Effects of nitrogen reduction combined with humic acid application on island cotton plants and soil nutrients

To investigate the effects of nitrogen reduction with humic acid application on island cotton plants and soil nutrients, in 2022−2023, a one-factor randomized block trial was conducted with island cotton ‘New 78’ as the material. Five treatments were set up, namely, no nitrogen application (CK), normal nitrogen application of 600 kg·hm⁻² (OF₁), reduced nitrogen application of 50% (300 kg·hm⁻², OF₂), and reduced nitrogen application of 300 kg·hm⁻² + humic acid 150 kg·hm⁻² (OF₃), and humic acid 300 kg·hm⁻² (OF₄). The effects of nitrogen reduction and humic acid on dry matter accumulation, nitrogen, phosphorus and potassium accumulation and distribution, soil nutrients and yield quality of island cotton plants were analyzed. The results showed that reduced nitrogen application combined with humic acid effectively promoted dry matter accumulation and optimized dry matter distribution in island cotton. Among them, OF₃ exhibited the highest dry matter accumulation, with a two-year average significantly higher than that of CK, OF₂, and OF₄ by 27.58%, 19.81%, and 12.99%, respectively, while showing no significant difference compared with OF₁. OF₃ effectively promoted the transport of nutrients to the reproductive organs and the distribution proportion gradually increased as the growth period progressed. Nitrogen reduction and humic acid application significantly increased the accumulation of total nitrogen, total phosphorus, and total potassium in island cotton plants and promoted the allocation of nitrogen, phosphorus, and potassium to the reproductive organs. At the flowering and boll period and boll open period, the accumulations of total nitrogen, total phosphorus, and total potassium in OF₃-treated plants were significantly higher than those in CK. However, the accumulations of total nitrogen were slightly lower than those in OF₁, and the differences were not significant. Nitrogen reduction with humic acid significantly increased soil organic matter and alkaline nitrogen content. During the flowering and boll period and boll open period, soil organic matter content was highest in OF₄ and second highest in OF₃. Soil alkaline nitrogen content was highest in OF₁ and second highest in OF₃. Soil available phosphorus and available potassium content were highest in OF₃ and second highest in OF₄. In addition, nitrogen reduction with humic acid not only improved the yield of island cotton but also maintained the fiber quality of island cotton at a high level. In the two-year experiment, the average seed cotton yield of OF₃ reached 6 581.15 kg·hm⁻², which was 36.91%, 4.36%, 11.57%, and 18.85% higher than that of CK, OF₁, OF₂, and OF₄, respectively. The average breaking tenacity improved by 3.35%, 1.31%, and 3.79% compared to CK, OF₂, and OF₄, respectively. The average micronaire value was improved by 8.75%, 2.23%, and 8.37% compared with CK, OF₂, and OF₄, respectively. There was no significant difference in all fiber quality parameters between OF₃ and OF₁. Therefore, the fertilization strategy of applying 300 kg·hm⁻² of nitrogen + 150 kg·hm⁻² of humic acid in the production of the main production area of island cotton in Xinjiang could significantly increase the nitrogen, phosphorus and potassium accumulation of island cotton plants, improve the content of soil organic matter and quick-acting nitrogen, and obtain a higher seed cotton yield to achieve the dual aims of efficient use of nitrogen fertilizer and yield enhancement.