Evaluating Drip Line Layout and N Application Rate to Improve Cotton Yield and N Partial Factor Productivity

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.