Synergistic Optimization of Root–Shoot Characteristics, Nitrogen Use Efficiency and Yield by Combining Planting Density with Nitrogen Level in Cotton (Gossypium hirsutum L.)

To address low nitrogen use efficiency (NUE) derived from excessive fertilization in cotton production in the Yellow River Basin, a field study was conducted to evaluate the effects of two planting densities and six nitrogen (N) rate levels. Key results show that a N rate of 225 kg ha−1 optimized root length density and root biomass density. High planting density (105,000 plants ha−1) improved the population-level root traits, photosynthetic radiation interception, and boll number per unit area, though it reduced individual plant root development. Total dry matter peaked at 225 kg ha−1 N, and density increased reproductive dry matter by 7.5–11.9%. Higher N rates reduced reproductive partitioning and root–shoot ratio. While the maximum seed cotton yield (SCY) was 225 kg ha−1, near-maximum yield was achieved at 150 kg ha−1. NUE declined with increasing N, but densification improved agronomic NUE and partial factor productivity by 1.5–6.6% and 3.3–39.3%, respectively. Under the “densification with N reduction” mode, combining a planting density of 105,000 plants·ha−1 with an N rate of 150 kg·ha−1 achieved conventional yield. At the same density, an N rate of 225 kg·ha−1 not only enabled high yield and maintained relatively high NUE but also showed better adaptability to the simplified cultivation mode in Yellow River Basin cotton-growing regions.