Objectives We investigated the light distribution across the canopy, and the dry matter and nitrogen uptake and distribution in vegetative and productive organs of mechanized sea island cotton under different nitrogen application rates and planting densities, to propose an optimum combination of N application rate and planting density for efficient sea island cotton production.

Methods The two-factor split field experiments were conducted in in Awati County, Xinjiang, in 2023 and 2024, the local main cultivar, sea island cotton Xin 78, was selected as the research object. The main plots were three planting densities, 20×10⁴ (D1), 24×10⁴ (D2), and 28×10⁴ plants /hm² (D3); and the subplots were four N application rates, 0 (N0), 160 (N1), 320 (N2), and 480 kg/hm² (N3). The interception rate of photosynthetically active radiation (PAR) in top 90−110 cm, middle 50−70 cm, and lower 10−30 cm layer of canopy were measured at the key growing stages, and the dry matter and nitrogen content in vegetative and reproductive organs were analyzed at the same stages. At harvest, the yield and yield components of sea island cotton were investigated.

Results Increasing N application rate and planting density enhanced PAR of cotton canopy, as well as the maximum accumulation of dry matter and nitrogen per plant, along with nitrogen use efficiency. In 2023 and 2024, as N application rate increased, the PAR interception rates in the lower and middle canopy layers during the boll-opening stage exhibited a parabolic response, reaching their maximum values at the N2 level. These maximum values represented increases of 18.97% to 31.7% and 35.1% to 108% compared to the N0 level, respectively. The trends in dry matter and N accumulation of whole plants, as well as N accumulation in vegetative organs, were consistent, all increasing with higher N application rates and reaching their maximum accumulation at the N3 level. N accumulation in reproductive organs showed an initial increase followed by a decrease, peaking at the N2 level, with the N2 level being 11.6% (in 2023) and 4.53% (in 2024) higher than the N3 level. The agronomic nitrogen use efficiency, nitrogen recovery rate, and partial factor productivity of nitrogen fertilizer (PFPN) all exhibited a linear decreasing trend with increasing N application rates. In 2024, the PFPN increased by 8.08% compared to 2023. Compared to the D2N3 treatment, the PFPN in D2N2 treatment increased by 67.39% in 2023 and 60.51% in 2024, with an average increase of 63.84% over the two years. Over the two years, as planting density increased, the PAR and N accumulation in the lower and middle canopy layers during the boll-opening stage, the N accumulation in reproductive organs, and yields of seed cotton and lint cotton all initially increased and then decreased, with peaks observed at the D2 density. The total N accumulation at the D2 density increased by 5.77% and 2.15% in 2023 and 6.57% and 2050% in 2024, compared to D1 and D3 densities, respectively. The dry matter accumulation amount per plant and the maximum dry matter accumulation rate (Vm) decreased with the increasing densities (D1>D2>D3). The N recovery rate and PFPN also exhibited an “increase-decrease” trend. Compared to the D2N3 treatment, the N recovery rate in the D2N2 treatment increased by 58.8% in 2023 and 69.6% in 2024, with an average increase of 62.5% over the two years.

Conclusions Planting density and nitrogen application rate influence the absorption, accumulation, and distribution of dry matter and nitrogen during cotton's growth stages. A reasonable combination of these factors can optimize the canopy structure of sea-island cotton, enabling the lower and middle canopy layers to intercept light energy and carry out photosynthesis more efficiently, thereby providing the energy and material basis for high crop yields. Comprehensive consideration of the accumulation and distribution of dry matter and nitrogen, yield, and nitrogen use efficiency across different growth stages, the suitable combination for the mechanically harvested sea island cotton is plant density 24×10⁴ plants/hm² and N application rate 320 kg/hm².