Effects of Dryland Tailwater Recharge on Paddy Rhizosphere Fungi and Phosphorus Reduction Mechanisms in Erhai Lake Basin

To investigate the effects of dryland tailwater recharge on rice rhizosphere fungi and phosphorus concentrations in the Erhai Lake Basin, two-year field experiments were conducted in Gusheng Village on the western shore of Erhai Lake in 2024 and 2025. Four water management treatments were established: conventional flooding (CK), controlled irrigation (C), lowwaterlayer tailwater recharge (CDC-L), and highwaterlayer tailwater recharge (CDC-H). High-throughput sequencing and the molybdenumantimony colorimetric method were used to analyze the dynamics of rhizosphere fungal communities and reveal the coupled responses among phosphorus fractions in surface water, shallow soil water (0–20 cm), and rhizosphere fungi. The results showed the following: (1) Compared with CK, the phosphorus activation coefficient (AC) in surface water under CDC-L and CDC-H decreased by 35.1% and 33.6%, respectively, in 2024, and by 4.5% and 18.4%, respectively, in 2025. The proportion of particulate phosphorus (PP%) increased by 67.5% and 66.1%, respectively, in 2024, and by 5.1% and 21.2%, respectively, in 2025. The ratio of dissolved phosphate (DP) to PP decreased by 77.3% and 79.2%, respectively, in 2024, and by 37.6% and 49.8%, respectively, in 2025. (2) Water management significantly reshaped the rhizosphere fungal community structure, with regulatory effects showing significant stage specificity. Tailwater recharge increased fungal OTU richness at the tillering stage. In 2024, richness at the tillering stage was 23.8% and 14.3% higher than that at the heading and milk stages, respectively; in 2025, the corresponding increases were 14.2% and 5.9%, respectively. (3) Controlled irrigation and tailwater recharge enriched Basidiomycota, which reduced surface water DP through biological immobilization and the promotion of particulate phosphorus sedimentation. Tailwater recharge also induced the enrichment of Mortierellomycota, which increased total phosphorus (TP) and DP in soil water by activating insoluble phosphorus. Although exogenous phosphorus input at the tillering stage caused a TP peak, wetdry alternation effectively inhibited the transformation of phosphorus into dissolved forms. In conclusion, the CDC-H treatment enabled the resource recovery of dryland tailwater while mitigating agricultural non-point-source phosphorus pollution. These synergistic outcomes demonstrate that CDC-H is more than a technical fix; it is a coupled waterfoodnvironment solution. This approach offers an optimized management model for water conservation and pollution reduction in the Erhai Lake Basin and similar regions. This study elucidates the mechanisms underlying phosphorus migration and transformation in tailwater-recharged paddies from the perspective of rhizosphere fungi, providing theoretical support and a practical paradigm for the green development of agriculture in plateau lake basins.