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

Context: Nitrogen (N) loss is becoming one of the main limitations for sustainable agricultural production globally, particularly for crops such as cotton. Research question: 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. Methods: Two field experiments were carried out to investigate cotton yield, N use efficiency (NUE), N accumulation characteristics, and N-15 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 (N-064), 1:6:3 (N-163), 2:6:2 (N-262, control), 3:6:1 (N-361) and 4:6:0 (N-460). Results: 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 N-262, a shift to later N application (N-064) 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 (Delta t) shortened by 15.2-24.4%. N-064 partitioned 15.6% more N-15 to reproductive organs. NUE was significantly positively correlated with K, t(1) (the day when Delta t started), Vt, Vm, Tm (the day on which Vm occurred) and N-15 accumulation in middle and upper canopy bolls, and negatively correlated with Delta t of total N accumulation in reproductive organs. Conclusions: 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. Significance: This N management strategy is a potential way to improve NUE and achieve sustainable and efficient production of cotton in arid areas.