Optimizing Nitrogen and Phosphorus Management under Different Irrigation Regimes and Plant Densities: Effects on Cotton Yield, Resource Use Efficiency, and Source: Sink Dynamics

This study addressed cotton (Gossipium hirsutum L.) production challenges in arid regions by identifying optimal irrigation, plant density, and nutrient management strategies. Field experiments in Feizabad, Iran (2022–2023) used a split-split plot design with irrigation levels (optimal, I0; severe restricted, Ir50), plant densities (5 and 10 plants/m2), and nutrient treatments (nitrogen [N], phosphorus [P], and phosphorus-solubilizing bacteria [PSB]). Under Iᵣ50, high density with N0P1PSB1 yielded 1785 kg/ha lint and 6.12 g/L water use efficiency in 2023, outperforming N1P0PSB0 (845 kg/ha, 3.08 g/L) by optimizing leaf area index-to-boll load ratio and reducing vegetative branches (3.6 per plant) during 2017's drought (46.0°C, 1.6 mm rain). Under I0, low density with N1P1PSB1 achieved 1924 kg/ha and 92.6 bolls/plant in 2018's milder climate (42.3°C, 3.5 mm rain), enhancing N uptake efficiency (0.75 kg/kg). P or PSB increased reproductive branches (16.2 per plant), P accumulation (44.8 kg/ha), and P use efficiency (280.7 g/g) under Ir50, while high density reduced N use efficiency (4.5 kg/kg) but raised P use efficiency, supported by higher relative water content (86.8%). The combination of biological and chemical fertilizers demonstrated synergistic effects on nutrient acquisition and utilization under water-limited conditions. Soil microbial activity was significantly enhanced by PSB inoculation. Conclusion: Tailoring management to climate improves cotton yield and efficiency, with high density excelling in drought and low density with N thriving under full irrigation. These findings provide practical guidelines for sustainable cotton production in water-scarce environments.