Objectives The optimal combination of irrigation and N application rate for cotton production was studied. The aim was to investigate the high N use efficiency and cotton yield under machine harvesting.

Methods Field experiments were conducted under drip irrigation conditions in Shihezi, Xinjiang Autonomous Region. According to local evapotranspiration (ETc), three irrigation schemes were set up: 60%ETc, 80%ETc, and 100%ETc. Under each irrigation scheme, five N application rates were set up: 0, 150, 225, 300, and 375 kg/hm² (N0, N150, N225, N300, N375), totalling 15 treatments. Cotton dry matter yield and quality, N uptake, water and N use efficiency were determined at the seedling, initial flowering, full flowering, full boll-setting, and full boll opening stages.

Results Cotton’s average dry matter quality in the opening stage was in the order 80%ETc>100%ETc>60%ETc. Except for 60%ETc + N375 and 100%ETc + N225, N application will increase the maximum increase rate of cotton dry matter to a certain extent, and then promoted the accumulation of cotton dry matter. 60%ETc+N150 and 60%ETc+N225 reduced the proportion of dry matter mass distribution to cotton bolls, and the dry matter mass and distribution rate to cotton bolls of other N application treatments were increased to varying degrees compared with N0. There was no significant (P > 0.05) difference in the average N uptake of cotton between 100%ETc and 80%ETc at the opening stage, which were increased by 26.64% and 25.55%, respectively, compared with 60%ETc. Under 60%ETc irrigation, N uptake increased with increasing of N application rate. Under 100%ETc and 80%ETc irrigation conditions, the N absorption of cotton at the opening stage was the highest (N300), and there was no significant difference between N375 and N300. Under the three irrigation schemes, the maximum increase rate of N uptake reached the maximum in N375 but under 60% ETc and 80% ETc drip irrigation conditions, the arrival time of the maximum N uptake increase rate of N375 was advanced by 10 and 3 days respectively, while under 100% ETc drip irrigation conditions, it was delayed by 5 days. Compared with 80%ETc and 100%ETc, 60%ETc reduced seed cotton yield. N application (P < 0.05) increased cotton yield, but there was no significant difference between N300 and N375 at 60%ETc. The cotton yield in N375 was 13.97% and 14.87% lower than N300 at 80%ETc and 100%ETc. Compared with 80%ETc+N300 and 100%ETc+N300 treatments, the N use efficiency of 60%ETc+N300 decreased by 18.36% and 14.64%, and irrigation water use efficiency increased by 5.14% and 36.68%, respectively. The average N use efficiency of the three irrigation schemes was 80%ETc>100%ETc>60%ETc, and the irrigation water use efficiency was 60%ETc>80%ETc>100%ETc.

Conclusions Irrigation and N application had substantial coupling effects on dry matter accumulation, N uptake, and yield of machine-harvested cotton. When the irrigation rate was set to 80%ETc, the dry matter accumulation, N absorption rate, and distribution ratio of cotton organs were optimum, corresponding to N 300 kg/hm² and suitable for mechanical harvesting mode. Boll number per plant and weight per boll were also higher than other treatments, maximizing the comprehensive benefits of yield, water, and N utilization.