Optimal phosphorus proportion increases cotton yields in low nitrogen environments by enhancing mid-upper canopy biomass and nitrogen utilization

To elucidate the physiological mechanism of optimal phosphorus (P) proportion under reduced nitrogen (N) environments on cotton yield, dry matter (DM) spatiotemporal distribution, and N utilization. A three-year experiment was conducted with reduced N (299 kg hm⁻², N3), four proportions of P fertilizer (25:75 % at squaring and bloom-bolling stages (P1), 50 %:50 % (P2), 75 %:25 % (P3), 100 %:0 % (P4)), and a traditional N application (402.5 kg hm⁻², N2) with P2 as control. Under the N3 treatment, the seed yield of P2 was 6.7–24.5 % higher than other P, and the yield of N3P2 was 1.6–2.6 % lower than N2P2. During the whole growth period, P2 demonstrated 12.4–46.9 % higher leaf and reproductive organs dry matter (LDM and RoDM) compared to other P, and primarily due to more mid-upper DM. Throughout the growth period, N3P2 maintained dominant LDM and exhibited RoDM 2.5 % higher at late-full bolling stage (LFB) compared to N2P2. Trends in leaf N content and reproductive organ N content were similar to DM, with N3P2 consistently maintaining higher levels. Regarding N metabolism activities, N3P2 displayed 8.5–16.1 % higher activity of nitrate reductase, 9.5–21.9 % higher activity of glutamine synthetase, and 7.5 %-15.6 % higher activity of glutamate synthase compared to other P across all growth stages, with no significant differences observed with N2P2. Moreover, N3P2 and N2P2 exhibited higher relative expression of NR, GS1, and ferredoxin-dependent glutamate synthase compared to other treatments. In Conclusion, at 25 % reduced N (N3), optimizing the proportion of P fertilizer (P2, 50 %:50 %) could lead to higher yields due to enhanced N re-utilization capabilities in the later growth stage and increased distribution of DM in leaves and reproductive organs, particularly in the middle and upper layers (especially the fruiting branches). The study findings hold significant theoretical implications for achieving sustainable and efficient production in drip-irrigated cotton fields under reduced N management.