Synergistic Effects of Triple Superphosphate, Mycorrhizal Fungi, and Phosphate-Solubilizing Bacteria on Cotton Yield, Fiber Quality, and Stress Tolerance under Variable Water Availability and Plant Density

Abstract A field experiment was conducted to evaluate the synergistic effects of triple superphosphate (TSP), arbuscular mycorrhizal fungi (AMF), and phosphate-solubilizing bacteria (PSB) on cotton growth, phosphorus acquisition, water relations, and yield under two irrigation regimes (full and deficit) and two plant densities (low: 5 plants/m2; high: 10 plants/m2). The triple combination (TSP + AMF + PSB) exhibited pronounced higher-order synergies, increasing lint yield by 26–56% over the control and 6–22% over the best dual treatments across environments. These benefits arose from complementary mechanisms: TSP provided immediate P supply, AMF extended P acquisition through extensive hyphal networks, and PSB mobilized fixed soil P via organic acid secretion and rhizosphere acidification. The triple treatment increased phosphorus uptake (accumulation in biomass) by up to 98% over the control under full irrigation and improved drought tolerance by elevating relative water content (8–11%), water use efficiency (34–42%), antioxidant enzyme activity (superoxide dismutase + 53–54%), and osmoprotectant accumulation (proline + 91–93%). Photosynthetic rate, chlorophyll content, and fiber quality (length + 14–17%, strength + 10–12%) were also significantly improved. Synergistic effects were strongest under full irrigation–low density (favorable conditions) and remained substantial under deficit irrigation–high density, confirming the robustness of integrated P management. Results demonstrate that combining mineral fertilizer with AMF and PSB offers a powerful, sustainable strategy to optimize cotton productivity and resilience in water-limited and high-density systems.