Optimizing nitrogen fertilizer for improved root growth, nitrogen utilization, and yield of cotton under mulched drip irrigation in southern Xinjiang, China

The root system plays a crucial role in water and nutrient absorption, making it a significant factor affected by nitrogen (N) availability in the soil. However, the intricate dynamics and distribution patterns of cotton (Gossypium hirsutum L.) root density and N nutrient under varying N supplies in Southern Xinjiang, China, have not been thoroughly understood. A two-year experiment (2021 and 2022) was conducted to determine the effects of five N rates (0, 150, 225, 300, and 450 kg N ha^-1) on the root system, shoot growth, N uptake and distribution, and cotton yield. Compared to the N0 treatment (0 kg N ha^-1), the application of N fertilizer at a rate of 300 kg N ha^-1 resulted in consistent and higher seed cotton yields of 5875 kg ha^-1 and 6815 kg ha^-1 in 2021 and 2022, respectively. This N fertilization also led to a significant improvement in dry matter weight and N uptake by 32.4% and 53.7%, respectively. Furthermore, applying N fertilizer at a rate of 225 kg N ha^-1 significantly increased root length density (RLD), root surface density (RSD), and root volume density (RVD) by 49.6-113.3%, 29.1-95.1%, and 42.2-64.4%, respectively, compared to the treatment without N fertilization (0 kg N ha^-1). Notably, the roots in the 0-20 cm soil layers exhibited a stronger response to N fertilization compared to the roots distributed in the 20-40 cm soil layers. The root morphology parameters (RLD, RSD, and RVD) at specific soil depths (0-10 cm in the seedling stage, 10-25 cm in the bud stage, and 20-40 cm in the peak boll stage) were significantly associated with N uptake and seed cotton yield. Optimizing nitrogen fertilizer supply within the range of 225-300 kg N ha^-1 can enhance root foraging, thereby promoting the interaction between roots and shoots and ultimately improving cotton production in arid areas.