Drip Irrigation of Phosphorus Fertilizer Enhances Cotton Yield and Phosphorus Use Efficiency

Root systems are pivotal for nutrient absorption, exhibiting high plasticity in phosphorus (P) acquisition, and significantly influencing soil phosphorus availability. However, the impacts of different P application methods on root parameters and P utilization efficiency in cotton (Gossypium hirsutum L.) under Xinjiang conditions are still not well understood. To identify optimal P fertilization strategies, a consecutive two-year field experiment (2023–2024) under mulched drip irrigation was conducted. Three P application methods were tested: no P (CK), basal P application (PB), and drip P application (PD). Results revealed that P application methods significantly affected cotton dry matter, P use efficiency, root morphology, and yield (p < 0.05). Over the two years, the optimized treatment (25% P applied at bud stage and 25% at flowering-boll stage, PD) increased yield by 13.62% and 9.50% compared to full basal application (PB), with P use efficiency improved by 22.04–31.51% and agronomic efficiency improved by 6.56–9.75 kg kg−1. PB significantly increased soil-available P in 0–20 cm (34.17–70.09%) and 20–40 cm layers (30.37–70.32%) compared to CK. During the bud stage, PD treatment exhibited higher soil-available P in the 20–40 cm layer than PB. PD enhanced P uptake and dry matter accumulation, with increases of 22.43–36.33% and 7.90–15.55% in reproductive organ P accumulation compared to other treatments. Root parameters followed PD > PB > CK across all treatments. At the seedling stage, PB increased total root length by 19.79% compared to CK, while PD increased root volume by 46.15% compared to PB. During the bud stage, PB increased root volume by 53.33% compared to CK, and PD enhanced root surface area and volume by 39.25% and 47.82% compared to PB. Root volume showed a significant positive correlation with phosphorus absorption across growth stages. The PD treatment significantly enhanced soil P availability and P use efficiency and optimized root spatial distribution. This treatment consistently increased cotton yield by 30.41–39.09% (p < 0.05) compared to CK, demonstrating stable positive effects. This study highlights that adjusting P application methods can establish sustainable, high-yield agricultural fertilization systems.