Late nitrogen fertilization improves cotton yield through optimizing dry matter accumulation and partitioning

Cotton (Gossypium hirsutum L.) production efficiency is constrained by issues such as a low nitrogen (N) utilization rate (30–35 %) and high N application rate (300–350 kg ha⁻¹) in particular arid climates, such as Xinjiang, the largest irrigated cotton-producing region in China. These issues could be alleviated by allocating more N to late application to satisfy plant needs for growth and development. Over a three-year (2019–2021) field experiment, 240 kg ha⁻¹ N was applied in 3 periods (squaring, flowering to peak boll, and late peak boll). The amount of N applied in the second period was fixed at 60 %, and the remaining 40 % was split between the first and the third periods, with five ratios (treatments), i.e., 0:6:4 (N₀₆₄), 1:6:3 (N₁₆₃), 2:6:2 (N₂₆₂), 3:6:1 (N₃₆₁) and 4:6:0 (N₄₆₀), to investigate cotton response in terms of growth, biomass accumulation, and yield. Compared with the conventional treatment (N₂₆₂), the N₀₆₄ treatment narrowed the plant width by 11.7–12.0 %, increased canopy light transmittance by 6.1–56.9 %, extended the boll growth period by 8.8–9.4 %, improved defoliation by 9.3–11.7 % and increased both seed cotton yield and N partial factor productivity by 7.1–8.1 %, depending on the year. N₀₆₄ accumulated 4.3–39.5 % more biomass (K) than N₂₆₂, with the average and maximum growth rates of the reproductive organs (Vt and Vm, respectively) increasing by 8.6–89.0 % and 6.9–125.7 %, respectively, while the fast growth duration (∆t) shortened by 9.9–31.6 %. Again, N₀₆₄ partitioned 1.5–35.8 % more biomass to bolls (higher partitioning index, PI_boll). Seed cotton yield was significantly positively correlated with K, t₁, Vt, Vm and PI_boll and negatively correlated with ∆t in reproductive organs. Collectively, the data suggest that allocating more N to late application could be an efficient N fertilizer management strategy in arid areas under N-reduced cultivation and fertigation.