Optimizing nitrogen application improves its efficiency by higher allocation in bolls of cotton under drip fertigation

Nitrogen (N) loss is becoming one of the main limitations for sustainable agricultural production globally, particularly for crops such as cotton. To alleviate this issue, a promising strategy may be to exploit the N-saving potential of cotton plants by optimizing N application ratios at different growth stages under lower N rate and higher planting density with wide-row spacing. Two field experiments were carried out to investigate cotton yield, N use efficiency (NUE), N accumulation characteristics, and 15N uptake and distribution in response to N fertilization. First, a two-year experiment was conducted with N fertilization of either 0, 120, 240, 360 (control) or 480 kg ha−1. A three-year experiment was then carried out with 240 kg N ha−1 being applied with different ratios across three periods (squaring, flowering to peak boll, and late peak boll), i.e., 0:6:4 (N064), 1:6:3 (N163), 2:6:2 (N262, control), 3:6:1 (N361) and 4:6:0 (N460). Compared with conventional N fertilization (360 kg N ha−1), moderate fertilization at 240 kg N ha−1 resulted in steady and higher seed cotton yields of 5945 and 5603 kg ha−1 in 2017 and 2018, respectively, and improved NUE by 49.1–53.6%. Compared with conventional N262, a shift to later N application (N064) increased both lint yield and partial factor fiber productivity of nitrogen (PFFPN) by 4.4–7.7%, and accumulated 6.1–14.4% more total N (K) in reproductive organs due to higher average (Vt) and maximum (Vm) accumulation rates by 25.2–49.0% and 15.0–48.7%, respectively, while the fast N accumulation period (∆t) shortened by 15.2–24.4%. N064 partitioned 15.6% more 15N to reproductive organs. NUE was significantly positively correlated with K, t1 (the day when ∆t started), Vt, Vm, Tm (the day on which Vm occurred) and 15N accumulation in middle and upper canopy bolls, and negatively correlated with ∆t of total N accumulation in reproductive organs. Allocating N from the squaring period to the late peak boll period led to an increase in N uptake during the reproductive growth stage and greater allocation of N to cotton bolls under reduced-N cultivation and drip fertigation. This N management strategy is a potential way to improve NUE and achieve sustainable and efficient production of cotton in arid areas.