was higher (p < 0.05), and soil NO₃⁻-N was mainly distributed in the 0–60 cm layer during one fertigation cycle and across the fertilization period, overlapping with the root distribution. W240 significantly reduced NO₃⁻-N leaching and residual soil NO₃⁻-N (p < 0.05) and increased root length density and nitrogen partial factor productivity (PFPN) (p < 0.05), without reducing boll biomass, total N uptake, or seed cotton yield (p > 0.05). Under the narrow-row layout, soil NO₃⁻-N migrated rapidly downward and accumulated in the 60–100 cm layer; N240 reduced NO₃⁻-N leaching and residual soil NO₃⁻-N (p < 0.05) but also reduced cotton N uptake and seed cotton yield (p < 0.05), whereas N300 increased residual soil NO₃⁻-N and leaching and reduced PFPN (p < 0.05). Overall, under the soil, climate, cultivar, and irrigation conditions of this two-year single-site experiment, the wide-row layout with 240 kg N ha⁻¹ maintained seed cotton yield while reducing the measured NO₃⁻-N leaching and residual soil NO₃⁻-N.